Pentode-Triode Sound
west
there is no discernible volume difference to the ears.
The way I cut the amps power in half is by switching the output tubes to
triode and adjusting the volume to have the same SPL in pentode.
I understand the theory but I would love to hear your opinions on what if
any differences can you hear between the 2 modes. I know this subject has
been beaten to the ground but the theory always obfuscated the sound changes
for me. Can we just talk about the sound in your words? It will help me
plenty. Thank you.
Cordially,
west
John Byrns
wrote:
There is a completely different change in sound depending on whether the
amplifier has negative feedback or not. You need to tell us whether the
amplifier in question uses negative feedback or not before we can
comment on the sonic differences between the two modes.
Regards,
John Byrns
--
Surf my web pages at, http://fmamradios.com/
Patrick Turner
The argument about pentode&beam VS triodes has been going on now
since about 1933, some 74 years.
Contestants in the argy bargies over the ssue usually
assume the triodes have little NFB, say 12 dB, and the
pentodes have heaps, say 20dB, which is 2.5 times more GNB
than the triode case. This way the pentode amp will have about the same
Rout
as the triode amp and a fair comparison can be made.
We should also incude comparisons with Ultralinear, and Local Cathode
FB, a la Quad-II amps.
All the people I know cannot tell the slightest difference between
triode sound and UL
sound when switching the screen supply to the anode, or OPT screen tap.
I've tried to get ppl to detect the differences, but none have.
Pure Pentode or beam tetrode SHOULD be far more easily detected by the
golden ear brigades
because the harmonic spectra of a pentode in PP or SE is a far richer
mix
of high numbered harmonics which all spoil music, especially
if the circuit works in near class B PP conditions.
But the GNF applied around a typical multigrid circuit reduces the
spuriae of all kinds
by the amount of NFB applied and this is usually by a factor of 0.1
even ib a near class B amp which usually produces a low amount of pure
class
of say 3 watts before the amp lurches into rougher working in class AB
which produces the
most objectionable spuriae, and IMD especially.
At 1 watt most ppl still hear ZERO difference between 2 x EL34 in class
A triode with 12dB GNFB, good for 12 watts max,
and the same tubes in pentode class AB good for 35 watts max.
The pentode may in fact measure less THD / IMD than the triodes ( or UL
for that matter )
although the mix of residual spuriae with pure pentode will be a richer
mix.
Triodes may have some 2H, 3H, and then very low levels of anything else,
but the PP pentodes will have 2,3,4,5,6,7,8,9H, but in declining
quantities.
The levels of say 4,5,6,7,8,9H at 1 watt from typvcal class AB PP
pentode amps at a watt
is usually negligible, along with resulting IMD.
Hence we don't hear the spuriae; its eclipsed by everything else.
But applying say only 10dB GNB around a pure pentode or beam output
stage will produce Rout that is
much above the triode case with 12dB GNB, so the speaker response won't
be the same, and the sound WILL change
with apparently more level wherever the speaker Z is high.
Triode without any GNFB are usually listenable at low levels, but
pentode amps are a disaster, and awful,
unless the load is kept at a constant R value, which is practically
unviable for most ppl to arrange.
UL or local CFB gives intermediate Rout; the Rout is usually about = RL,
with no GNFB.
Pure pentode Rout can be typically 40 ohms with a pair of EL34.
Triodes without GNFB are about 4.5 ohms with the same OPT ratio used in
pentode.
Triode connection prefers 10k RLa-a instead of 5k a-a, and if this is
the case
triode Rout = 2.3ohms approx, including some OPT winding resistances.
The NFB or not arguments revolving around percieved sound imageing
effects is all anecdotal.
When careful response measurements are made the sound changes probably
are due to varying
Rout and response contours.
Where the Rout is kept low for either case of with/without GNFB,
and where the response is maintained, with the same phase shifts, I bet
nobody notices the NFB
if its used.
Putting on my flame suit about right now...
Patrick Turner.
Anonyma
know it is never this simple!
west <rest...@verizon.net> wrote:
> Setup: Speakers efficient enough that if I cut the amp's power
in half,
> there is no discernible volume difference to the ears.
> The way I cut the amps power in half is by switching the output
tubes to
> triode and adjusting the volume to have the same SPL in pentode.
> I understand the theory but I would love to hear your opinions on what if
> any differences can you hear between the 2 modes. I know this subject has
> been beaten to the ground but the theory always obfuscated the sound changes
> for me. Can we just talk about the sound in your words? It will help me
> plenty. Thank you.
>
> Cordially,
> west
The fly in your ointment is, as always, global negative
feedback, which can homogenize anything. That's great for
those who eat preprocessed, pre-sliced, predigested cheese
on tasteless white bread. But it isn't why you spent the money
on a tube amp.
We'll assume that your amp is playing in Class A at all times,
which may or may not mean modest volume, and that your
system is well matched. Assuming on top of that there there is
NFB in the order of 20dB, you will hear almost no difference.
Assuming 12dB of NFB, which is then added to the native NFB
of the tube when it is trioded, I still don't think you will hear much;
Patrick in his long screed has about a dozen other things that
might or might not make a difference.
However, assuming Class A operation and only 6dB of NFB in
the amp, chances are that you will hear that the triode amp is
smoother, warmer, more pleasing, easier to listen to for a long t
ime. If you could manage to remove the NFB altogether (it might
mean snipping two or four wires or disconnecting some
components on a PCB and fitting jumpers instead -- you gotta
put it back the way it was before you operate it in pentode again)
you will definitely hear a difference. At these modest volumes
we're talking about, average listening volume, you will find the
pentode amp a little unrefined, you will find a UL or CFB amp
more refined, but your dissatisfaction with them will grow over
time; they will make you edgy. Notice that we're talking about
a good amp here, not one in which you can consciously hear
distortion in any mode of operation. We're in fact talking of a
subliminal effect, where in the pentode amp the lower level
(absolute) mix of higher harmonics after NFB impacts on
your enjoyment in a way that you will find difficult to pinpoint. I
n fact, as Patrick may have been trying to say, the triode version
with low or zero negative feedback cannot be made as t
heoretically clean on a distortion meter as the high-NFB
pentode and especially the UL or CFB types, which can be
made stunningly clean. But that's an irrelevance. It isn't the
meter that is the arbiter here but your ear.
To cut to the chase, never mind all the other complications
Patrick has brought to the table (he'll soon get onto damping
factors at even greater length), there are already too many
complications here at a much more fundamental level because
you didn't take control of the NFB in your amp before you started
the test.
It is of the subtleties of the electronic interactions, so rudely
called complications above, which make tube sound endlessly
fascinating -- and sometimes so frustrating you want to scream.
HTH.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review
Borked Pseudo Mailed
know it is never this simple!
********
west <rest...@verizon.net> wrote:
> Setup: Speakers efficient enough that if I cut the amp's power
in half,
> there is no discernible volume difference to the ears.
> The way I cut the amps power in half is by switching the output
tubes to
> triode and adjusting the volume to have the same SPL in pentode.
> I understand the theory but I would love to hear your opinions on what if
> any differences can you hear between the 2 modes. I know this subject has
> been beaten to the ground but the theory always obfuscated the sound changes
> for me. Can we just talk about the sound in your words? It will help me
> plenty. Thank you.
>
> Cordially,
> west
**********
Patrick Turner
The ability for listeners to subjectively discern NFB artifacts is not
clear after 74 years of arguments,
so tonight's discussions will not provide any certainty.
But I would say its up to diyers, audiophiles, tinkerers to decide what
they wish
with regard to NFB, and maybe they either agree with Andre, or myself,
or with neither of us,
or have an idea that is elsewhere.
I must raise the issue of damping factor as A has predicted I would, and
ask the group what
is best, a pentode amp with Rout = 40 ohms, as well as the THD = 0.5%
and IMD at 5% at 2 watts,
or is the music to sound better, less tiring, etc, with Rout = 1ohm, and
THD = 0.05% and IMD = 0.5% at 2 watts?
In about 1999, I demonstrated a 5050 UL PP stereo amp to the Audiophile
Society in NSW.
This had 2 x GE6550A tubes per channel and 6CG7 drivers. About 16dB of
GNFB was used.
On the same day, another guy demonstrated his Forced Symmetry circuit
with a quad of 6550 per channel
but in triode, and a claimed total of 6dB NFB, enough to halve the raw
triode Rout,
but do little to quell any THD & IMD.
The 30 club members present were not overly impressed with the triode
amp and in every
way the UL amp with 1/2 the bottles was said to sound smoother at the
top end.
Arguments continued between the no NFB camp and the pro camps for some
time, and later when I
had another demo with SEUL amps with 13Ei tubes again with about 16dB
GNFB,
During the coffee break I overheard a guy say to his mate, "well, this
proves you CAN use NFB with an SE amp."
It is a fashionable idea that SEamps must not use any NFB. Bollocks. It
ain't necessarily so.
However, please remember this is only my opinion, and any here in the
group are free to find out for
themselves.
Everyone liked those SEUL amps; there was sea of contended smiles.
They said the midrange was quite especially outstanding, but in fact it
was all true full range sound,
free of the artifacts of an ill concieved SE amp without enough power
and a
poor technical performance with high Rout.
There were 30 ppl present, the venue is the upstairs section of an old
library,
and much louder levels than anyone would use at home were used.
All quite OK with only 25 watts/channel.
The 13Ei is a perfectly HORRID non linear tube used as a pure bean
tetrode.
I applied 66% of the anode signal from a tap on the OPT to wrench the
performance
towards triode op but with a higher power output nearing beam output
power.
Rout was still too high.
GNFB did the rest of the job to reduce Rout to around 0.6 ohms at that
time.
UL taps are NFB from anode to the screen, and triode is just maximum
screen FB
unless you had an "extension of the anode winding" to derive a signal
GREATER
than the anode signal which is then fed back to the screen.
I see nothing to recommend this idea, because of further power output
reductions lower than triode.
The triode connection is a form of NFB.
There IS NFB within a triode even without a screen.
Time and time again ppl tell me the dynamics and pizzazz are bettered
with NFB.
I just refuse to use too much NFB. I make sure the open loop bandwidth
of the OPT is at least equal or greater than the closed loop BW.
Devices have an adequate amount of class A power.
If your amp has more NFB than mine, and measures slightly better, I am
not
terribly interested, because mine sound well enough.
What I have seen occur often is customer preference for a tube amp
with say a reasonable 0.05% THD at 2 watts maybe 0.2% at 40 watts,
with NFB over a solid state amp with 0.002% at 100 watts, and such low
THD at
2 watts its almost un-measurable.
To my mind the tube amp artifacts in a FB amp don't matter at all.
Please don't believe me.
Find out with your own experiments.
Patrick Turner.
Iain Churches
"west" <rest...@verizon.net> wrote in message
news:zpkGh.215$Ih.208@trnddc02...
> Setup: Speakers efficient enough that if I cut the amp's power in half,
> there is no discernible volume difference to the ears.
> The way I cut the amps power in half is by switching the output tubes to
> triode and adjusting the volume to have the same SPL in pentode.
Well, not even half.
Switching my UL pentode amps to triode
reduces the overall power from 50W to 32W, so in terms of
power, by shirtcuff calculation that's less than 2dB. With a
complex music signal, many people might be hard pressed
to notice the level change:-)
> I understand the theory but I would love to hear your opinions on what if
> any differences can you hear between the 2 modes. I know this subject has
> been beaten to the ground but the theory always obfuscated the sound
> changes
> for me. Can we just talk about the sound in your words? It will help me
> plenty. Thank you.
I have, on several occasions set up listening sessions with
groups of musicians and studio colleagues, all people
whose level of aural perception I respect.
None has been able to differentiate between the UL
and triode "sound". I would add that this amp has
18dB NFB.
However, sometime ago, I modified a VAC80W tube
amp from UL to triode for a friend who uses it with a pair
of open baffle speakers. The amp has very little NFB,
and a low damping factor.
Conversion to triode did not seem to alter any of the
performance parameters much except for ironing out a
lift of some 3dB at 20kHz.However, there was a marked
reduction in Zout and hence and improvement in the DF. See:
http://www.kolumbus.fi/iain.churches/Pics/Test%20Data/VAC_Zo.png
I returned the amplifier to its owner with one channel
only modified to allow him to compare triode with UL
and decide if he wanted to continue with the modification.
I did not mention anything to him about the comparison
measurements I had made. He claimed he could hear a
distinct difference, and mentioned that in triode mode
the bass was "leaner and tighter"
Regards to all
Iain
sho...@trailing-edge.com
> Triode without any GNFB are usually listenable at low levels, but
> pentode amps are a disaster, and awful,
> unless the load is kept at a constant R value, which is practically
> unviable for most ppl to arrange.
For "low-fi" and "mid-fi" applications (a couple watts output from
phonographs, car radios, etc), single-ended pentode miniature tubes
(e.g. 6AK6, 6AQ5) are commonly used and do pretty well with zero or
smallish amounts of feedback. Of course they're going into pretty
lousy speakers in abysmal environments, and a common theme is to shunt
a resistor across the load (usually on the speaker side of the output
transformer but sometimes the tube side of the output transformer). I
always thought the shunt resistor was there in case the speaker comes
unconnected, but maybe you have a point about keeping a more constant-
R load.
Tim.
Peter Wieck
> Blather without support.
McCoy: your opinions are aways welcome, even to me (whether you
believe that or not). But, West was looking for such opinions
supported by specifics and reasons. That has always been hard for you.
You will not cite specifics as you tend to be quite wrong when you do.
And reasons typically require a 'because'. Oops, more specifics are
then required. Best to stay out of discussions such as this one and
let those with actual knowledge contribute. Then, you might actually
learn something such that you might get your little KISS UltraFi amp
to actually pass signal.
Peter Wieck
Wyncote, PA
Patrick Turner
The old trick of shunting the a-a with R
will give a load when the speaker isn't connected.
Bass resonances of hi Z or other high Z peaks are controlled a bit, but
not a lot.
Then power is wasted into the shunt R.....
If R = 3 x speaker nominal Z, the DF is much better than having no R at
all,
and not too much power is wasted.
Patrick Turner.
> Tim.
Bret Ludwig
>
> The fly in your ointment is, as always, global negative
> feedback, which can homogenize anything. That's great for
> those who eat preprocessed, pre-sliced, predigested cheese
> on tasteless white bread. But it isn't why you spent the money
> on a tube amp.
>
> We'll assume that your amp is playing in Class A at all times,
> which may or may not mean modest volume, and that your
> system is well matched. Assuming on top of that there there is
> NFB in the order of 20dB, you will hear almost no difference.
> Assuming 12dB of NFB, which is then added to the native NFB
> of the tube when it is trioded, I still don't think you will hear much;
> Patrick in his long screed has about a dozen other things that
> might or might not make a difference.
It's more bullshit.
Amplifiers with no feedback are just not any good, whilst too much is
harmful. But 20 dB is normal and seems to beabout the right level. Now
there are amplifiers that have no GLOBAL nfb that work fine, and in
fact triode stages have a certain built in feedback, as it were.
The amplifier must have a damping factor appropriate for the speaker
used (and higher is not always better.) But the pure open loop Restrum
Erectric circa 1938 amp just does not cut it. Yes the ollies, like our
friend wlaud73 who seems bent on buying up every single 19C Oscillator
from dumb Amellicans who haven't figured out the little oliver will
probably pay a grand apiece if ww drive it up there, pay stupid money
for them. But measure the standard Japanese prophylactic and you will
know why. Sound is not the reason.
Bret Ludwig
West needs to run some tests with a scope and a sig gen first to see
what approximately he's looking for, then rig up a Hafler bridge and
null out some real world signals and listen to the diff signal.
What he's looking for will mostly be apparent when the amp is
between its true Class A power point and clipping.
In order not to deafen himself while listening, he will need to
build some L-pad loads to enable using a speaker while running the
beast at high power.
Jon Yaeger
Ludwig at bret...@yahoo.com wrote on 3/6/07 4:53 AM:
Good advice, Bret, but you miss the point. West wants YOU to do the tests,
and report back to him.
Jon
Ian Bell
> Now
> there are amplifiers that have no GLOBAL nfb that work fine, and in
> fact triode stages have a certain built in feedback, as it were.
>
Can you explain how this 'built in' NFB occurs?
Cheers
IAn
Bret Ludwig
> in article 1173174812.592743.287...@p10g2000cwp.googlegroups.com, Bret
> Ludwig at bretld...@yahoo.com wrote on 3/6/07 4:53 AM:
I am reminded of the story, perhaps bogus, told by the opportunistic
lawyer-leech Engelberg who was house counsel and majordomo to Joe
DiMaggio in his last decades of life. One day while in Southern
California, Joe asked him to drive to "the cemetery"-there was no need
to ask which one-to visit "the grave"-likewise.
When they got there Joe had Engelberg go in and look at it and come
back. DiMag asked him about how it looked. Engelberg described the
scene, and said, Joe, why don't you go over in there and visit?
Joe replied, "For thirty years there has been a guy who goes there
every day and waits for me so he can take my picture by the grave. Let
him wait."
Bret Ludwig
Pentodes have very much more gain because the screen grid in effect
removes this "feedback". More precise explanations exist but I am a
two finger typist so will direct you to the books such as those for
free at www.pmillett.com.
Ian Bell
I am very familiar with this site and have all the tube books it mentions in
one form or another. Not one of them mentions built in feedback in triodes.
So I ask again, can you explain how this 'built in' NFB occurs.
Cheers
Ian
west
Profile try, let me know, Jon ... you're basically diminutive in size and
got your ass whipped many times growing up ... correct? Not a gift of mine,
just your obvious transparency.
"Jon Yaeger" <jon...@bellsouth.net> wrote in message
news:C212D66B.9672B%jon...@bellsouth.net...
Patrick Turner
Probably, Bret cannot explain the NFB that does exist in every triode,
or pentode strapped as such, but he is on the right track about the
existance of the NFB that sure enough isn't explained
as widely as it should be in so many old books.
Terman's Radio Engineering of 1937 has a long discussion of the way
tubes work in detail
penned by Professor Child, and wherin the prof describes the "self
regulation"
of triodes which is equated to NFB today.
The NFB in triode exists because the electrostatic field effect of the
grid upon
the electron space charge around the cathode and hence electron current
flow
is opposed by the oppositely phased field effect of the anode on the
space charge.
The triode's current flow is the result of the net effects of the two
fields
operating simultaneously upon the space charge.
So when a 12AX7 grid moves +1V, there may be a -70V change at the anode
and its RL.
The effect on the cloud of electrons around the cathode is the compined
effect of the +1V from the nearby grid, and the -70V from the much more
distant anode.
Patrick Turner.
Bret Ludwig
>
> Probably, Bret cannot explain the NFB that does exist in every triode,
> or pentode strapped as such, but he is on the right track about the
> existance of the NFB that sure enough isn't explained
> as widely as it should be in so many old books.
> Terman's Radio Engineering of 1937 has a long discussion of the way
> tubes work in detail
> penned by Professor Child, and wherin the prof describes the "self
> regulation"
> of triodes which is equated to NFB today.
>
> The NFB in triode exists because the electrostatic field effect of the
> grid upon
> the electron space charge around the cathode and hence electron current
> flow
> is opposed by the oppositely phased field effect of the anode on the
> space charge.
> The triode's current flow is the result of the net effects of the two
> fields
> operating simultaneously upon the space charge.
>
> So when a 12AX7 grid moves +1V, there may be a -70V change at the anode
> and its RL.
> The effect on the cloud of electrons around the cathode is the compined
> effect of the +1V from the nearby grid, and the -70V from the much more
> distant anode.
If you understand why the presence of the screen grid enables the
tetrode and pentode tubes to have far higher amplification factors
than triodes, you then understand it is analogous to, or just as if,
the triode had a built in feedback circuit which the pentode does not.
It's AS IF the screen grid sucked out the feedback force. This is
well explained in many of those books. It was and is not usually
explained as "feedback" because no one back then had an irrational
fear of negative feedback: they just didn't use it because gain was a
scarce commodity.
Read pp 7-9 of the RCA Receiving Tube manual- I just happened to
pick up RC-24. I think they are all the same from RC20 on. RC24 is
copyright 1965.
Ian Bell
>
> If you understand why the presence of the screen grid enables the
> tetrode and pentode tubes to have far higher amplification factors
> than triodes, you then understand it is analogous to, or just as if,
> the triode had a built in feedback circuit which the pentode does not.
> It's AS IF the screen grid sucked out the feedback force. This is
> well explained in many of those books. It was and is not usually
> explained as "feedback" because no one back then had an irrational
> fear of negative feedback: they just didn't use it because gain was a
> scarce commodity.
>
> Read pp 7-9 of the RCA Receiving Tube manual- I just happened to
> pick up RC-24. I think they are all the same from RC20 on. RC24 is
> copyright 1965.
So what you are saying is that you consider the effect of plate voltage
changes on anode current in a triode to be negative feedback. That cannot
be the case because that would mean under small signal conditions (when the
plate voltage hardly changes) the gain of a triode would be as high as
pentode which is clearly not the case.
Ian
Patrick Turner
Feedback was assumed to be always something operating around some
external network.
Nobody had the modern fear of FB, but that wasn't the reason why FB was
not the term used for
internal triode self regulation at all frequencies.
The reason was to keep the distinction between the alterable external
loop FB and the
unavoidable fixed FB internally with all triodes.
However, at HF the situation is that the NFB in triodes is constantly
being refered to due to the Miller effect, ie, current flow from
anode to grid and into the grid from the stage before it by a
capacitance path.
in RF apps, positive FB is used to apply an oppositely phased sign to
the that at the anode back to the grid
to neutralize the gain losses caused by the NFB. Such circuit tricks
were a PITA for
engineers and users of equipment with variable tuned circuits, since
oscillations and all manner of difficulties arose.
Pentode and beam tube invention got around the problem of both
capacitance path NFB
and LF FB due to electrostatic forces acting like a shunt NFB set up.
>
> Read pp 7-9 of the RCA Receiving Tube manual- I just happened to
> pick up RC-24. I think they are all the same from RC20 on. RC24 is
> copyright 1965.
My copy of the RCA tube manual has the info on how tubes work, but not
why there is NFB in the triode
at LF, but does mention the Miller C without mentioning Mr Miller.
You have to search further for the proof on NFB, but once you
understand,
you cannot see any other way a triode can work so uniquely amoung all
the devices known.
Many don't accept there is NFB in a triode.
Heresy abounds on a BS riddled Internet.
Patrick Turner.
Patrick Turner
The size of the voltage is irrelevant, triode gain is always
the same until distortion reduces it.
The triode FB always acts under large, medium, and small signal
conditions
Where there is an RL connected to the anode which permits the voltage to
change
with changing Ia, there will always be some effect of the anode voltage
change upon the
Ia. Both grid and anode have transconductance.
More astonishingly, WITHOUT ANY Ia change, when the triode has a
constant current source
as its anode load, the anode voltage will still change when the grid
voltage has changed.
And the gain with a CCS = µ .
And the amount of inmternal NFB applied becomes the highest possible
amount.
If you were to instal a screen to a given triode and it perfectly
screened off the
electrostatic field effects between anode and the cathode space charge,
then you'd have a perfect tetrode, and gain would be simply Gm x Ra,
and Ra would be extremely high, so the gain would be too, and the gain
is predictable if we knew the electrode distances and internal behaviour
formulas.
The very same distance information determines the triode gain with its
FB because the electrode distances,
anode to cathode, and grid to cathode
set the quantifiable amounts of the electrostatic fields and their net
effects around the cathode at distances from it
and to where the grid is located.
A pentode is regarded as a current source because its Ra is often say
1Mohm, while the RL connected is say 50k.
It can be this for say a 6AU6, at about 2mA for Ia.
Gm is say 0.003A/V, so since µ = gm x Ra, the µ for 6AU6 is 0.003 x
1,000,000 = 3,000,
and if you have a CCS load on a 6AU6, you get truly enormous gain.
If you have a shunt NFB loop though, the gain is then set by the R1 and
R2 values, and where the gain with FB
is say 30, then there is a 100 fold gain reduction from the CCS load
figure.
And the gain with NFB will be approximately the same
if gain without NFB or any load is between 100 and 10,000.
With a triode, we are never to know what gain it WOULD HAVE if it had a
perfect screen to allow Ra to be extremely high
when RL was a CCS, unless we understand more complex formulas the real
designers of tubes were used to using
all those years ago.
We can say that whatever the triode gain was with a perfect screen, the
gain would be enormous.
But if the triode has a gain of say 25 with a CCS, then whatever NFB
within the triode is the product of electrostatic
field effects having relative intensity ratio of 1:25.
The triode FB isn't a perfect FB path because anode current varies to a
rule dependant on a constant x the square root of the
anode voltage change cubed.
Its called the 3/2 rule for anode current vs anode voltage, abd Ra
curved lines seen in the data sheets
for the fixed Eg grid bias voltage values are graphs of a line dran
where Ia = K x Voltage change to the power of 3/2
This formula and its discussion IS in the old books fairly widely where
they talk about diodes, then triodes.
If you never read a book, you'll never learn anything.
Patrick Turner.
> Ian
Mark Aitchison
> So what you are saying is that you consider the effect of plate voltage
> changes on anode current in a triode to be negative feedback. That cannot
> be the case because that would mean under small signal conditions (when the
> plate voltage hardly changes) the gain of a triode would be as high as
> pentode which is clearly not the case.
>
> Ian
I don't think you mean *small signal* conditions so much as output into
a near short circuit. The parameter to compare between triode and
pentode then would be transconductance, and it varies a lot from pentode
to pentode (e.g. 1.85, 2.2, 9, 6.4, 7.4, 6, 2.2mA/V for EF40 to EF86) or
triode to triode (e.g. 2.9, 5.5, 2.2, 1.6, 6.1, 12.5mA/V for ECC40 to
ECC88), and of course varies with operating conditions. The
representative sample of valves shows the "gain" (not voltage gain,
because we want to avoid the complication of voltage nfb for the
comparison) is no worse - in fact slightly better on average - for
triodes (who get a maximum 38% higher than pentodes in this small
sample, and an average of 5.1 compared with 4.4mA/V for the pentodes).
Triodes (or pentodes in "triode mode") really do behave like pentodes
with negative feedback in all important respects, but the main
complicating factor is that the negative feedback is fed into a place
normal amplifiers don't have: in triodes it is an internal electric
field we can only imagine, in triode-strapped pentodes it is g2 - it
isn't exactly shunt nor series nfb, and the screen grid has a
significant nonlinearity (when fed from a large voltage swing, as it
normally will be) so the resultant voltage transfer function is slightly
strange (still more linear, but not exactly what you might expect from
plain conventional negative feedback).
If you compare plate current-for-grid voltage curves (where the plate
voltage is held constant) the basic shape is identical for triodes and
pentodes. I have seen it claimed that the triode curve is pretty much a
straight line except for a bit of a curve, but really they all (triode,
tetrode, pentode, JFET etc) are essentially a parabola in the main
(middle/useful) section but loosing it at the extremes, approaching a
horizontal line for large negative grid voltages and approaching a
limited slope as the grid voltage approaches zero. That limited slope
means it approaches a straight line, which is good news on the linearity
front, but in real amplifiers with real load resistors this will be
modified by the plate voltage dropping a lot, hence nonlinearities for
other reasons. So triodes are not magically more linear. Some triode
might be a bit better than some pentode+nfb combinations for linearity
for small-medium output voltages, but the cult following of triodes
("one grid good, two grids bad!") is a bit silly, and that coupled with
the idea of any negative feedback being sonic suicide is difficult to
accept if we (gasp!) publicly acknowledge the heresy that triodes have
(in effect) negative feedback built in.
The two best ways to appreciate that triodes have de facto negative
feedback are:
1. consider how a triode-connected pentode very closely resembles a
triode (output impedance, voltage gain, lower noise, dominant terms in
the polynomial relating output to input, general appearance of curves).
A pentode with the screen grid connected to the output (anode/plate)
obviously has voltage negative feedback, yet acts like a triode, so it
strongly suggests a relationship between triode characteristics and nfb.
2. the proof, though, comes from the physical effects going on; this
has been described already, and show an increase in plate voltage
results in an extra (superimposed) electric field that tends to oppose
the increase, which is pretty much negative feedback in a nutshell.
My 2 cents worth,
Mark A
Arny Krueger
news:45ee6b83.0@entanet
You've missed the fact that its not the absolute magnitude of the changes,
but the relative magnitude of the changes that matters.
Arny Krueger
Either all amplifiers sound the same, or exactly which amplifiers you use
will be signficant in this test.
There are a great number of other potentially significant, unspecified
parameters.
IOW, your proposed experiment is woefully under-specified.
Jon Yaeger
Hmmm. . . . I see . . . It's OK for you to malign my character, ethics,
etc. unseen but you cry foul when I point out your obvious flaw.
Didn't get in many fights as a kid . . . kicked ass as many or more times
than mine was. Try again, Dr. Freud. Your profiling ability is bullshit.
There are givers and takers. Patrick is a giver. You're obviously a taker,
and a lazy one at that.
Have a nice day.
Jon
n article EgrHh.1888$3i.840@trnddc01, west at rest...@verizon.net wrote on
3/6/07 11:33 PM:
Iain Churches
> Ludwig at bret...@yahoo.com wrote on 3/6/07 4:53 AM:
>
>> build some L-pad loads to enable using a speaker while running the
>> beast at high power.
>>
> Good advice, Bret, but you miss the point. West wants YOU to do the
> tests,
> and report back to him.
>
Hi Jon. Strangely enough, I don't think West's request
was an unreasonable one.
Amp comparison and analysis is much better done as a
group than alone.
As mentioned earlier, I doubt that many will be able to differentiate
between a pentode and a triode PP amp that has >12dB NFB.
Regards to all
Iain
Arny Krueger
news:f_DHh.11025$sA3....@reader1.news.saunalahti.fi
Hearing the difference between the amp operational modes portrayed at
http://www.kolumbus.fi/iain.churches/Pics/Test%20Data/VAC_Zo.png
might be possible. With the right speaker loads it would probably be easy.
Ian Bell
>
> As mentioned earlier, I doubt that many will be able to differentiate
> between a pentode and a triode PP amp that has >12dB NFB.
>
I am puzzled by this regular mention of so many dBs of negative feedback as
if it were something you can set. The feedback fraction sets the closed
loop gain and is normally relatively fixed by a resistor ratio. Open loop
gain depends on various device parameters and therefore varies quite a lot.
The amount of feed back is simply open loop gain less feedback fraction
(all expressed in compatible units) and therefore varies as much (in dB) as
open loop gain.
Or have I missed something?
Ian
Iain Churches
"Arny Krueger" <ar...@hotpop.com> wrote in message
news:B5SdnTRIa-VkjXLY...@comcast.com...
> Hearing the difference between the amp operational modes portrayed at
> http://www.kolumbus.fi/iain.churches/Pics/Test%20Data/VAC_Zo.png
> might be possible. With the right speaker loads it would probably be
> easy.
Hi Arny. Yes I think you are right.
Unfortunately I was not present when the modified (to triode) and
unmodified (still pentode) channels were compared.
The owner of the amp is a man whose musical skill and
perception I hold in high esteem. As I mentioned earlier,
he had no knowledge of the measured differences in the amplifier,
and so I feel his comment about a "leaner and tighter LF"
was probably totally accurate.
His speakers are French-built open baffle units.
They are not to my taste.
Regards to all
Iain
Ian Iveson
Ian Bell wrote:
> So what you are saying is that you consider the effect of plate
> voltage
> changes on anode current in a triode to be negative feedback. That
> cannot
> be the case because that would mean under small signal conditions
> (when the
> plate voltage hardly changes) the gain of a triode would be as high
> as
> pentode which is clearly not the case.
There is no point in arguing about this with those who still cling to
the idea, Ian, as you will see if you look back over the "triode
feedback" debates over the past couple of years.
Just as feedback theory can be used to arrive at ohms law for a
resistor, so it can be used in deriving the triode function. It is a
trivial application of the term "feedback", is merely notional, and
understanding is better off without it.
Particularly as those who tout the notion tend to argue that a tetrode
is merely a triode with the "internal feedback" removed. This leaves
comprehension of the tetrode obscured by fantasy.
cheers, Ian
Ian Iveson
> ...
> The two best ways to appreciate that triodes have de facto negative
> feedback are:
>
> 1. consider how a triode-connected pentode very closely resembles a
> triode (output impedance, voltage gain, lower noise, dominant terms
> in the polynomial relating output to input, general appearance of
> curves). A pentode with the screen grid connected to the output
> (anode/plate) obviously has voltage negative feedback, yet acts like
> a triode, so it strongly suggests a relationship between triode
> characteristics and nfb.
>
> 2. the proof, though, comes from the physical effects going on;
> this has been described already, and show an increase in plate
> voltage results in an extra (superimposed) electric field that tends
> to oppose the increase, which is pretty much negative feedback in a
> nutshell.
No it absolutely is not. Variation in output due to load does not
require feedback in the sense we usually use the term. Quite the
reverse, in fact.
The notion of internal feedback is neither necessary nor useful in
explaining either the triode or the tetrode. It may feel and sound OK
to think of it like that, but it does not bear analysis, and obscures
whatever the truth might be.
As for strict definition, a triode is not a feedback system because
the output is not referred to the input. You may posit some internal
subsystem in which that is true, but that becomes a trivial exercise
because there is no delay.
A triode with an anode load and a resistance at its grid becomes a
feedback system via the miller capacitance, which is real,
non-trivial, and significant, since we must take into account the
delay it introduces.
cheers, Ian
Bret Ludwig
wrote:
Which is why we use the disclaimer, "as if". The old books talk about
the action of screen nad suppressor grids until they are blue in the
face and apparently to little avail. I am a poor typist and don't care
to copy by hand (and, now that the CapShare scanner has apparently
been quietly removed from existence,don't have a scanner.)
Read the books. It's in there.
west
I'm just trying to swat you away. I would never bring initial attention to
an mosquito. Mine are ALWAYS responses to your unsolicited and immature
remarks. I know you're attracted to my CO2, Perhaps it's the Methane you're
really after. Just go away asshole. Patrick still loves you, don't worry.
"Jon Yaeger" <jon...@bellsouth.net> wrote in message
news:C2146CBF.967C7%jon...@bellsouth.net...
Ian Bell
>
> Which is why we use the disclaimer, "as if". The old books talk about
> the action of screen nad suppressor grids until they are blue in the
> face and apparently to little avail. I am a poor typist and don't care
> to copy by hand (and, now that the CapShare scanner has apparently
> been quietly removed from existence,don't have a scanner.)
>
>
> Read the books. It's in there.
Actually it isn't. What you are mistaking for feedback is the simple result
of anode resistance. The reason the books don't call it feedback is because
it isn't.
Ian
Patrick Turner
If you compared a class A pentode amp and class A triode amp
each with 12dB NFB, there may be profound differences.
12dB of NFB is not much for a pentode amp.
Using a pair of EL34, typically we'd have 5k for the pentode RLa-a,
and without GNFB we'd get 20 watts of PP class A, at about 3% THD, and
if the
the OPT was 5k:5 ohms, Rout = 24 ohms.
In triode without NFB, the EL34 produces 1% THD at 12 watts of class A
into say 9k RLa-a.
If the OPT is 9k:5 then Rout = 1.5 ohms
The 12dB NFB is just the gain reduction due to NFB, or 4 times so that
the pentode amp with NFB has 0.75% THD at 20 watts, and thus at 12 watts
gives 0.57% THD.
The triode amp with 12dB NFB gives 0.25% at 12W, less than 1/2 the
pentode case,
and with less numerous harmonics.
12dB of NFB makes the Rout of the pentode PP amp become nearly as low
as the triode amp without NFB.
The triode with its 12dB NFB has Rout < 0.5 ohms.
Clearly, the triode amp with its 12dB of global NFB will perform
with much lower Rout and 1/2 the THD as the pentode amp
but only up to 12 watts.
Which sounds better isn't my argument, since nobody has answered such
arguments
once and for all since 1933.
Patrick Turner.
Patrick Turner
Ian Iveson wrote:
>
> Ian Bell wrote:
>
> > So what you are saying is that you consider the effect of plate
> > voltage
> > changes on anode current in a triode to be negative feedback. That
> > cannot
> > be the case because that would mean under small signal conditions
> > (when the
> > plate voltage hardly changes) the gain of a triode would be as high
> > as
> > pentode which is clearly not the case.
>
> There is no point in arguing about this with those who still cling to
> the idea, Ian, as you will see if you look back over the "triode
> feedback" debates over the past couple of years.
>
> Just as feedback theory can be used to arrive at ohms law for a
> resistor, so it can be used in deriving the triode function.
Since you contend that it is possible to use NFB theory to proove a
resistor has NFB,
could you expand a little and proove it?
Remember that the public you speak to will only be quite merciless
if you cannot proove what you say is true.
We all know you have stubbornly refused to accept that a clearly
identifiable
mechanism for NFB exists within a triode, which unlike a resistor, has 3
terminals, not just 2.
> trivial application of the term "feedback", is merely notional, and
> understanding is better off without it.
Understanding is NEVER better for dismissing notional ideas if they be
true,
or if they have any possibility of being untrue.
Sooner or later, something IS TRUE, or IS NOT TRUE. Its simply no use
trying to
trivially dismiss triode NFB as "notional", which implies you suspect it
could even exist.
So don't try to have a foot in either camp, it does you no favours in
front of an intelectually
capabale crowd of people who I hope WILL be MERCILESS by having the
opportunity yet again to
indicate your'e wrong, again, and others would have no reasons to follow
you.
>
> Particularly as those who tout the notion tend to argue that a tetrode
> is merely a triode with the "internal feedback" removed. This leaves
> comprehension of the tetrode obscured by fantasy.
Those of us who understand 3 terminal devices don't need to reach for a
4 terminal device to rescue our arguments.
However, tetrodes and pentodes were invented to overcome the
problems the triode NFB caused in RF amps, and thus allowed efficient RF
amplifiers well above the limits of triodes.
The alternative was to have to use cascode circuits, and by about 1930,
using two triodes where one could be made to work but with one added
electrode
was seen as stupid and a waste. And even if cascoded triodes could have
been used
instead of having tetrodes pentodes, they were always going to be more
expensive to implement,
and not quite as good in many apps where the high Ra of the multigrid
was such a benefit to tuned circuit Q.
But after pentodes came a potpourie of other multigrids, hexodes,
septodes, octodes, even nonodes.
These were mainly frequency converter tubes for which 101 uses were
found.
All had a screen, some had two screens, and the screen function was to
prevent the
internal NFB between the driven output terminal from interacting with
the input circuit.
( Very much after 1930, Erno Bobelli for one does wax lyrical about
cascode advantages
in his j-fet designs with cascoded j-fets to overcome the miller effect.
None of the SS devices have the internal NFB of triodes though, and most
SS devices resemble
pentodes, so external loop FB must be routinely used in every SS
circuit. )
The pentode invention bonus was higher gain at AF and LF, and if we
wanted the lower gain of a triode in a
in a stage then a resistance shunt NFB network could be placed around
pentode
to duplicate the internal NFB in a real triode, OR we could simply
triode connect the pentode, and have the shunt NFB applied internally
and electrostatically.
within the pentode from screen to grid.
Until you proove there is no NFB within a triode, its safe to say there
is.
And you rarely ever proove anything Ian, we just get waffle and BS.
So none of us will hold our breath waiting for your careful and
extensive and convincing
arguments.
We expect to get a short tirade of anti personal expletives so generic
from those
so un-careful as to not explain their position fully.
The podium awaits you.
Patrick Turner.
>
> cheers, Ian
Patrick Turner
Ian Iveson wrote:
>
> Mark Aitchison wrote:
>
> > ...
> > The two best ways to appreciate that triodes have de facto negative
> > feedback are:
> >
> > 1. consider how a triode-connected pentode very closely resembles a
> > triode (output impedance, voltage gain, lower noise, dominant terms
> > in the polynomial relating output to input, general appearance of
> > curves). A pentode with the screen grid connected to the output
> > (anode/plate) obviously has voltage negative feedback, yet acts like
> > a triode, so it strongly suggests a relationship between triode
> > characteristics and nfb.
> >
> > 2. the proof, though, comes from the physical effects going on;
> > this has been described already, and show an increase in plate
> > voltage results in an extra (superimposed) electric field that tends
> > to oppose the increase, which is pretty much negative feedback in a
> > nutshell.
>
> No it absolutely is not. Variation in output due to load does not
> require feedback in the sense we usually use the term.
This states the obvious. you can either have or not have NFB, and to get
a
voltage change at an anode, you don't need NFB.
> Quite the
> reverse, in fact.
And what is that Ian?
>
> The notion of internal feedback is neither necessary nor useful in
> explaining either the triode or the tetrode. It may feel and sound OK
> to think of it like that, but it does not bear analysis, and obscures
> whatever the truth might be.
Great, place your head in the sand. I think you are Egyptian, Ian.
You are in de nile!
YOU are denying a truth. YOU are the one to obscure the truth,
to tell us not to enjoy its beauty and presence.
You feel the NFB isn't there, you like the simple sound of your mind
saying it isn't there.
We don't mind the analysis.
>
> As for strict definition, a triode is not a feedback system because
> the output is not referred to the input.
Wrong, it IS refered to the input.
> You may posit some internal
> subsystem in which that is true, but that becomes a trivial exercise
> because there is no delay.
Please speak the English of your country Ian.
You've drifted in gibberish.
We can't find a translator for Gibberish.
The people of Gibber have a lousy time running their nation state
and cannot conduct family affairs or run businesses.
>
> A triode with an anode load and a resistance at its grid becomes a
> feedback system via the miller capacitance, which is real,
> non-trivial, and significant, since we must take into account the
> delay it introduces.
More gibberish. And its also a red-herring.
At AF, the Miller effect is quite negligible
because it depends on capacitances to function, and signal triode Cag
are very small, and have no effect at AF unless the source R driving a
grid
is very high.
What you fail to accept or understand is the net interactive
elecrostatic effects
of both grid and anode voltages upon Ia, and where Ia remains constant
upon Ea,
when the triode becomes a pure voltage operated device with maximal
internal NFB.
Such electrostatic effects are utterly distinct from the Miller
effect which does generate current flow even at LF although it is
negligible.
Patrick Turner.
>
> cheers, Ian
Patrick Turner
Oh and also don't forget the mountain of pro-NFB-in-tiodes evidence on
the Web.
http://www.next-power.net/next-tube/articles/Stockman/Stockman.pdf
Unfortunately for Ian Iveson, there are very few sources of Web based or
book information that would support
his idea than no NFB exists in a triode.
Patrick Turner.
Patrick Turner
Well, looks like you don't get it like Ian Iveson.
Read some MORE books. I would suggest Terman's 1937 Radio Engineering.
Or study the thoughts at
http://www.next-power.net/next-tube/articles/Stockman/Stockman.pdf
This is not an easy tome to understand because its full of math.
But you don't need math or Stockman to lead to concluding NFB exists
as a reality in every triode.
The low Ra of triodes is the direct result of the NFB.
Patrick Turner.
Ian Bell
>
> Oh and also don't forget the mountain of pro-NFB-in-tiodes evidence on
> the Web.
> http://www.next-power.net/next-tube/articles/Stockman/Stockman.pdf
>
Is that the best you can do? It is completely erroneous.
> Unfortunately for Ian Iveson, there are very few sources of Web based or
> book information that would support
> his idea than no NFB exists in a triode.
>
That is silly. You don't expect to references to something that does not
exist. None of the major texts mention it simply becuase it does not exist.
And they knew all about NFB back then; they had whole chapters on the
subject.
Ian
Ian Bell
>
> This states the obvious. you can either have or not have NFB, and to get
> a
> voltage change at an anode, you don't need NFB.
>
Precisely, so the fact that the anode current varies with plate voltage does
NOT prove NFB exists in a triode.
Ian
Ian Bell
>
>
> Ian Bell wrote:
>>
>> Bret Ludwig wrote:
>> >
>> > Which is why we use the disclaimer, "as if". The old books talk about
>> > the action of screen nad suppressor grids until they are blue in the
>> > face and apparently to little avail. I am a poor typist and don't care
>> > to copy by hand (and, now that the CapShare scanner has apparently
>> > been quietly removed from existence,don't have a scanner.)
>> >
>> >
>> > Read the books. It's in there.
>>
>> Actually it isn't. What you are mistaking for feedback is the simple
>> result of anode resistance. The reason the books don't call it feedback
>> is because it isn't.
>>
>> Ian
>
> Well, looks like you don't get it like Ian Iveson.
>
> Read some MORE books. I would suggest Terman's 1937 Radio Engineering.
>
I have read lots of them and not one talks about inherent NFB in a tride.
Name one respectable book that does.
> Or study the thoughts at
> http://www.next-power.net/next-tube/articles/Stockman/Stockman.pdf
>
> This is not an easy tome to understand because its full of math.
> But you don't need math or Stockman to lead to concluding NFB exists
> as a reality in every triode.
I am very good at math. This article is full of errors.
>
> The low Ra of triodes is the direct result of the NFB.
>
No it isn't. If triodes were pentodes with NFB then their distortion spectra
would be identical, just altered on overall amplitude by the NFB, but the
relative amplitudes would be the same. As their spectra are in fact quite
different one can only conclude that triodes are not like pentodes with
NFB.
Ian
Patrick Turner
Ian Bell wrote:
>
> Patrick Turner wrote:
> >
> > Oh and also don't forget the mountain of pro-NFB-in-tiodes evidence on
> > the Web.
> > http://www.next-power.net/next-tube/articles/Stockman/Stockman.pdf
> >
>
> Is that the best you can do? It is completely erroneous.
You have zero credibility. You never bother to proove anything,
let alone the erroneous nature of Stockman's article.
>
> > Unfortunately for Ian Iveson, there are very few sources of Web based or
> > book information that would support
> > his idea than no NFB exists in a triode.
> >
>
> That is silly. You don't expect to references to something that does not
> exist. None of the major texts mention it simply becuase it does not exist.
> And they knew all about NFB back then; they had whole chapters on the
> subject.
Unfortunately, you are again pissing into the wind.
You scorn far greater minds than yours, or mine, and
illustrate your abject ignorance.
Is it not strange how you cannot find ONE article you didn't write
that scientifically prooves NFB within a triode is hogwash?
NOT ONE!
Patrick Turner.
>
> Ian
Patrick Turner
This post tells us all you can see 3 metres into the fog in front of
you.
There is a fence post there, watch out.
If the fog ever lifts, you will see further, and see the other fence
posts,
and you'll see wires connecting the fence posts together,
and then you'll see the fence goes around a farm called Negative
Feedback.
Maybe if we are lucky, you'll see there are interactions
between the animals on the farm; there's Anode, a large hairy beast, and
he pulls the plough,
Cathode, a smelly small hot creature, he likes to root around in the
Earth, and Grid,
a small lively animal which hardly eats much, but controls the lot.
Then Farmer B+ brings the food for them all,
and the Electrostatic fields provide the very sustenance for all the
creatures on the farm while they
plod around on the farm.....
And that's your beddie bye story for tonight, sweet dreams.
Patrick Turner.
Patrick Turner
The feedback paths in a triode are the result of the 3/2 rule, and are
slightly non linear
paths, so applying NFB within a triode leads to different spectra to a
pentode.
If you ever managed to do the complex math involved, you'd probably find
the internal FB within triode can be proven to exist because of the
spectral differences.
Even if we have a device X and apply NFB externally with resistances
unaffected by the
impedances to which they connect, the spectra after NFB compared to
before
will be different, and differ with the amount of NFB applied,
open loop phase shift and bandwidth.
What you claim indicates there is no FB in triodes does not indicate any
such thing.
In short, triodes are surely not like pentodes, and yes their spectra
differs, but its because of the NFB
within a triode that makes the difference.
Using a shunt resistance NFB network around a pentode to force the
pentode to have the same gain
and perhaps Ra as a given triode won't make that pentode produce the
same spectra that the triode makes,
indicating yet again there is NFB within the triode.
I'm too dumb for the complex math, and learning all that just to prove
something dead simple to YOU
would be a monumental waste of time.
The reason why scant attention is given to the NFB in triodes in many
books is because its a
parameter set by the tube designer, and the outcome of his efforts with
slide rule
is locked up in vacuum sealed bottles, and thus utterly un-alterable,
so we never need to have the triode FB up front in gain/FB equations for
the circuits employing
triodes. We don't have to include the formulas for the glass making
either.
You need to read MORE old books and old reference material where the
boffins of the 1930s
had triodes sussed out.
The simple interaction of electrostatic field behaviour within a triode
becomes more
obvious when you examine exactly how a tube is made, what happens with
electrons,
what the field intensity is at different locations,
and the way the electrode distances determine Ra, gm, µ and the amount
of NFB.
Patrick Turner.
Arny Krueger
news:45F00473...@turneraudio.com.au
Well-argued, Patrick.
Cheers!
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
Patrick, why do you persist in arguing with Ian, for years he has
consistently demonstrated in this group that he has very little gray
matter between his ears, there is no benefit to be gained by trying to
teach him anything.
> The alternative was to have to use cascode circuits, and by about 1930,
> using two triodes where one could be made to work but with one added
> electrode
> was seen as stupid and a waste. And even if cascoded triodes could have
> been used
> instead of having tetrodes pentodes, they were always going to be more
> expensive to implement,
> and not quite as good in many apps where the high Ra of the multigrid
> was such a benefit to tuned circuit Q.
Doesn't the cascode circuit also have a high effective Ra like a tetrode
or pentode, or have I got it wrong?
Regards,
John Byrns
--
Surf my web pages at, http://fmamradios.com/
John Byrns
wrote:
> Patrick Turner wrote:
> >
> > The low Ra of triodes is the direct result of the NFB.
>
> No it isn't. If triodes were pentodes with NFB then their distortion spectra
> would be identical, just altered on overall amplitude by the NFB, but the
> relative amplitudes would be the same. As their spectra are in fact quite
> different one can only conclude that triodes are not like pentodes with
> NFB.
One can't conclude any such thing from the spectra. This statement
underscores your complete lack of understanding of triode operation and
the negative feedback within. Adding feedback to a circuit or device
does not simply alter the overall amplitude of the distortion spectra,
it interacts with the nonlinearity in the device or circuit to not only
completely change the relative amplitudes of the spectral components,
but can even add new spectral components that weren't present in the
output of the circuit or device without NFB applied.
Since you claim to be good at math, try putting feedback around a simple
square law nonlinearity and see what happens to the distortion spectrum.
Arny Krueger
news:byrnsj-556898....@newsclstr02.news.prodigy.com
> In article <45eff2e5.0@entanet>, Ian Bell
> <ruffr...@yahoo.co.uk> wrote:
>
>> Patrick Turner wrote:
>>>
>>> The low Ra of triodes is the direct result of the NFB.
>>
>> No it isn't. If triodes were pentodes with NFB then
>> their distortion spectra would be identical, just
>> altered on overall amplitude by the NFB, but the
>> relative amplitudes would be the same. As their spectra
>> are in fact quite different one can only conclude that
>> triodes are not like pentodes with NFB.
>
> One can't conclude any such thing from the spectra.
Agreed.
> This statement underscores your complete lack of understanding
> of triode operation and the negative feedback within.
Mostly agreed. There's an additional subtlety in this case, which Patrick
pointed out. The subtlety is that the feedback is applied through a
mechanism that is itself nonlinear. This is relatively rare. In fact its so
rare that I managed to do most of my post-graduate work in engineering and
control systems without it being mentioned.
> Adding feedback to a circuit or device does not simply
> alter the overall amplitude of the distortion spectra, it
> interacts with the nonlinearity in the device or circuit
> to not only completely change the relative amplitudes of
> the spectral components, but can even add new spectral
> components that weren't present in the output of the
> circuit or device without NFB applied.
In short, even if the feedback path is linear, there's a tendency for the
error components to simulataneously be reduced and also to pick up more high
order components. The usual effect is that all orders of distortion are
reduced, but there is a shift of relative amounts to higher orders.
> Since you claim to be good at math, try putting feedback
> around a simple square law nonlinearity and see what
> happens to the distortion spectrum.
That's a good first exercise. The second exercisemight be to apply the
feedback through a path that is itself highly nonlinear.
Ian Iveson
> Patrick, why do you persist in arguing with Ian, for years he has
> consistently demonstrated in this group that he has very little gray
> matter between his ears, there is no benefit to be gained by trying
> to
> teach him anything.
Not like the archetypal crony to fly his true colours. Are you
standing on a box?
Ian
Ian Iveson
>> Unfortunately for Ian Iveson, there are very few sources of Web
>> based or
>> book information that would support
>> his idea than no NFB exists in a triode.
>
> That is silly. You don't expect to references to something that does
> not
> exist. None of the major texts mention it simply because it does not
> exist.
> And they knew all about NFB back then; they had whole chapters on
> the
> subject
It's no good trying, truly. It can only get sillier when there is
commercial and personal interest in obfuscation and
self-aggrandisement.
Those who cite Stockman appear not to understand the meanings of
"factitious" and "imagined", as used in his summary.
Stockman himself seems disingenuous in the way he shifts his claim as
the text progresses. He is quite clear at first in contending only
that feedback theory can be applied to the triode, but later confuses
this with the contention that some real feedback circuit actually
exists. Considering the summary must have been written last, it is odd
that he returns to "factitious" and "imagined". He should have gone
back and edited the wilder passages.
As an exercise in mathematics, it scans OK to me. As a contribution to
practical understanding it has zero merit and has never been used in
the real development of the thermionic valve. It doesn't seem to have
gained any academic respect either.
There was a fashion, possibly about the time, for applying control
systems theory to absolutely everything. Kind of interesting for
academics to illuminate things from a different angle. Engineers are
generally too busy to complicate matters unnecessarily, and only use
the theory where necessary, which is when the feedback is real, so
there is delay in the loop. Otherwise a simple time-invariant forward
function is sufficient to describe the system, as with a triode.
> Is that the best you can do? It is completely erroneous.
It may be the best, but I'm sure there is plenty more to come,
regardless...
One trait of the fantasy merchants is that they resolutely refuse to
read even a basic introduction to control systems engineering. There
are others...and other harboured notions...that non-existence should
be demonstrable; that the world is controlled by "bean counters"
and/or "marketing", and all manner of other conspiracy theories;
imagined crowds of intellectuals at their feet, hanging on every word;
all that kind of gang stuff. And here it comes....
cheers, Ian
John Byrns
"Ian Iveson" <IanIves...@blueyonder.co.uk> wrote:
> There was a fashion, possibly about the time, for applying control
> systems theory to absolutely everything. Kind of interesting for
> academics to illuminate things from a different angle. Engineers are
> generally too busy to complicate matters unnecessarily, and only use
> the theory where necessary, which is when the feedback is real, so
> there is delay in the loop. Otherwise a simple time-invariant forward
> function is sufficient to describe the system, as with a triode.
Why is delay required for NFB to exist? In any case there is delay in a
triode, it is called the "electron transit time effect" IIRC, and is the
reason the performance of triodes falls off at microwave frequencies.
Even if delay is required for negative feedback to exist, the fact that
there is delay in a triode invalidates your argument.
George Orwell
The confessed garage vermin Jon (aka John and Jono) Yaeger <jon...@bellsouth.net> wrote to West:
> There are givers and takers. Patrick is a giver. You're obviously a taker,
> and a lazy one at that.
Crap. West is entirely entitled to ask those with experience or
ability any question he likes. Moreover, that is the historical
pattern of this newsgroup, that questions are free and answers
are voluntary. That is good. What is also true is that there are
usually a few clowns like you who want to wield their mickey
mouse little bit of doubtful knowledge like a club, idiots who
want to create entry barriers.
West's question was not only interesting and relevant,
it sparked off a spirited discussion. That makes it a lot more
useful than your juvenile interjections.
Unsigned out of contempt
> n article EgrHh.1888$3i.840@trnddc01, west at rest...@verizon.net wrote on
> 3/6/07 11:33 PM:
>
> > Jon, you never cease an opportunity for a "dig." Immaturity ...remember?
> > Profile try, let me know, Jon ... you're basically diminutive in size and
> > got your ass whipped many times growing up ... correct? Not a gift of mine,
> > just your obvious transparency.
Gee, Dr Freud, *of course* the snippy asshole Yeager suffers from
Duck's Disease, historically a violently disruptive condition of
which the symptom is that the patient's ass drags the ground. An
analysis that easy won't get you one-quarter of a point in any
paper I ever set, unless you wear a 38 D-cup, in which case it
might have made 5 per cent towards a pass mark. Mind you,
what makes Duck's Disease interesting is that it is one of the
few conditions for which modern psychiatry doesn't even pretend
to offer a chemical solution, so Yaeger, otherwise a worthless
garage trader whom no one will miss, could have some novelty
value as an experimental subject for the injection of growth
hormones. -- Andre Jute
Peter Wieck
> Its usual stream of virulent drivel.
Interesting words from an unrepentant liar, poseur and charlatan.
Once again, in a discussion purportedly around facts and well-
supported opinions, you should stand aside and watch. I guess at the
tender age of 62 you have decided that you have nothing more to learn
and have become the fountainhead of received wisdom. I suppose we
should all sit on our fingers and worship you from afar.
If I send you a bit of cloth, will you please bless it?
Peter Wieck
Wyncote, PA
Ian Iveson
>> There was a fashion, possibly about the time, for applying control
>> systems theory to absolutely everything. Kind of interesting for
>> academics to illuminate things from a different angle. Engineers
>> are
>> generally too busy to complicate matters unnecessarily, and only
>> use
>> the theory where necessary, which is when the feedback is real, so
>> there is delay in the loop. Otherwise a simple time-invariant
>> forward
>> function is sufficient to describe the system, as with a triode.
>
> Why is delay required for NFB to exist? In any case there is delay
> in a
> triode, it is called the "electron transit time effect" IIRC, and is
> the
> reason the performance of triodes falls off at microwave
> frequencies.
> Even if delay is required for negative feedback to exist, the fact
> that
> there is delay in a triode invalidates your argument.
You have taken a paragraph out of context. I was talking of Stockman.
There is no delay in his account, so on that count it is at best a
trivial application of control systems theory. On other counts it is
also obviously only notional.
No your delay does not invalidate my argument. Just because all
feedback control systems incorporate delay does not mean that all
things which incorporate delay are feedback control systems.
Elementary logic: the difference between a necessary and a sufficient
condition.
There is always delay between the input and output of a real system. I
haven't argued that it is necessarily true, simply that it is true. It
is a matter of fact. I don't know why. God would know, perhaps. Why
ask me when you just said in another post that I have no brains?
Perhaps brains aren't necessary. Maybe god just knows without needing
to mull it over.
In mechanical systems it is necessary that an extant thing has
extension, and as a matter of fact all materials have elasticity and
mass and some element of damping. Consequently every system made from
real materials will incorporate some delay. In electronics, extension,
capacitance, inductance and resistance are equally inescapable. Can
you cite a system in which there is no delay? Perhaps without such
delay there would be no time in the world. It's hard to speculate
without a brain.
But all of these concern the meaning of "delay" that we rather take
for granted in audio, and in control system theory in general: group
delay. We are generally safe in the assumption that our outputs will
start at the same time as our inputs, and only need to worry about the
fact that they take longer to happen, and so we get phase shift.
Delay is the raison d'etre of feedback control theory. If there is no
delay, then you are left with only the common time-invariant "feedback
equation" which can be derived in one line of simple algebra. Nothing
else applies. Bode, Laplace, Nyquist, et al....the whole body of
analysis returns nothing.
A delay arising from the (presumably light-speed?) travel of
electrostatic fields within a valve would be absolute AFAIK, in the
sense that the output would start some time after the beginning of the
input. That makes it rather a different animal from that arising from
combinations of reactance and resistance. Something we don't normally
need to bother with, fortunately, since I don't find it so easy to fit
such delay into systems theory.
What kind of delay is yours? Is it considered to be a propagation
delay, the time it takes to establish a standing wave perhaps? In that
case I guess it would be group delay. Or is there considered to be an
absolute gap in time between the departure and the arrival of
electrons? How are the velocities of electrons distributed across the
population at a particular point in time? Perhaps it can be seen
either way. I am not a physicist, and have never worked in an area of
engineering where such small time intervals are significant.
In any case, it is not what Stockman is on about, and has no bearing
on whether a triode has real internal feedback or not. Delay happens
in both open and closed loop systems.
Anyone know how a maser works? That might cast some light on the
issue.
Incidentally, I am using this term "absolute delay" in the hope that
the meaning is obvious. Is there an accepted term in electronics
engineering?
Ian
Ian Bell
> In article <45eff2e5.0@entanet>, Ian Bell <ruffr...@yahoo.co.uk>
> wrote:
>
>> Patrick Turner wrote:
>> >
>> > The low Ra of triodes is the direct result of the NFB.
>>
>> No it isn't. If triodes were pentodes with NFB then their distortion
>> spectra would be identical, just altered on overall amplitude by the NFB,
>> but the relative amplitudes would be the same. As their spectra are in
>> fact quite different one can only conclude that triodes are not like
>> pentodes with NFB.
>
> One can't conclude any such thing from the spectra. This statement
> underscores your complete lack of understanding of triode operation and
> the negative feedback within. Adding feedback to a circuit or device
> does not simply alter the overall amplitude of the distortion spectra,
> it interacts with the nonlinearity in the device or circuit to not only
> completely change the relative amplitudes of the spectral components,
> but can even add new spectral components that weren't present in the
> output of the circuit or device without NFB applied.
>
I suggest you check out pages 308 and 309 of the radiotron Designers
Handbook (1960 edition) and readily available on line for the math that
demonstrates my point. Its conclusion on the effect of voltage negative
feedback on harmonic distortion is:
"That is, the magnitudes of *all* the harmonics (and of course the
intermodulation products) introduced by the amplifier are reduced by
negative voltage feedback in the same proportion that the gain is reduced."
Ian
Ian Bell
Fine, name ONE, that describes NFB within a triode.
Ian
Arny Krueger
Arny Krueger
The RDH makes a correct statement. However that correct statement means that
the relative amplitudes of the distortion spectra will remain the same, only
if the reduction in gain at all frequencies is identically the same. A
triode amplifier will tend to have flatter response before loop feedback is
applied, because the plate resistance of the tubes that are changed to be
triodes is lower. Thus there will be differences in the reduction in gain at
some frequencies.
Patrick Turner
John Byrns wrote:
>
> In article <45EFD04D...@turneraudio.com.au>,
> Patrick Turner <in...@turneraudio.com.au> wrote:
>
> Patrick, why do you persist in arguing with Ian, for years he has
> consistently demonstrated in this group that he has very little gray
> matter between his ears, there is no benefit to be gained by trying to
> teach him anything.
I recall it wasn't always as if he was unwilling to learn.
>
> > The alternative was to have to use cascode circuits, and by about 1930,
> > using two triodes where one could be made to work but with one added
> > electrode
> > was seen as stupid and a waste. And even if cascoded triodes could have
> > been used
> > instead of having tetrodes pentodes, they were always going to be more
> > expensive to implement,
> > and not quite as good in many apps where the high Ra of the multigrid
> > was such a benefit to tuned circuit Q.
>
> Doesn't the cascode circuit also have a high effective Ra like a tetrode
> or pentode, or have I got it wrong?
I sure wasn't around when the cascode was invented, but it must have
been tried early
in the history by someone wondering what series connection could do.
But you are dead right about the high output resistance looking into the
anode
of the top gain tube of a cascode pair.
The Ra of the triode and the internal NFB remains intact, and is is
there but the top gain tube
has another triode as its cathode resistance.
The top cascode is basically a grounded grid tube, or in common grid
mode.
The cathode input has low Rin, it is the top tube load / gain so its
capable of fair bandwidth.
The bottom tube anode resistance can be high if it has no bypass on its
cathode bias R,
and so a low µ tube like 6SN7 with Rk = 2k2 has effective Ra of about
56k,
and this is the effective Rk for the top tube, so its effective Ra is
10k + (21 x 56) = 1,186k approx, or about the same
as a pentode, but where it is shunted by the load of say 100k, the
actual Rout of such a circuit is
about 90k, since Ra effective is in parallel with RL.
The 6SN7 in this case acts like a pure current source, instead of like a
voltage source with Rout of approx 10k
at a few mA dc.
The gain of the bottom tube is low,especially with Rk unbypassed, so
bypassing is always usually used,
because the cascode is mainly used for low signal work well below the 1V
out level.
if the load on 6SN7 is 100k, its gain will be about 18, so the cathode
load Rin for the bottom tube =
100k / 18 = 5.5k, and the bottom tube gain = 7 approx, and if the Rk of
2k2 is unbypassed, then bottom tube
gain = 2 approx.
Thus the 6SN7 cascode can be tailored to produce a range of gain from
say 126 to 36 with top RL = 100k.
If the top tube RL is only say 22k,
then the bottom tube may have no anode gain at all, but act merely
as an inverting gm converter with high Rin at its grid.
This is how the cascode is used in many RF circuits and the Miller
effect
is defeated because of the low gain from g to a of the bottom tube, and
the anode V swing
of the top tube has little effect on the grid held at dcV.
In fact the top tube has its anode screened off from the cathode by the
grid.
THD in cascodes isn't a pretty picture espcially as Vout rises, but they
are great for some apps,
and TV tuners and FM tuners used 6BQ7 and 6AQ8, 12AT7 in cascode in
countless tunes at HF up to 100MHz and beyond.
Cascode phono amp inputs are also very nice, especially if the lower
device is a high gm j-fet, and top tube
then becomes non critical and can be almost any triode as long as it can
run with
the wanted Ia that gives the j-fet good idle dc conditions of about 5mA
for 2SK369/2SK147/2SK170 etc.
Where Rout is high, and you have a tuned circuit for a load, the highest
Q of the tuned circuit is maintained
when the R shunting the tuned circuit is much higher than the Z of the
tuned circuit at its Fo.
Best Q for tuned circuit loads occurs when the source is a pure current
source.
Patrick Turner.
Patrick Turner
Arny Krueger wrote:
>
> "John Byrns" <byr...@sbcglobal.net> wrote in message
> news:byrnsj-556898....@newsclstr02.news.prodigy.com
> > In article <45eff2e5.0@entanet>, Ian Bell
> > <ruffr...@yahoo.co.uk> wrote:
> >
> >> Patrick Turner wrote:
> >>>
> >>> The low Ra of triodes is the direct result of the NFB.
> >>
> >> No it isn't. If triodes were pentodes with NFB then
> >> their distortion spectra would be identical, just
> >> altered on overall amplitude by the NFB, but the
> >> relative amplitudes would be the same. As their spectra
> >> are in fact quite different one can only conclude that
> >> triodes are not like pentodes with NFB.
> >
> > One can't conclude any such thing from the spectra.
>
> Agreed.
>
> > This statement underscores your complete lack of understanding
> > of triode operation and the negative feedback within.
>
> Mostly agreed. There's an additional subtlety in this case, which Patrick
> pointed out. The subtlety is that the feedback is applied through a
> mechanism that is itself nonlinear. This is relatively rare. In fact its so
> rare that I managed to do most of my post-graduate work in engineering and
> control systems without it being mentioned.
The non linear delivery of the internal NFB path in a triode is
countered
by the amount of NFB actually present.
In a triode with no RL, so no anode voltage change is possible,
and only current change is possible, the gm can be the same as a pentode
in the same load condition.
Both tubes would have gain = 0, but have the same gm transfer character,
and both produce very similar current distortion spectra.
As soon as we allow the triode to have some voltage change at its anode,
there is NFB action within it. In power triodes, it isn't a huge amount
for when the rated load is used, but its better than nothing, despite
its shortcomings of non linear delivery, ie, as in the ase of 2A3/300B.
But when the load is a CCS, as in the case of a signal amp, the load
line has become horizontal,
and then the amount of NFB has skyrocketed to maximum, and a 300B can
make 150Vrms output
at only 1.5% and linearity is in fact limited by the test measuring
gear, as it was
in 1930, when the 300B data was first drawn up.
In fact the linearity of an SE 300B or KT88/6550 strapped as a triode
with CCS load is usually
better than the data curves indicate ( because the curves are plain
wrong ).
So where 150Vrms of signal at <1.5% THD can be had without trying very
hard just by using a CCS load,
then at 1.5Vrms out the THD could be theoretically 0.015%.
But 6SN7 can also be extraordinarily linear for a device without loop FB
for where 1.5Vrms or less is wanted, in SE mode, when the load is a CCS,
and internal FB is high.
Using a balanced pair makes THD just about vanish below the noise
floor.....
>
> > Adding feedback to a circuit or device does not simply
> > alter the overall amplitude of the distortion spectra, it
> > interacts with the nonlinearity in the device or circuit
> > to not only completely change the relative amplitudes of
> > the spectral components, but can even add new spectral
> > components that weren't present in the output of the
> > circuit or device without NFB applied.
>
> In short, even if the feedback path is linear, there's a tendency for the
> error components to simulataneously be reduced and also to pick up more high
> order components. The usual effect is that all orders of distortion are
> reduced, but there is a shift of relative amounts to higher orders.
The NFB will produce new components that were not present before NFB was
applied
when the amount of NFB is low, and is worst when the open loop gain has
low bandwidth and high phase shift.
The mechanism for the higher order H artifacts after only moderate NFB
application
is the intermodulation of distortion fed back.
A 300B amp with only 10dB GNFB may measure only very slightly better,
but sound worse for the FB, but usually this is to occur at high output.
Its a negligible concern at low outputs for say horn speakers.
Ppl say they hear the NFB being applied, and it spoils thew sound, but
unless ppl are
told of its presence, they usually cannot detect if its there or not
especially at low levels.
>
> > Since you claim to be good at math, try putting feedback
> > around a simple square law nonlinearity and see what
> > happens to the distortion spectrum.
>
> That's a good first exercise. The second exercisemight be to apply the
> feedback through a path that is itself highly nonlinear.
Such experiemnts and math have already all been attempted in past
articles in Wireless World
and later when that became Electronics World, when clever electronics
writers
who actually had real knowledge were paid well to write and conduct
measured experiments.
Everything you can possibly think about NFB has already been well
researched, logged, measured, and argued
about by many leading brains of ppl who are now mostly dead, or soon
will be.
I am indebted to the wisdom of my forefathers who asked awkward
questions about their surroundings
and who wouldn't rest until answers were found, described by accurate
concept descriptions and mathematics,
and published carefully. They were celebrated until they were forgotten
by a keyboard wielding generation of PC dominated dumbclucks who won't
fucking well
ever read a fucking book or do any real fucking experiments!!!!
Patrick Turner.
John Byrns
"Ian Iveson" <IanIves...@blueyonder.co.uk> wrote:
My question about the necessity of delay for negative feedback to exist
was just an aside. My point was that there is delay in the triode,
which invalidates the argument that you put forward that one reason
negative feedback couldn't exist in a triode was because there was no
delay in a triode.
Patrick Turner
Ian Iveson wrote:
>
> Ian Bell wrote:
>
> >> Unfortunately for Ian Iveson, there are very few sources of Web
> >> based or
> >> book information that would support
> >> his idea than no NFB exists in a triode.
> >
> > That is silly. You don't expect to references to something that does
> > not
> > exist. None of the major texts mention it simply because it does not
> > exist.
> > And they knew all about NFB back then; they had whole chapters on
> > the
> > subject
>
> It's no good trying, truly. It can only get sillier when there is
> commercial and personal interest in obfuscation and
> self-aggrandisement.
Just what are you suggesting?
Its no use even hinting that because I make amps commercially that
I bend the powers of intellectual persuasion to better myself.
You can't prove anything to me or anyone else.
>
> Those who cite Stockman appear not to understand the meanings of
> "factitious" and "imagined", as used in his summary.
>
> Stockman himself seems disingenuous in the way he shifts his claim as
> the text progresses. He is quite clear at first in contending only
> that feedback theory can be applied to the triode, but later confuses
> this with the contention that some real feedback circuit actually
> exists. Considering the summary must have been written last, it is odd
> that he returns to "factitious" and "imagined". He should have gone
> back and edited the wilder passages.
I don't need Stockman to believe in the NFB within a triode.
But there is no reason not to consider the triode operation without
the effect of the anode voltage upon the electron stream.
>
> As an exercise in mathematics, it scans OK to me. As a contribution to
> practical understanding it has zero merit and has never been used in
> the real development of the thermionic valve. It doesn't seem to have
> gained any academic respect either.
But other ppl have gained respect.
Maths wiz kids don't get all the glory.
>
> There was a fashion, possibly about the time, for applying control
> systems theory to absolutely everything. Kind of interesting for
> academics to illuminate things from a different angle. Engineers are
> generally too busy to complicate matters unnecessarily, and only use
> the theory where necessary, which is when the feedback is real, so
> there is delay in the loop. Otherwise a simple time-invariant forward
> function is sufficient to describe the system, as with a triode.
In the case of a triode, the delays you claim(?) are so short as to be
negligible, and that the FB effect is virtually instaneous.
If not, how much phase shift are you talking about?
Please conduct your speaches in language of the masses please,
lest you be lynched as a sorcerer.
Engineering language should be curt, to the point, and with each
statement backed up with math.
>
> > Is that the best you can do? It is completely erroneous.
>
> It may be the best, but I'm sure there is plenty more to come,
> regardless...
>
> One trait of the fantasy merchants is that they resolutely refuse to
> read even a basic introduction to control systems engineering.
You demonstrate appalling ignorance, and cannot anyone who support
your point of view.
So how about less breathtaking hypocrisy please!!!
There
> are others...and other harboured notions...that non-existence should
> be demonstrable; that the world is controlled by "bean counters"
> and/or "marketing", and all manner of other conspiracy theories;
> imagined crowds of intellectuals at their feet, hanging on every word;
> all that kind of gang stuff. And here it comes....
Blather will only lead to medications being applied eventually.
Final relief from the condition known as blatheritis is System Expiry...
Patrick Turner.
>
> cheers, Ian
Patrick Turner
Ian, you are standing higher, on the roof top, and shouting BS to the
whole street...
Patrick Turner.
John Byrns
wrote:
The point you are missing about the distortion spectra is that the
introduction of negative feedback introduces new distortion products and
that changes the relative spectral amplitudes. Your analysis assumes
that the feedback signal itself does not produce any new distortion when
passing through the amplifier.
Patrick Turner
Well OK, a delay IS present. All FB circuits have delay, but that does
not mean you
can listen to the music before the delay occurs, or that if you remove
the FB then
you can listen without any delay at all, so it must sound better.
But this idiotic argument has been trotted out in the past.
Meanwhile, if you abolish NFB, then what you usually get is a shirtload
of
increased delays called phase lags in engineering terms.
Such delays are easily cured with NFB.
If the typical systems used for analog music recording had been stripped
of all their NFB,
methinks the sound we would have had from tape and vinyl would have been
mostly abysmal.
But I digress, because the ethics of NFB use have nothing to do with FB
within a triode.
Patrick Turner.
Anonyma
Ian Iveson <IanIves...@blueyonder.co.uk> wrote:
> Mark Aitchison wrote:
>
> > ...
> > The two best ways to appreciate that triodes have de facto negative
> > feedback are:
> >
> > 1. consider how a triode-connected pentode very closely resembles a
> > triode (output impedance, voltage gain, lower noise, dominant terms
> > in the polynomial relating output to input, general appearance of
> > curves). A pentode with the screen grid connected to the output
> > (anode/plate) obviously has voltage negative feedback, yet acts like
> > a triode, so it strongly suggests a relationship between triode
> > characteristics and nfb.
> >
> > 2. the proof, though, comes from the physical effects going on;
> > this has been described already, and show an increase in plate
> > voltage results in an extra (superimposed) electric field that tends
> > to oppose the increase, which is pretty much negative feedback in a
> > nutshell.
>
> No it absolutely is not. Variation in output due to load does not
> require feedback in the sense we usually use the term. Quite the
> reverse, in fact.
>
> The notion of internal feedback is neither necessary nor useful in
> explaining either the triode or the tetrode. It may feel and sound OK
> to think of it like that, but it does not bear analysis, and obscures
> whatever the truth might be.
>
> As for strict definition, a triode is not a feedback system because
> the output is not referred to the input. You may posit some internal
> subsystem in which that is true, but that becomes a trivial exercise
> because there is no delay.
As usual, you're up the river without a paddle, Iveson; these are
"trivial" exercises only to leftwing beardies who long since binned
Ogham's razor for fear of cutting themselves on plain common
sense; I can remember when you discovered delay for
the first time and immediately declared yourself an expert on slew
rate. BTW, if you can find those pages I advised you to tear out of
the RDH to make it more digestible, they will tell you in the RF
pages somewhere that a triode does have built-in delays too; we
don’t bother about them much on RAT because they don’t bother
the audio frequencies much. But they’re there.
However, I'm glad to see you have since made a
huge leap in the direction of enlightenment:
> A triode with an anode load and a resistance at its grid becomes a
> feedback system via the miller capacitance, which is real,
> non-trivial, and significant, since we must take into account the
> delay it introduces.
That too, as a character in a longrunning radio comedy I created
used to say wisely when he was stunned by the blustering ignorance
of the fools around him. Still, for you, that’s a huge advance. Those
of us who actually know about this have long since worked out that at
AF the contribution of Miller must be rather small.
However, if you now admit that the three-terminal interaction of a
triode generates some form of NFB via Miller, however small I
(and everyone else capable of doing the math) may consider it, then
you must ask the next obvious question, “Okay, so if Miller
doesn’t account for more than a fraction of the observed effect, where
is the rest of the NFB which by the results is clearly at work here to
the extent of some 12-14dB?” This isn’t Marxist dialectic, pal, in which
an anomaly is an all-purpose get-out clause for intellectual inadequates,
this is physics in which every phenomenon has an explanation — and
the explanation for this one is internal NFB.
> cheers, Ian
Slainte.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review
Peter Wieck
> More regugitated tripe cribbed from Patrick Turner.
a) Let us all see you do the math.
b) Thought so.
Ooze aside and observe others who can think do this. You might learn.
I am, certainly.
Peter Wieck
Wyncote, PA
Patrick Turner
Please be more scientific in all your discussions.
All menbers of the group cannot decipher gibberish.
Unmitigated *** BULL SHIT *** from Ian Iveson!!!
Patrick Turner.
Patrick Turner
This is quoted out of context.
On page 309, the Bible goes on to state 3 conditions under which the
statement above is correct.
Nearly all amplifiers operate outside the conditions stated
for full equal reductions of harmonic content.
And RDH4 does not explore to what extent new additional higher numbered
harmonics of frequencies above the open loop
spectra are produced by the NFB.
There is much the RDH4 hasn't said because of the space and time
constraints on its team or authors.
But many bright minds came along after RDH4 to shed a much brighter
light
upon the subject, but their combined wisdom was never covened into ONE
BOOK
because the world was a changing for where old traditions were being
dismantled,
and solid state had become so invasive to the status quo, and so rapid
changing
that writing one really good about electronics was pointlesss, because
by the time
the book was published, many techniques were obsolete.
And the sheer amount of electronic information became so vast....
Patrick Turner.
electronics was total modern for ,
y .
Ian
Patrick Turner
Miller C is however fairly responsible for the phase lag in tube amp
open loop character and hence the cause of instability,
so it is no easy trick sometimes to stabilise amps with GNFB.
Using pentodes and high Ra triodes always favours substantial Miller C
caused reductions in AF gain beginning around 8kHz in many amp's open
loop gain
character.
To avoid the problems of phase lag, ie, real delay,
ppl use low µ signal/input/driver triodes and triode output stages, and
wide bandwidth IST and OPT,
ie, they try to minimize all low shunt capacitances.
>
> However, if you now admit that the three-terminal interaction of a
> triode generates some form of NFB via Miller, however small I
> (and everyone else capable of doing the math) may consider it, then
> you must ask the next obvious question, “Okay, so if Miller
> doesn’t account for more than a fraction of the observed effect, where
> is the rest of the NFB which by the results is clearly at work here to
> the extent of some 12-14dB?” This isn’t Marxist dialectic, pal, in which
> an anomaly is an all-purpose get-out clause for intellectual inadequates,
> this is physics in which every phenomenon has an explanation — and
> the explanation for this one is internal NFB.
The chance is, he'll stumble over the NFB sometime but let's
not hold the breath waiting. He fears loosing face over the issue, and
enjoys being
obstinate for the sake of being obstinate, and never for the sake of
exposing the cause, or the truth.
The electrostatic feedback makes possible the ectsatic quality of music
without loop FB should we desire it.
But I assure everyone Truth is an easy companion later, if not all that
comfortable sooner.
Patrick Turner.
s '
Chris Hornbeck
<in...@turneraudio.com.au> wrote:
>Heresy abounds on a BS riddled Internet.
Kinda says it all.
Didn't we just do all this less than a year ago? Must
be a slow year.
The only new argument I've read is the "non-linear
feedback inherent in triodes" one. (The rest are
booooooring; sorry.)
Anybody care to try to make a case for the "non-linear
feedback" model? Be particular!
We obviously have nothing better to do right now,
Much thanks,
Chris Hornbeck
John Byrns
Chris Hornbeck <chrishornbe...@att.net> wrote:
> On Wed, 07 Mar 2007 09:54:09 GMT, Patrick Turner
> <in...@turneraudio.com.au> wrote:
>
> >Heresy abounds on a BS riddled Internet.
>
> Kinda says it all.
>
> Didn't we just do all this less than a year ago? Must
> be a slow year.
>
> The only new argument I've read is the "non-linear
> feedback inherent in triodes" one. (The rest are
> booooooring; sorry.)
>
> Anybody care to try to make a case for the "non-linear
> feedback" model? Be particular!
I think I posted an old fashioned ASCII art drawing of how that works
about a year ago.
Regards,
John Byrns
Chris Hornbeck
wrote:
>> Anybody care to try to make a case for the "non-linear
>> feedback" model? Be particular!
>
>I think I posted an old fashioned ASCII art drawing of how that works
>about a year ago.
I'd hoped for something more technical than artistic,
but you're our only hope, Obi Wan. Can you reproduce it?
No Black Boxes need apply, natch.
Much thanks, as always,
Chris Hornbeck
Ian Bell
>these are
> "trivial" exercises only to leftwing beardies who long since binned
> Ogham's razor for fear of cutting themselves on plain common
> sense;
Do you mean Occam's razor? or was Ogham some kind of audiophile?
Ian
Ian Bell
>
> The RDH makes a correct statement. However that correct statement means
> that the relative amplitudes of the distortion spectra will remain the
> same, only
> if the reduction in gain at all frequencies is identically the same. A
> triode amplifier will tend to have flatter response before loop feedback
> is applied, because the plate resistance of the tubes that are changed to
> be triodes is lower. Thus there will be differences in the reduction in
> gain at some frequencies.
This is true, and in general the gain will be lower at higher frequencies
which mean the higher harmonics will be reduced less. This again means that
the triode internal NFB theory fails because pentodes have relatively more
third harmonic distortion than second.
Ian
Ian Bell
I asked about this because I am new to this group. I was hoping to get some
help and explanation but I am beginning to wish I had not bothered.
Ian
Ian Bell
>
> The point you are missing about the distortion spectra is that the
> introduction of negative feedback introduces new distortion products and
> that changes the relative spectral amplitudes.
How am I missing that? The maths is correct is it not?
> Your analysis assumes
> that the feedback signal itself does not produce any new distortion when
> passing through the amplifier.
>
>
The math makes no such assumption.
Ian
Ian Bell
>
> Ian Bell wrote:
>>
>> I suggest you check out pages 308 and 309 of the radiotron Designers
>> Handbook (1960 edition) and readily available on line for the math that
>> demonstrates my point. Its conclusion on the effect of voltage negative
>> feedback on harmonic distortion is:
>>
>> "That is, the magnitudes of *all* the harmonics (and of course the
>> intermodulation products) introduced by the amplifier are reduced by
>> negative voltage feedback in the same proportion that the gain is
>> reduced."
>
> This is quoted out of context.
>
> On page 309, the Bible goes on to state 3 conditions under which the
> statement above is correct.
>
> Nearly all amplifiers operate outside the conditions stated
> for full equal reductions of harmonic content.
>
Within the audio band those conditions are true.
> And RDH4 does not explore to what extent new additional higher numbered
> harmonics of frequencies above the open loop
> spectra are produced by the NFB.
>
Are you saying the math in RDH is WRONG? or INCOMPLETE? If so please point
to to a reference where it is corrected.
Ian
Patrick Turner
Chris Hornbeck wrote:
>
> On Wed, 07 Mar 2007 09:54:09 GMT, Patrick Turner
> <in...@turneraudio.com.au> wrote:
>
> >Heresy abounds on a BS riddled Internet.
>
> Kinda says it all.
>
> Didn't we just do all this less than a year ago? Must
> be a slow year.
Once in about 2002, when Dennis Assaneyev converted me to the faith.
He had great trouble doing that, because although he really knew what
the FB mechanism was,
he could hardly explain it at all.
Before that I believed triodes were glorified vacuum diodes with a grid.
diode
At that time he was a firm admirer of local
cathode voltage FB done like Quad-II, only better, so he and I had a lot
in common
in what we knew worked technically well and which sounded great.
Now I have no idea what happenned to Dennis, maybe he got married and
had a child,
and in Russia that means you give up time you'd spend on the Net
and a lot of other things to make ends meet.....
The Google record has some of the early discourse on the matter for all
to read,
and I read it all without too much shame.
Twice occured in about 2003-4, I can't find the post I composed to
convince Andre about NFB, and which he threatened to frame because it
summed it up so precisely. and thrice in 2006..
Just dial in "negative feedback in triodes" and search.
>
> The only new argument I've read is the "non-linear
> feedback inherent in triodes" one. (The rest are
> booooooring; sorry.)
>
> Anybody care to try to make a case for the "non-linear
> feedback" model? Be particular!
Well, the NFB in a triode IS non linear.
Its because its like the Ea to Ia transfer function in a diode.
When you look at a set of "triode curves", what are the curves?
The curves are the graph of Ea vs Ia for FIXED values of Eg1.
The curves are no different to diode anode curves and for most triodes
the line we get follows a plotted set of points that can be calculated
from Ia = constant x square root of Ea cubed.
cubed and squared things are very common in the natural world of
physics.
The force required to propel yourself on a bicycle through the air
varies at the rate of speed squared.
A torpedo speed through water is limited by its "hull speed" and a
formula
which denies the Navy much real speed underwater, unless the torpedo is
huge, and has
unlimited power to propel it.
>
> We obviously have nothing better to do right now,
> Much thanks,
There are plenty of better things to do, and many other things to do,
and NEVER enough time to savour doing them.
Your statement is that of an ageing man, and although I am aged,
I am happy to forget much youthful BS, but regret not having the speed I
once had.
As years pass, we struggle to learn more than we forget.
And what we learn matures when we learn it again.
>
> Chris Hornbeck
Ian Bell
>
> Do you have any problem accepting that Ultra Linear taps connected to
> a pentode's screen constitute negative feedback?
>
> If not then why do you have a problem with the exact same mechanism in
> a triode?
Because a triode has no screen therefore that cannot be the mechanism.
Ian
Patrick Turner
With a triode, THD without FB is mainly 2H, with 3H at perhaps -20dB
below that,
and other H well below the 3H.
If the 2H is 1%, you can work out the significance of the
other H.
Now if you apply say 10dB of external loop FB, you are feeding back some
2H to the input
and here it is amplified to cancel itself somewhat.
But because the triode has SOME NON LINEARITY, and isn't as pure as Snow
White,
there are intermodulation products produced by the triode, and you get
the additional harmonics equal to the sum and difference between
the other frequency present, calculated as 2H - 1H, and 2H + 1H, where
1H is the fundemental input frequency,
and 2H is the other frequency present. 2H could be a large equal second
input voltage,
but here in this case its merely the tiny bit of distortion.
So the products to the above calculation is 1H and 3H respectively.
This means after MODERATE FB is applied, you get 3H vastly increased,
and the fundmental 1H isn't
reduced quite exactly by 10dB.
The 3H interacts with the 2H present and creates more 1H and 5H, and
levels are extremely low though,
and insignificant, if the 2H was low to begin with, ie, under 1%, not at
10%, which could be possible without clipping
with some triodes.
The intermodulation products, IMD, are called "second order products"
where they are produced by
re-cycled THD artifacts, the IMD spectra increases measurably with
increasing THD
of any given device, and are truly horrid in BJT and j-fet amps without
NFB.
But the triode IMD second order junk is negligible when THD < 1%.
I suggest you all measure what goes on to be sure you know. Don't just
take my word for it all!
Its because of these IMD phenomena that some beloved SE pentode amps can
sound like crap
when used to replace solid state crap. I have had to re-wire EL34 SE
pentode amps made in Thailand
which have about 8% THD at 7 watts.
The little NFB within them doesn't make the sound any better, because
the second order
products are high, even when measure with sine wave, but with Motzart
test signals its
a lot worse, because there are more tones to produce vastly more
artifacts,
and if you could hear all this crap without the undistorted music, its
like someone tearing up paper in rythym with the music. This destroys
music.
Conversion of such amps to triode and KT88 tubes is the better option.
If the same test is done at the same level, the paper tearing sound is a
lot less.
But the load on the PS has to be watched.
It would seem to me that many asian makers have ZERO idea about plate
loading,
bandwidth in OPTs, and stability issues, and contribute
to the pentode's terrible reputation. As long as money flows, they like
the $$$ relief from
getting only $2 per day, since nobody cares about what they don't
know....
RDH4 has some comparisons between Small Signal pentode and triode THD
production, and below 10Vrms,
one could be forgiven for believing pentodes had less THD than triodes,
but at higher levels triodes were better.
RCA who furnished funds for RDH4 to be produced were not to
come out strongly in favour of triodes; pentodes were the state of the
art device,
and were promoted for profits.
Nevertheless, I suggest everyone test a few pentodes and triodes to find
their own truth about the matter.
Please DO NOT RELY fully on a Bible like RDH4 any more than a religious
person should think
that amoung the animals on Noah's Ark, there were a pair of every
dinosaur, and whales in an aquarium.
( Noah din't have much ink, and ran out of papyrus during the writing of
the bible as the storm approached ...)
My own tests of signal pentodes prooves they have more THD with more
complex spectra
at any signal level than an optimised triode with the same Ia dc and
voltage output.
The pentode will have perhaps 20 times the voltage gain as the triode
with this same Vo
and Ia dc, so if an external loop of NFB is applied around the pentode,
its going
to be equipped with a linear NFB network, and its gain can be reduced to
equal the
triode without loop FB.
I suggest you compel yourselves to your workshop, and arm yourselves
with a breadboard
and THD meter, and a 6SN7, and a 6SH7/6AU6.
Old radio chassis cleared of its parts except for tube sockets and a
power supply will do.
Make sure the PS has good filtering of the B+, lest hum make it
impossible to
view waveforms clearly, and for distortion voltages to be also displayed
on your CRO.
None will have any excuse not to build a one stage line level preamp as
follows......
It won't be difficult to get the 6SN7 to make 1% thd at some voltage
maybe 10Vrms into
say 22k with Ia dc = 4mA, bypassed k, no loop FB, gain about 13.
Is measured gain = µ x RL / ( Ra + RL ) from the data book?
Why isn't it equal?
The 6SH7 will have gain at 4mA = about 88 with k and screen bypassed.
At what voltage does 1% THD occur ? what is the spectra of each tube?
If the pentode has a shunt FB network set up to reduce the gain of 88
to 13, what is the network? what resistors would be used to not load the
pentode much more
than with its 22k?
What is the open loop gain with the NFB loop connected?
What is the spectra after FB has been aplied compared to the 6SN7?
Try loading the pentode so the dc arrives through a 1/2 6SN7 follower,
so you have a
µ follower, and with Rk between pentode anode and triode cathode = 15k.
Take the output from the triode cathode.
Bias the triode carefully for optimum dc idle conditions.
if oscillation at LF or HF occurs, apply stabilisation techniques.
What is the pentode voltage gain now? What is the RL of the pentode
with the active load of the triode? What is the THD, and spectra?
What means can RL be changed when using a µ follower?
What RL produces the least 2H?
Connect the loop of FB used with just the lone pentode to give a gain of
13.
What is the new gain with NFB? what has happened to THD and its spectra?
How much NFB has been applied in the two experiments.
Which is better of the 3 circuits?
What are your reasons for the betterments?
Which ciruit sounds better with music?
I suggest I have ruined everyone's weekend; you all thought you were
going
to sit peacefully unchallenged on fat arses typing crap into a PC.
But now you have a real answer producing project to keep you occupied.
To answer questions, you must look.
If you don't look, you won't know.
If you never ask, you remain ignorant.
Patrick Turner.
Patrick Turner
Read the archives, there is a heck of a lot there.
Patrick Turner.
Patrick Turner
Only God could be Complete, or Perfect.
His Creations and his Destructions have reasons we cannot understand,
and no book can enlighten us fully, not even the Bible.
It is our Human Condition to be alone with whatever information exists.
We are mere men, and so were our fathers who wrote RDH4.
Forgive them their shortcomings, as they forgave us when they
brought us up.
I cannot point to everything you need to find.
You must rely on your ability to search high and low for knowledge,
and on your ability to work out the math, not ours.
Patrick Turner.
Ian Bell
snp
>
> This means after MODERATE FB is applied, you get 3H vastly increased,
> and the fundmental 1H isn't
> reduced quite exactly by 10dB.
>
I agree. Trouble is, an open loop pentode already has more 3rd then 2nd
harmonic, so if a triode is a pentode with NFB the 3rd harmonic in a triode
would be should be even higher than it is in a pentode. In fact the
converse is true.
Ian
Ian Bell
>
> Only God could be Complete, or Perfect.
> His Creations and his Destructions have reasons we cannot understand,
> and no book can enlighten us fully, not even the Bible.
> It is our Human Condition to be alone with whatever information exists.
>
> We are mere men, and so were our fathers who wrote RDH4.
>
> Forgive them their shortcomings, as they forgave us when they
> brought us up.
>
> I cannot point to everything you need to find.
> You must rely on your ability to search high and low for knowledge,
> and on your ability to work out the math, not ours.
>
> Patrick Turner.
I am pleased you admit your are not perfect and that you could possibly be
wrong.
Ian
Patrick Turner
Well, Ian, just what is the effect of the screen in a UL amp?
How does the screen manage to reduce odd number harmonics?
How does the screen make the pentode achieve Dn spectra approaching
triode?
Patrick Turner.
Ian Iveson
point in telling you again.
Ian
"John Byrns" <byr...@sbcglobal.net> wrote in message
news:byrnsj-FDC87B....@newsclstr02.news.prodigy.com...
> In article <gm%Hh.55232$nW6....@fe2.news.blueyonder.co.uk>,
> "Ian Iveson" <IanIves...@blueyonder.co.uk> wrote:
>
>> John Byrns wrote:
>>
> My question about the necessity of delay for negative feedback to
> exist
> was just an aside. My point was that there is delay in the triode,
> which invalidates the argument that you put forward that one reason
> negative feedback couldn't exist in a triode was because there was
> no
> delay in a triode.
Patrick Turner
Pentode and beam tetrode have varying 2H and 3H and other H according to
whatever load value is being used.
2H is like a triode when RL is low, and the 2H phase relative to
the fundemental is like the triode's.
As RL is raised, 2H gets lower, reaches a null, then increases again as
RL
is raised above the value for minimum 2H.
But at the high RL values, the 2H phase relative to the 2H below the RL
for least 2H is reversed,
and careful load line plotting will reveal this behaviour.
Triodes don't suffer the phase reversal of their 2H as RL rises; 2H
merely diminishes
to a minima.
But anyway, usually pentodes and beam tubes have a lot of distortion at
their
clipping power quoted for the load, and much more than a triode, and the
mix is more complex, so you must apply 20dB to at least get the Rout
down to acceptable values.
And behold, the second order IMD products begin to be suppressed well
enough.
but 10dB NFB around a pentode amp only has a mild effect on distortions.
3 times more than 10dB is 20 dB.
The dynamic Dn of a pentode amp can make transients more impressive;
each drum beat
causes a spray of extra IMD...
But in lousy SE pentode SE amps such as a lone 6V6 in a mantle radio
with no NFB,
the speaker has rising response until it cuts off at about 8kHz, and
the Dn products compete with the attenuated real HF products in the
music or speach.
( The RF parts of old AM radios rarely produced more than 3kHz of AF
bandwidth ).
Hence we have unique "old radio" sound, with artificial boost to the HF
content.
Guitar amps get by on a similar principle.
All devices are basically ridled with non linear behaviours without any
FB of any kind.
BJTs have linear current in/out curves, but make lousy voltage amps
without
huge amounts of both current and voltage NFB.
J-fets and mosfets have attrocious voltage linearity, but Dn is mainly
2H, and
still you need I and V NFB.
Tubes are more linear than SS devices, but are useless without NFB.
Every piece of gear used in the analog recording chain from mic to
cutting head had lots of NFB.
I doubt one single recording any company ever made with a full bandwidth
recording
after 1955 when some fabulous recording were made without loop NFB.
Companies using triode based gear used less loop FB than the pentode
based products full of EF86, EL84, KT66 etc.
All devices depend on correctional facilities available.
The self regulating triode is the most forgivable criminal,
and he controls his urge to act uglililly in public.
The pentode or a beam tetrode requires a stern and firm handed warden,
Senior Loopaldo Neegartiff Feedbak, who is a fair minded giant.
But even he cannot ever eradicate all spurious behaviours.
Patrick Turner,
Dept of Custodial Singing Lessons.
Patrick Turner
Being perfect requires lots of people to lie about you.
One can only ever really be partially correct, and partially wrong.
Newton gave us laws of motion which seemed SO RIGHT.
but you could never get a rocket to visit planets of the solar system
just relying on Momentum = 1/2 m x V squared et all.
He was right within his limited views, and for our little world, Newton
was pretty right,
if all we want to do is pull a train with a locomotive.
Then along came Einstein, and he had a few visions about the universe,
and he knew a good mate with much better
math ability to work out how bits of the universe related to each other,
along with energies
of many varieties. Some things he discovered existed only in theory, to
be verified much later by observations.
Einstein made a few blunders too. Women flocked to him because he must
have been a great
conversationalist.....
I heard he gave up playing the violin at 70. There are some things
you can't stay doing when they turn out wrong.
As the known universe becomes far bigger the more we look, it would seem
we cannot ever comprehend the infinite amount of information describing
it
because we have only finite brains, only marginally better than a dog.
As we disect atoms, we get Russian Dolls, ever more smaller bits and
pieces and they last
for shorter and shorter amounts of time, and we begin to see we know
less, rather than more.
There would always be something a quantum physicist may say about a
triode, but
amoung us no quantum guy is to be heard...
Newton would have been fascinated were he around now, not to mention
having Leonardo Da Vinci
on the NFB problem. I'd much like to have Leonardo, Isaac, and Albert
in for dinner one evening.
I'd probably never follow what the heck they were talking about.
But when Liz Hurley visits me (for dinner), she doesn't have a clue
about triodes,
but she likes a big pentode.
Patrick Turner, triodolically addicted.
Ian Bell
Yes, but you have ignored my point about the relative levels of 3H and 2H in
each type.
Ian
Ian Bell
>
>
> Ian Bell wrote:
>>
>> flipper wrote:
>> >
>> > Do you have any problem accepting that Ultra Linear taps connected to
>> > a pentode's screen constitute negative feedback?
>> >
>> > If not then why do you have a problem with the exact same mechanism in
>> > a triode?
>>
>> Because a triode has no screen therefore that cannot be the mechanism.
>>
>> Ian
>
> Well, Ian, just what is the effect of the screen in a UL amp?
>
> How does the screen manage to reduce odd number harmonics?
>
I am not sure but UL is a special case where even harmonics are already
cancelled by the basic push pull topology. This confuses the issue rather
than helps resolve it.
Instead consider an EF86 connected as a triode. Can you connect it as a
pentode with feedback to obtain the identical characteristics? I don't
think you can.
Ian
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> One can only ever really be partially correct, and partially wrong.
>
> Newton gave us laws of motion which seemed SO RIGHT.
> but you could never get a rocket to visit planets of the solar system
> just relying on Momentum = 1/2 m x V squared et all.
> He was right within his limited views, and for our little world, Newton
> was pretty right,
> if all we want to do is pull a train with a locomotive.
Is this really true, that it requires more than Newtonian Physics to
send a rocket anywhere in the solar system? I don't know one way or the
other, I never thought about it and just assumed that Newton was all we
needed for this simple task, so I am curious.
John Byrns
wrote:
> Patrick Turner wrote:
There are two big problems with this assertion. First the statement
that a "pentode already has more 3rd then 2nd harmonic" is not obviously
true as you have failed to specify any details about the particular
tubes involved and the condition under which you have measured the
harmonic spectrum. At what level is this oft quoted triode vs. pentode
distortion spectrum measured? I suspect it is measured at a high output
levels and in particular at an output level for the pentode beyond which
an equivalent triode could even muster the same amount of power. I
think it is only fair to level the playing field and measure equivalent
triodes and pentodes at the same power level, a power level within the
range that the triode can handle without excessive distortion. By
equivalent triodes and pentodes I don't mean tubes of each type that
have the same rated power output, but rather I mean a pair of tubes
where the characteristics of the pentode when it is triode strapped are
similar to the characteristics of the equivalent triode. If the two
tubes are then measured at the same power level, say max power for the
triode, the pentode will be loafing along and will likely have a
different distortion spectra than the one usually assumed. The pentode
is capable of greater power output than an equivalent triode because of
the effect of the fixed potential on the screen grid. Another point is
that saying a pentode has more third harmonic than second harmonic is
somewhat akin to saying that a triode push-pull amplifier has more third
harmonic relative to second harmonic than does a single ended amplifier
using the same triode tubes. The push-pull circuit configuration
cancels the second harmonic. Similarly in a pentode the presence of the
screen grid allows the loading to be optimized in a way not possible
with a triode such that the second harmonic is canceled at a specific
power level.
I have a vague memory that someone was building a triode version of the
EL34 a few years ago, I assume this was built as a true triode around
the cathode-grid assembly of the EL34 pentode. If these tubes exist it
would be interesting to compare the distortion spectra of this tube with
an actual EL34 operating at the same level. Does anyone out there
remember this tube or know if it really existed?
The second problem with your statement is that it isn't at all obvious
how your conclusion "so if a triode is a pentode with NFB the 3rd
harmonic in a triode would be should be even higher than it is in a
pentode" follows from the hypothetical fact that a pentode has higher
third harmonic distortion than it has second harmonic distortion. Can
you explain your reasoning on this point? You have stated several times
that negative feedback reduces all harmonics equally, if that is true
wouldn't it mean that at worst the relative harmonic structure would
remain unchanged?
John Byrns
wrote:
> Patrick Turner wrote:
> >
> > And RDH4 does not explore to what extent new additional higher numbered
> > harmonics of frequencies above the open loop
> > spectra are produced by the NFB.
> >
>
> Are you saying the math in RDH is WRONG? or INCOMPLETE? If so please point
> to to a reference where it is corrected.
The RDH4 is incomplete, it does not consider new harmonics are generated
by the feedback, see the third assumption on page 309.
Ian Bell
> In article <45f146b3.0@entanet>, Ian Bell <ruffr...@yahoo.co.uk>
> wrote:
>
>> Patrick Turner wrote:
>>
>> snp
>>
>> >
>> > This means after MODERATE FB is applied, you get 3H vastly increased,
>> > and the fundmental 1H isn't
>> > reduced quite exactly by 10dB.
>> >
>>
>> I agree. Trouble is, an open loop pentode already has more 3rd then 2nd
>> harmonic, so if a triode is a pentode with NFB the 3rd harmonic in a
>> triode would be should be even higher than it is in a pentode. In fact
>> the converse is true.
>
> There are two big problems with this assertion. First the statement
> that a "pentode already has more 3rd then 2nd harmonic" is not obviously
> true as you have failed to specify any details about the particular
> tubes involved and the condition under which you have measured the
> harmonic spectrum.
I am not making any assumption about high levels of power. In fact I am
considering and EF86 in a class A preamp.
>
> The second problem with your statement is that it isn't at all obvious
> how your conclusion "so if a triode is a pentode with NFB the 3rd
> harmonic in a triode would be should be even higher than it is in a
> pentode" follows from the hypothetical fact that a pentode has higher
> third harmonic distortion than it has second harmonic distortion. Can
> you explain your reasoning on this point? You have stated several times
> that negative feedback reduces all harmonics equally, if that is true
> wouldn't it mean that at worst the relative harmonic structure would
> remain unchanged?
>
I see two possibilities. First the relative harmonic structure remains
unchanged. If the harmonic structures of open loop triodes and pentodes are
different then the harmonic structure of a pentode with feedback must be
different to that of a triode.
If, as some other posters have suggested, NFB changes the relative harmonic
structure and accentuates the higher order harmonics then the answer
depends on the actual open loop relative harmonic structures of the two
types of valve. My contention is that the harmonic structure of an open
loop pentode has a higher 3H relative to 2H than is the case for a pentode.
If NFB increases the relative level of higher harmonics, then a pentode
with NFB will have an even higher 3H relative to 2H than an open loop one
so its distortion spectrum is even further removed from that of a triode.
At present I am conducting some experiments with an EF86 in triode and
pentode configurations to try to put some figures to these assertions.
ian
Ian Bell
> In article <45f10d0c.0@entanet>, Ian Bell <ruffr...@yahoo.co.uk>
> wrote:
>
>> Patrick Turner wrote:
>> >
>> > And RDH4 does not explore to what extent new additional higher numbered
>> > harmonics of frequencies above the open loop
>> > spectra are produced by the NFB.
>> >
>>
>> Are you saying the math in RDH is WRONG? or INCOMPLETE? If so please
>> point to to a reference where it is corrected.
>
> The RDH4 is incomplete, it does not consider new harmonics are generated
> by the feedback, see the third assumption on page 309.
>
>
Yes, a lot depends on 'how hard' the valve is driven. In preamps the open
loop distortion is small to begin with because the signal levels are
relatively small so the third assumption is not an issue. In power amps
things are different - however, that should be immaterial if the 'triode is
a pentode with NFB' contention is true.
Ian