High-ohm grid resistor in tube (pre)amp design
Richard Rasker
I regularly repair tube amplifiers, and there's one thing I occasionally
encounter that doesn't appear to make much sense: a high resistor (> 100K)
in series with one of the first 12AX7 preamp tube grids.
OK, I fully understand that this resistor, together with the Miller
capacitance between grid and anode, can act as a high cut-off filter, and
that this can prevent HF oscillations and the likes, but it also introduces
noise -- a LOT of noise.
I have here exhibit A, a Peavey 5150 EVH. Shorting the aforementioned 470K
grid resistor in the second preamp triode cut the noise level by as much as
15dB(!). OK, the sound got slightly more agressive as well -- definitely
more treble in clean mode -- but that didn't matter too much in overdrive
mode (and in clean mode, just small adjustment of the Treble control would
take care of that).
So my question is if anyone can explain why any designer would use such a
ludicrously high grid resistor -- IMHO, there are far better ways to create
a high cut-off circuit instead of this messy, noisy "solution". Or is there
some sort of extra or hidden advantage to this?
Thanks in advance, best regards,
Richard Rasker
--
http://www.linetec.nl
Rich Grise
Have you tried contacting Peavey?
Good Luck!
Rich
Jan Panteltje
<spam...@linetec.nl> wrote in
<4afb3280$0$729$7ade...@textreader.nntp.internl.net>:
>So my question is if anyone can explain why any designer would use such a
>ludicrously high grid resistor --
And then also explain why the moron used tubes, and not a good opamp and / or semiconductors.
Richard Rasker
> On Wed, 11 Nov 2009 22:54:08 +0100, Richard Rasker wrote:
>
>> Hi all,
>>
>> I regularly repair tube amplifiers, and there's one thing I occasionally
>> encounter that doesn't appear to make much sense: a high resistor (>
>> 100K) in series with one of the first 12AX7 preamp tube grids.
...
>> I have here exhibit A, a Peavey 5150 EVH.
...
> Have you tried contacting Peavey?
No. But I know from experience that it takes a LOT of perseverence and
patience before you get to speak to someone who remembers "when that guy
drew up the preamp stage, 25 years ago." Not to mention the fact that
manufacturers are very rarely inclined to discuss matters of design --
especially when it comes to possible shortcomings.
I'm under the impression that I'm dealing with a kind of almost "genetic"
flaw, where whole schools of Peavey (and other brands') designers simply
copied the tube preamp stage from one model to the next, without
questioning it, because "it worked". Never mind the noise, these things are
meant to be played loud anyway.
So in short, my bets are rather more on this news group than on Peavey.
Dave Platt
Richard Rasker <spam...@linetec.nl> wrote:
>So my question is if anyone can explain why any designer would use such a
>ludicrously high grid resistor -- IMHO, there are far better ways to create
>a high cut-off circuit instead of this messy, noisy "solution". Or is there
>some sort of extra or hidden advantage to this?
At a guess, it's done because it's simple and cheap, and "good enough"
for the specific application in which the product is used. The design
was Muntzed.
(ref: http://en.wikipedia.org/wiki/Madman_Muntz)
Or, possibly, the designer was drunk or insane at the time :-)
--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Tim Wescott
Is the resistor in the path from grid to ground?
Tubes exhibit an effect where the electrons boiling off the cathode want
to settle on the grid. Make the path to ground with a high enough DC
bias and the tube will self-bias. The resistor is then called a "grid
leak"
(and before all you old tubies jump on me -- no, I don't remember if 470k
is a high enough resistance for this to work, or if you need multi-
megohms).
Could that be it? If so, shorting out the resistor would overbias the
tube and cause excessive current. Shorting the resistor out with a cap
should cause the same effect in the audio (if it's a big enough cap).
Such a resistor would also tend to reduce the bias on the tube in the
event that it gets over-driven - as soon as the grid starts to conduct
the stage would get biased for less plate current, gain, output, etc.
Fred Bartoli
Sure! How could they use a grid resistor with opamps?
--
Thanks,
Fred.
Ross Herbert
Schematic might be in order then...
http://www.blueguitar.org/new/schem/peavey/5150evh.pdf
Adrian Tuddenham
[...]
> Tubes exhibit an effect where the electrons boiling off the cathode want
> to settle on the grid. Make the path to ground with a high enough DC
> bias and the tube will self-bias. The resistor is then called a "grid
> leak"
>
> (and before all you old tubies jump on me -- no, I don't remember if 470k
> is a high enough resistance for this to work, or if you need multi-
> megohms).
It's usually 10 megohms and develops about 1 to 2 volts of negative
bias. If the valve becomes the slightest bit 'soft', the grid can pick
up ions and go positive, which leads to runaway if there is no current
lmiting factor elsewhere in the circuit. Because of this, this method
of biassing is only really suitable for low level signal stages with
high value anode resistors, which will work adequately with such low and
unpredicatable levels of negative bias.
Even 470k can sometimes be too high a grid leak for power stages, even
with normal cathode bias, which might become slightly soft due to the
high temperatures out-gassing the anode structure.
The series resistor described by the original poster is called a 'grid
stopper' (at least, it is on this side of the Pond), but I have never
seen one as high as 470k. Usually they are in the range 10k to 47k. I
don't suppose the OP has mis-read the colour code or measured one that
is on its way to going O/C?
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
Phil Allison
>
> I regularly repair tube amplifiers, and there's one thing I occasionally
> encounter that doesn't appear to make much sense: a high resistor (> 100K)
> in series with one of the first 12AX7 preamp tube grids.
>
> OK, I fully understand that this resistor, together with the Miller
> capacitance between grid and anode, can act as a high cut-off filter, and
> that this can prevent HF oscillations and the likes, but it also
> introduces
> noise -- a LOT of noise.
** Not true, when it is in a later stage of the circuit than the input
stage.
Cos the noise generated by the first tube stage swamps that resistor's
noise.
BTW:
470kohms has a self noise of about 10uV in the audio band - much less in
the narrower band guitar amps use.
The audio band INPUT noise level of a typical 12AX7 triode section is 3
V - multiplied by the gain of 50 or more at the plate.
> I have here exhibit A, a Peavey 5150 EVH. Shorting the aforementioned 470K
> grid resistor in the second preamp triode cut the noise level by as much
> as
> 15dB(!).
** Not true, in normal use.
> So my question is if anyone can explain why any designer would use such a
> ludicrously high grid resistor
** Simple - to prevent the previous tube stage from being asymmetrically
loaded by grid current when following stage is overdriven.
Peavey's clearly has a delicate touch with tube guitar circuits.
..... Phil
Phil Allison
"Phil Allison"
** Hmmm.... two typos crept in:
" The audio band INPUT noise level of a typical 12AX7 triode section
is 3 uV - multiplied by the gain of 50 or more at the plate."
" Peavey's designer clearly has a delicate touch with tube guitar
circuits."
..... Phil
ChrisQ
>
>
> Schematic might be in order then...
> http://www.blueguitar.org/new/schem/peavey/5150evh.pdf
Quite a complex front end and if you take a quick look at the phase
splitter on page 2, (V4), it looks like they are using some weird
assymetrical drive to get core saturation on the output stage. It's all
about the sound on valve guitar amps and it doesn't always follow the
usual expected rules of electronics. I don't know why thay used the 470k
to the grid of v1b, but it could be somthing to do with the behaviour of
the triode under severe overdrive conditions, where it may be drawing
some grid current. Whether the design engineer realised this at the
time, or whether it was just a hack to stop some squegging problem on
the prototype, who knows.
On Marshall amps, for example, they used to have the output transformers
wound with the primary centre tap offset to ensure non linearity and
core saturation at high levels :-)...
Regards,
Chris
Phil Allison
"Chris Quack"
>
>> Schematic might be in order then...
>> http://www.blueguitar.org/new/schem/peavey/5150evh.pdf
>
>
> Quite a complex front end and if you take a quick look at the phase
> splitter on page 2, (V4), it looks like they are using some weird
** Nonsense - it is a 100% standard, time honoured circuit .
Almost identical to that used in most Fender models.
> I don't know why thay used the 470k to the grid of v1b, but it could be
> somthing to do with the behaviour of the triode under severe overdrive
> conditions, where it may be drawing some grid current.
** So you read my post just 10 minutes ago .............
Bloody plagiarist.
> On Marshall amps, for example, they used to have the output transformers
> wound with the primary centre tap offset to ensure non linearity and core
> saturation at high levels :-)...
** 1000% purest BOLLOCKS !!!!
Not true of any tube guitar amp ever made.
Where do trolling fuckwits like Mr C. Quack get their insane shit from ??
And why do they have to spread it around all over the fucking net ???
..... Phil
Jan Panteltje
<phi...@tpg.com.au> wrote in <7m2fn3F...@mid.individual.net>:
>> So my question is if anyone can explain why any designer would use such a
>> ludicrously high grid resistor
>
>
>** Simple - to prevent the previous tube stage from being asymmetrically
>loaded by grid current when following stage is overdriven.
How much sense does that make?
If any grid current flows the wave form will be distorted.
I have looked at that diagram, and maybe for a guitarist it is a great diagram.
But I would put the thing with the trash if I found it.
Even 10 years before it as designed.
You can make the same effect with a few diodes....
LOL
ChrisQ
> "Chris Quack"
>>> Schematic might be in order then...
>>> http://www.blueguitar.org/new/schem/peavey/5150evh.pdf
>>
>> Quite a complex front end and if you take a quick look at the phase
>> splitter on page 2, (V4), it looks like they are using some weird
>> asymmetrical drive to get core saturation on the output stage.
>
> ** Nonsense - it is a 100% standard, time honoured circuit .
>
Yes, (yawn) it's long tailed pair with cathode coupling, but the anode
loads are different and there's an added input to the second stage via
c51 from next2, which comes from the first page. The asymmetrical anode
values may be there just to balance the gain, (They are usually the same
value) but who can tell without measuring the voltages under various
operating conditions ?. My guess is that it's an intentional imbalance.
100k vs 82k ?.
>> I don't know why thay used the 470k to the grid of v1b, but it could be
>> somthing to do with the behaviour of the triode under severe overdrive
>> conditions, where it may be drawing some grid current.
>
> ** So you read my post just 10 minutes ago .............
>
> Bloody plagiarist.
>
What makes you think that you are the only one that knows anything about
'tube' amps ? or tube characteristics in general ?. Be assured, I really
don't need to plagiarise anything of yours.
>
>> On Marshall amps, for example, they used to have the output transformers
>> wound with the primary centre tap offset to ensure non linearity and core
>> saturation at high levels :-)...
>
>
> ** 1000% purest BOLLOCKS !!!!
>
> Not true of any tube guitar amp ever made.
>
None, ever, I guess you've seen them all then ?. If you are so clever,
why don't you measure the primary inductance of both halves of an early
1970's Marshall OP transformer, or do a turns ratio test, to see for
yourself ?.
Having grown up with 'tubes' and working in pro audio, 1968-1980, I may
just remember part of a clue, even now, despite what you may think...
Regards,
Chris
Phil Allison
"Chris Quack"
>>>> Schematic might be in order then...
>>>> http://www.blueguitar.org/new/schem/peavey/5150evh.pdf
>>>
>>> Quite a complex front end and if you take a quick look at the phase
>>> splitter on page 2, (V4), it looks like they are using some weird
>>> asymmetrical drive to get core saturation on the output stage.
>>
>> ** Nonsense - it is a 100% standard, time honoured circuit .
>>
>
> Yes, (yawn) it's long tailed pair with cathode coupling, but the anode
> loads are different
** Just as in nearly every Marshall tube amp ever sold.
> and there's an added input to the second stage via c51 from next2, which
> comes from the first page.
** For the "presence" pot - as used in Marshalls as well.
> The asymmetrical anode values may be there just to balance the gain,
** Duhhhhhh ...........
(They are usually the same value)
** 100% BOLLOCKS !!
> but who can tell without measuring the voltages under various operating
> conditions ?. My guess is that it's an intentional imbalance. 100k vs 82k
> ?.
** Too hard for you go look at a Marshall schem ??
Or an early Fender model that Marshall copied ??
I do understand you would have to quit masturbating for a couple of
minutes.
Such sacrifices have to be made in the cause of science .....
>>> I don't know why thay used the 470k to the grid of v1b, but it could be
>>> somthing to do with the behaviour of the triode under severe overdrive
>>> conditions, where it may be drawing some grid current.
>>
>> ** So you read my post just 10 minutes ago .............
>>
>> Bloody plagiarist.
>>
>
> What makes you think that you are the only one that knows anything about
> 'tube' amps ? or tube characteristics in general ?. Be assured, I really
> don't need to plagiarise anything of yours.
** But you did anyway - just for the heck of it right ?
You fucking liar.
>>> On Marshall amps, for example, they used to have the output transformers
>>> wound with the primary centre tap offset to ensure non linearity and
>>> core saturation at high levels :-)...
>>
>>
>> ** 1000% purest BOLLOCKS !!!!
>>
>> Not true of any tube guitar amp ever made.
>>
>
> None, ever, I guess you've seen them all then ?.
** Just about.
> If you are so clever, why don't you measure the primary inductance of both
> halves of an early 1970's Marshall OP transformer, or do a turns ratio
> test, to see for yourself ?.
** Fact is - I have.
That is how I am absolutely sure that YOU are a nothing but a
fucking, trolling BULLSHITTER !!!
> Having grown up with 'tubes' and working in pro audio, 1968-1980,
** Doing what ? Cable winding.
> I may just remember part of a clue, even now, despite what you may
> think...
** You have no clues at all.
Just delusions.
..... Phil
Phil Allison
"Jan Panteltje"
>>> So my question is if anyone can explain why any designer would use such
>>> a
>>> ludicrously high grid resistor
>>
>>
>>** Simple - to prevent the previous tube stage from being asymmetrically
>>loaded by grid current when following stage is overdriven.
>
> How much sense does that make?
** Lots actually.
> If any grid current flows the wave form will be distorted.
** But not the one at the previous plate and neither will the coupling cap
charge up much.
Matters in tube guitar amps.
Little thing sometime called "blocking" .
> I have looked at that diagram, and maybe for a guitarist it is a great
> diagram.
> But I would put the thing with the trash if I found it.
> Even 10 years before it as designed.
** Jan - why were you ever born ??
Was you mum a cheap whore who got careless ??
..... Phil
Ralph Barone
Ross Herbert <rher...@bigpond.net.au> wrote:
My guess is that it's there to reduce the grid current that flows when
you overdrive the tube so badly that the grid goes positive wrt the
cathode.
ChrisQ
>
> ** For the "presence" pot - as used in Marshalls as well.
>
And um, what effect is the presence pot having and how does it function
in term of signal path, system gain and frequency response ?.
>
> ** Too hard for you go look at a Marshall schem ??
It's 30+ years since I even saw a Marshall amp with the cover off, let
alone a schematic and the schematic doesn't define the o/p transformer
characteristics or the design rational.
>
<jealous, bitchy cucumber rant, snipped>
>> If you are so clever, why don't you measure the primary inductance of both
>> halves of an early 1970's Marshall OP transformer, or do a turns ratio
>> test, to see for yourself ?.
>
> ** Fact is - I have.
Really ? when exactly was this ? :-).
If you've been involved in the business for that long, you must remember
a company called IES London and Mavis mixers ?...
Regards,
Chris
Jan Panteltje
<phi...@tpg.com.au> wrote in <7m2lbmF...@mid.individual.net>:
>
>"Jan Panteltje"
>
>>>> So my question is if anyone can explain why any designer would use such
>>>> a
>>>> ludicrously high grid resistor
>>>
>>>
>>>** Simple - to prevent the previous tube stage from being asymmetrically
>>>loaded by grid current when following stage is overdriven.
>>
>> How much sense does that make?
>
>** Lots actually.
>
>> If any grid current flows the wave form will be distorted.
>
>
>** But not the one at the previous plate and neither will the coupling cap
>charge up much.
>
> Matters in tube guitar amps.
Everything matters in a guitar amp, the guitarist heard a sound that he liked,
and that design was his sound from then on.
Copied by many.
Could have been banging trash cans too, as you know :-)
> Little thing sometime called "blocking" .
>
>
>> I have looked at that diagram, and maybe for a guitarist it is a great
>> diagram.
>> But I would put the thing with the trash if I found it.
>> Even 10 years before it as designed.
>
>** Jan - why were you ever born ??
Well, I had a deep moment of sadness reading the conversation, especially your reply to the OP.
But this now really cheers me up, at least you ask solid questions that you need to answer in your life.
So WHY were you born?
Let's simplify this, I say: to be happy.
It is not to reproduce only, and that may be a dead end street anyways, as any dinosaur would be able to tell you.
Now the next question, and the important one, is how to always be happy.
As the efficiency (of you), in percent, is 100x days_happy / days_lived.
There are no official standards yet, many would score really low and have to be ..ahum...
And probably there never will be a law about that.
But is is beneficially to work on that yourself, introspection, meditation, I recommend it.
So how does all this relate to guitar? Well obviously if you are happy and make music,
then it will make others happy who listen to it.
A sad song will make others sad..
Nature.
How, and how much, the thing distorts is not even relevant, although one sort of distortion may
sound a lot more pleasant then the other,.
But I would have put the tube thing with the trash anyways, because from the above it follows that
you can also make nice distortion in many ways with semiconductors... with the same effect on yourself and humanity.
And it has been that way since the first Ge transistors came on the market.
So, and you were wrong about that load issue, it is the signal that counts.
ChrisQ
In fact, it's very difficult to get the required musical characteristics
from a solid state amp because of the hard limiting with a typical
transformerless output stage. A valve preamp and output stage gives a
softer saturation characteristic. I guess the closest you could get to a
solid state version would be to use power mosfets driving a transformer,
but valves are very rugged as well in terms of short and medium term
gross overload and a power mosfet design would need to have quite a bit
of protection to handle shorted output, open circuit speaker lead at max
volume etc.
No idea what the current state of art is now, but even in the mid to
late 70's, it was getting difficult to find good quality output valves
like 6L6 and EL34's. Most if not all the western manufacturers were out
of the valve business and the russian and bulgarian substitutes were
poor quality and rarely lasted for more than a show or two. The Marshall
amps of 1979 timeframe overdrove the anode voltage at no load (~500v dc,
iirc) as well and the valves blue glowed from new. Failure was a red hot
anode and subsequent blown ht fuse. But there again, the sound was
perfect for the job :-)...
Regards,
Chris
Jim Thompson
>Jan Panteltje wrote:
>> On a sunny day (Wed, 11 Nov 2009 22:54:08 +0100) it happened Richard Rasker
>> <spam...@linetec.nl> wrote in
>> <4afb3280$0$729$7ade...@textreader.nntp.internl.net>:
>>
>>> So my question is if anyone can explain why any designer would use such a
>>> ludicrously high grid resistor --
>>
>> And then also explain why the moron used tubes, and not a good opamp and / or semiconductors.
>>
>
>In fact, it's very difficult to get the required musical characteristics
>from a solid state amp because of the hard limiting with a typical
>transformerless output stage.
Real men know how to shape clipping so that it is "soft", yet maintain
output damping ;-)
>A valve preamp and output stage gives a
>softer saturation characteristic.
Real men don't use valves ;-)
>I guess the closest you could get to a
>solid state version would be to use power mosfets driving a transformer,
>but valves are very rugged as well in terms of short and medium term
>gross overload and a power mosfet design would need to have quite a bit
>of protection to handle shorted output, open circuit speaker lead at max
>volume etc.
Real men don't use transformers ;-)
>
>No idea what the current state of art is now, but even in the mid to
>late 70's, it was getting difficult to find good quality output valves
>like 6L6 and EL34's.
Did that shit when I was 16-18 years old... I'll never go back ;-)
>Most if not all the western manufacturers were out
>of the valve business and the russian and bulgarian substitutes were
>poor quality and rarely lasted for more than a show or two. The Marshall
>amps of 1979 timeframe overdrove the anode voltage at no load (~500v dc,
>iirc) as well and the valves blue glowed from new. Failure was a red hot
>anode and subsequent blown ht fuse. But there again, the sound was
>perfect for the job :-)...
>
>Regards,
>
>Chris
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
The Democrat theory is that spend.. spend.. spend.. and government
jobs will bolster the economy and employment will improve. If that
were actually true, wouldn't France have the world's best economy?
ChrisQ
>> and there's an added input to the second stage via c51 from next2, which
>> comes from the first page.
>
> ** For the "presence" pot - as used in Marshalls as well.
>
>
Just in case that was too much, just redrew the circuit with v4 the
right way up and the presence pot included.
What we seem to have is a 0.047 between the grid across r53 and r51,
with the junction of r51, r57 and c31 connected to the series pot and a
couple of caps, one of which is 0.033. Starts to look a bit wein bridgy
to me, in a rough sort of way. Result would be a frequency dependent
amplitude into V4a grid, which might produce some interesting non linear
effects in the op stage.
Anyway, stop being such a misery Phil and tell us how you think it works :-)
Regards,
Richard Rasker
> "Richard Rasker"
>>
>> I regularly repair tube amplifiers, and there's one thing I occasionally
>> encounter that doesn't appear to make much sense: a high resistor (>
>> 100K) in series with one of the first 12AX7 preamp tube grids.
>>
>> OK, I fully understand that this resistor, together with the Miller
>> capacitance between grid and anode, can act as a high cut-off filter, and
>> that this can prevent HF oscillations and the likes, but it also
>> introduces noise -- a LOT of noise.
>
> ** Not true, when it is in a later stage of the circuit than the input
> stage.
OK, but in this case, the resistor is found in series with the grid of the
second triode -- and believe me, it's responsible for a lot of noise.
> Cos the noise generated by the first tube stage swamps that resistor's
> noise.
>
> BTW:
>
> 470kohms has a self noise of about 10uV in the audio band - much less in
> the narrower band guitar amps use.
>
> The audio band INPUT noise level of a typical 12AX7 triode section is 3
> V - multiplied by the gain of 50 or more at the plate.
>> I have here exhibit A, a Peavey 5150 EVH. Shorting the aforementioned
>> 470K grid resistor in the second preamp triode cut the noise level by as
>> much as 15dB(!).
>
> ** Not true, in normal use.
I measured this with the "pre" gain turned 1/4 up and the "post" gain
halfway up. In a silent environment, it makes the difference between
audible noise and almost total silence. OK, at normal playing volume, it's
hardly noticeable, but during quiet passages, it's rather annoying --
especially in a studio environment.
>> So my question is if anyone can explain why any designer would use such a
>> ludicrously high grid resistor
>
>
> ** Simple - to prevent the previous tube stage from being asymmetrically
> loaded by grid current when following stage is overdriven.
That seems a bit far-fetched in this case, considering the placing in the
circuit -- it's R82, behind the very first triode, with some filtering and
the volume control in between.
Anyway, I'll do some more measuring, experimenting and listening; perhaps
the best thing to do is to decrease the resistor value to 100K or
thereabouts.
Thanks everyone for their input; I've learned a few new things about tube
amp design :-)
ChrisQ
>
> Real men don't use valves ;-)
Or eat quiche, from what I understand :-).
>
> Real men don't use transformers ;-)
>
Not even ferrite ?. I might, even now, though the rest of it would be
solid state. Not such a new idea either. They were using some
transistorised plug in tube replacements for 12ax7 etc in some amps as
far back as the 70's, though tubes and transformers always in the
output. I guess you could consider the amp itself as part of the musical
instrument. Thus, almost everything is subjective.
>
> Did that shit when I was 16-18 years old... I'll never go back ;-)
>
Go back to what, Jim :-). Was all part of the ongoing book of the
progress of electronics. Look back at it as a great time to be working
in the art, with the techniques of the day having something to teach us
even now...
Regards,
Jan Panteltje
<me...@devnull.com> wrote in <8zXKm.33887$uf7....@newsfe12.ams2>:
>Jan Panteltje wrote:
>> On a sunny day (Wed, 11 Nov 2009 22:54:08 +0100) it happened Richard Rasker
>> <spam...@linetec.nl> wrote in
>> <4afb3280$0$729$7ade...@textreader.nntp.internl.net>:
>>
>>> So my question is if anyone can explain why any designer would use such a
>>> ludicrously high grid resistor --
>>
>> And then also explain why the moron used tubes, and not a good opamp and / or semiconductors.
>>
>
>In fact, it's very difficult to get the required musical characteristics
>from a solid state amp because of the hard limiting with a typical
>transformerless output stage. A valve preamp and output stage gives a
>softer saturation characteristic. I guess the closest you could get to a
>solid state version would be to use power mosfets driving a transformer,
>but valves are very rugged as well in terms of short and medium term
>gross overload and a power mosfet design would need to have quite a bit
>of protection to handle shorted output, open circuit speaker lead at max
>volume etc.
It all depends.
If we leave the combination speaker-amplifier out of the picture for a short moment,
I will tell you what happened to me some time ago.
I was listening to some old recording on the PC, and that was an old analog recording.
And I thought: 'Hey analog sounded better'.
Now I caught myself thinking that, and realized that I was listening to, YES, an
mp3 version, digitally processed, made from a wave recording of the old analog tape...
So the essence is of course that if you have an amplifier with sufficient low distortion, or
amplifier-speaker combination with the same, then you can *reproduce* any sound.
Somehow you seem to think, well that seems to me at least, that the sound or
specific distortion so to speak ( :-) ) needs to be created in the output stages?
>No idea what the current state of art is now, but even in the mid to
>late 70's, it was getting difficult to find good quality output valves
>like 6L6 and EL34's. Most if not all the western manufacturers were out
>of the valve business and the russian and bulgarian substitutes were
>poor quality and rarely lasted for more than a show or two. The Marshall
>amps of 1979 timeframe overdrove the anode voltage at no load (~500v dc,
>iirc) as well and the valves blue glowed from new. Failure was a red hot
>anode and subsequent blown ht fuse. But there again, the sound was
>perfect for the job :-)...
Yes I have build a nice tube amp for a band, and the lead guitarist liked the distortion
when overdriving it :-)
But really, with modern processing techniques you can make a lot more, and perhaps
better, effects at the low signal level and then play it via a good solid state amp and high
quality speakers...
Probably with more volume too...
It would be interesting to somehow characterise the transfer curves of some of those old tube things,
so it can be mimicked in FPGA or with a DSP for example.
But with all those pots it would be difficult.
But then, listening to Jimmy Hendrix in *my* opinion it would not really matter if it was a few percent off.
Regards
Jan
>Regards,
>
>Chris
>
John Walliker
> But really, with modern processing techniques you can make a lot more, and perhaps
> better, effects at the low signal level and then play it via a good solid state amp and high
> quality speakers...
> Probably with more volume too...
> It would be interesting to somehow characterise the transfer curves of some of those old tube things,
> so it can be mimicked in FPGA or with a DSP for example.
Many years ago (probably 1977 or 1978) Charlie Watkins of WEM gave a
talk to my university audio society. One of the questions he answered
was "why not use non-linear networks to simulate the valve sound" in
guitar amps.
The answer was that WEM had tried hard to do this, as there would have
been considerable robustness and cost advantages. Unfortunately, they
could not get the sound right, so they decided to carry on making
valve amps. I think the problem was not just getting the right
transfer function, but also the time dependencies like power supply
sag and even valve microphonics that affect the overall sound under
severe overload.
John
Jan Panteltje
Walliker <jrwal...@gmail.com> wrote in
<c35f8d6c-f5dc-4916...@r24g2000yqd.googlegroups.com>:
If you can describe it, then you can program it.
Jim Thompson
>Jim Thompson wrote:
>
>>
>> Real men don't use valves ;-)
>
>Or eat quiche, from what I understand :-).
You understand wrong. With quiche, real men eat strawberry-spinach
salads ;-)
>
>>
>> Real men don't use transformers ;-)
>>
>
>Not even ferrite ?. I might, even now, though the rest of it would be
>solid state. Not such a new idea either. They were using some
>transistorised plug in tube replacements for 12ax7 etc in some amps as
>far back as the 70's, though tubes and transformers always in the
>output. I guess you could consider the amp itself as part of the musical
>instrument. Thus, almost everything is subjective.
No transformers in the signal path.
As for "instruments", I designed and built a 400W guitar amplifier
once-upon-a-time ;-)
>
>>
>> Did that shit when I was 16-18 years old... I'll never go back ;-)
>>
>
>Go back to what, Jim :-).
6V6 => 6L6 => EL34, KT66, KT88 => Motorola Power Trannies
>Was all part of the ongoing book of the
>progress of electronics. Look back at it as a great time to be working
>in the art, with the techniques of the day having something to teach us
>even now...
>
>Regards,
I can still design with tubes, though I now limit myself to 1KV and
upwards stuff. Why waste all that power?
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Liberal, Leftist and Democrat are polite aphorisms for YELLOW.
ChrisQ
>
> No transformers in the signal path.
>
Like all engineering, it's what gets the job done in the most effective
way and a transformer is a perfectly respectable component to use for
such an application, especially considering it's history of getting the
job done well at low cost. It even adds weight to the box, so the
roadies think it's been properly screwed together :-). Such stuff
matters and you build the product, imperfections and all, to sell into
that market. I got into micros in the late 70's, so you can see where my
head was going. There's only so much you can do with a class ab
amplifier and it was time to move on.
If you could accurately model a Marshall 100 or Watkins AC30 or similar
and then dsp the resulting signal chain, you can bet that some muso's
would still say they could still tell the difference and to be honest,
they probably could :-)...
Regards,
Chris
Phil Allison
"Chris Quack is a bullshitting LIAR "
>> ** For the "presence" pot - as used in Marshalls as well.
>>
>
> And um, what effect is the presence pot having and how does it function in
> term of signal path, system gain and frequency response ?.
** Look at the schems yourself - wanker.
Shame you have to give up on fat chick porn for a few minutes.
>> ** Too hard for you go look at a Marshall schem ??
>
> It's 30+ years since I even saw a Marshall amp with the cover off,
** Schems are readily available on the net - wanker.
Shame you have to give up on fat chick porn for a few minutes.
>>> If you are so clever, why don't you measure the primary inductance of
>>> both halves of an early 1970's Marshall OP transformer, or do a turns
>>> ratio test, to see for yourself ?.
>>
>> ** Fact is - I have.
>>
>> That is how I am absolutely sure that YOU are a nothing but a
>>
>> fucking, trolling BULLSHITTER !!!
> Really ? when exactly was this ? :-).
** Done regularly as part of amp testing.
> If you've been involved in the business for that long, you must remember a
> company called IES London and Mavis mixers ?...
** Don't confuse me with damn pommies.
..... Phil
Phil Allison
"Jan Panteltje"
( snip piles of putrid crapping on )
** Jan - why were you ever born ??
Was you mum a cheap whore who got careless ??
What a shame this gross mental defective was not aborted.
.... Phil
Phil Allison
"Richard Rasker"
> Phil Allison wrote:
>
>> ** Not true, when it is in a later stage of the circuit than the input
>> stage.
>
> OK, but in this case, the resistor is found in series with the grid of the
> second triode -- and believe me, it's responsible for a lot of noise.
** No way do I "believe" any such assertions.
>> ** Not true, in normal use.
>
> OK, at normal playing volume, it's
> hardly noticeable, but during quiet passages, it's rather annoying --
> especially in a studio environment.
** Get a noise gate - fool.
>>> So my question is if anyone can explain why any designer would use such
>>> a
>>> ludicrously high grid resistor
>>
>>
>> ** Simple - to prevent the previous tube stage from being asymmetrically
>> loaded by grid current when following stage is overdriven.
>
> That seems a bit far-fetched in this case,
** You are completely missing the point.
Grid current causes the coupling cap to become charged and when the signal
stops it will bias off the following triode stage until it discharges.
Something best avoided in a guitar amp.
..... Phil
christofire
"ChrisQ" <me...@devnull.com> wrote in message
news:CF%Km.9817$RG....@newsfe20.ams2...
What's a 'Watkins AC30'? The well-known AC30 certainly wasn't one of
Charlie Watkins's creations.
Chris(a different one)tofire
christofire
"John Walliker" <jrwal...@gmail.com> wrote in message
news:c35f8d6c-f5dc-4916...@r24g2000yqd.googlegroups.com...
The requisite DSP techniques hadn't been rummaged by 1978. WEM carried on
making the 'Copycat' with a tape loop and an AC induction motor long after
bucket-brigade devices had become available, and then started using them
after the rest of the industry had gone digital. Line 6, for one, have made
a reasonable job of simulating 'valve sound' in DSP, but that's only my
personal opinion. As noted in another thread in this Usenet group, it's
believed that Line 6 upsample before applying a non-linear function (e.g.
tanh) so the highest of the resulting harmonics don't create noticeable
enharmonic aliases when they beat with the sampling frequency.
Chris(tofire)
christofire
"christofire" <chris...@btinternet.com> wrote in message
news:YdednTxZ3IMLJ2HX...@bt.com...
PS: of course that should have been 'Copicat'.
Phil Allison
"christofire"
> WEM carried on making the 'Copycat' with a tape loop and an AC induction
> motor long after bucket-brigade devices had become available,
** The original valve WEM Copicat was never more than a cheap and cheerful
device for impoverished guitarists - subsequent WEM models were only
marginally better.
Much better preforming but more expensive units existed, like the Klempt and
the famous Binson Ecorec - the latter using a rim driven drum which
eliminated most of the horrible problems experienced with tape loops.
There are still no readily available digital units that produce the same
kinds of sound effects possible with these mechanical units.
.... Phil
christofire
"Phil Allison" <phi...@tpg.com.au> wrote in message
news:7m3ud1F...@mid.individual.net...
I've seen your photo of the (futuristic?) 120 mm drum elsewhere but I've
never had the opportunity to examine one in detail. Was the rim of the drum
coated with oxide in some way like tape, or was it just a steel drum?
Chris
Phil Allison
"christofire"
> "Phil Allison" "christofire"
>>
>>> WEM carried on making the 'Copycat' with a tape loop and an AC induction
>>> motor long after bucket-brigade devices had become available,
>>
>> ** The original valve WEM Copicat was never more than a cheap and
>> cheerful device for impoverished guitarists - subsequent WEM models were
>> only marginally better.
>>
>> Much better preforming but more expensive units existed, like the Klempt
>> and the famous Binson Ecorec - the latter using a rim driven drum which
>> eliminated most of the horrible problems experienced with tape loops.
>>
>> There are still no readily available digital units that produce the same
>> kinds of sound effects possible with these mechanical units.
>>
>
> never had the opportunity to examine one in detail. Was the rim of the
> drum coated with oxide in some way like tape, or was it just a steel drum?
>
** The Binson drum was made up from several parts.
1. A machined aluminium ring of about 10mm square cross section and 120 mm
in diameter.
2. A thin, *continuous* steel band about 7mm wide, shrink fitted onto the
above ring.
3. A pressed steel disk that held a 12 mm steel centre shaft and the above
ring.
The surface speed of the steel band was about 410 mm per second or 16 ips.
.... Phil
christofire
Amazingly complicated, but it did the job so effectively. It's a shame the
Boss RE-20 has feedback from all the delays instead of just the longest one,
otherwise it could have made a good job of the Meazzi.
Chris
ChrisQ
>
> What's a 'Watkins AC30'? The well-known AC30 certainly wasn't one of
> Charlie Watkins's creations.
>
> Chris(a different one)tofire
>
>
Quite right, memory fades. The AC30 was by Vox, from the 60's initially
and later reintroduced by the company in the late 70's. 4 x EL84 in the
output stage, iirc...
Regards,
Chris
ChrisQ
>
>
> ** Done regularly as part of amp testing.
Sure, man, sure and the cheque's in the post, right ?...
Regards,
Chris
Richard Rasker
>
> "Richard Rasker"
>> Phil Allison wrote:
>>
>>> ** Not true, when it is in a later stage of the circuit than the input
>>> stage.
>>
>> OK, but in this case, the resistor is found in series with the grid of
>> the second triode -- and believe me, it's responsible for a lot of noise.
>
>
> ** No way do I "believe" any such assertions.
>
>
>>> ** Not true, in normal use.
>>
>> OK, at normal playing volume, it's
>> hardly noticeable, but during quiet passages, it's rather annoying --
>> especially in a studio environment.
>
> ** Get a noise gate - fool.
OK, I'll get a noise gate allright -- I believe it's called a "killfile",
and I'm testing it with your messages right now. OK, it seems to work
pretty well. Thanks once again for your helpful insights.
Best regards,
Phil Allison
"Richard Rasker is a TROLL "
** Never come back.
.... Phil
Michael A. Terrell
Fred Bartoli wrote:
>
> Jan Panteltje a �crit :
> > On a sunny day (Wed, 11 Nov 2009 22:54:08 +0100) it happened Richard Rasker
> > <spam...@linetec.nl> wrote in
> > <4afb3280$0$729$7ade...@textreader.nntp.internl.net>:
> >
> >
> > And then also explain why the moron used tubes, and not a good opamp and / or semiconductors.
> >
>
There 'are' tube based op amps. Not very common anymore, but the
first op amps were all vacuum tube based.
--
The movie 'Deliverance' isn't a documentary!