6SN7=6CG7?=?6FQ7
Ian Bell
its B9A direct equivalent. Looking for sources of the 6CG7 on the
internet I found more examples of its supposed equivalent the 6FQ7 than
of the 6CG7 itself. Looking at the data sheets the two seem extremely
similar and the 6FQ7 even has slightly less plate to grid capacitance.
So, is the 6FQ7 a worthy alternative to the 6CG7? Are there any real
differences between the two? Presumably there must be some difference
else why have a different number.
Cheers
Ian
Peter Wieck
I see in all my substitution books that the 6CG7/FQ7/GU7 are
functionally equivalent and may be substituted for each other.
*Purely* speculative SWAG here - it is entirely possible (and based on
other similar tubes) that the original model was developed and
patented for a specific use. Other makers wanted "in" on the
substitution market, or created a similar device requiring that sort
of tube. So, several iterations were developed ultimately combined in
the data-base after the 'separate need' went away.
Or, the original tube was refined over the years by different makers -
therefore they assigned different numbers to it.
On the evolutionary side: A 6GU7 shows a 6FQ7 and a 6EV7 as possible
subs. But *NOT* a 6CG7. 6EV7 reverses to only those two as well. A
6CG7 only goes to 6FQ and 6GU....
Peter Wieck
Melrose Park, PA
John Byrns
Ian Bell <ruffr...@yahoo.com> wrote:
The main difference that I am aware of between the 6CG7 and the 6FQ7 is that the
6CG7 includes a shield between the two sections. Presumably this added shield
is helpful in some types of applications, such as RF and others.
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/
Ian Bell
I had not noticed that but now that you mention it the data sheets I
have show a shield on pin 9 for the 6CG7 but NC for the 6FQ7. I agree
with your supposition that this may have been for rf work. The
ECC88/6dJ8 for example also has this shield and they were typically used
in cascode for VHF front ends. Makes sense to me.
Cheers
Ian
Ian Bell
> On Nov 24, 3:10 pm, Ian Bell <ruffreco...@yahoo.com> wrote:
>> The 6SN7 is said to be a good tube for audio and the 6CG7 is apparently
>> its B9A direct equivalent. Looking for sources of the 6CG7 on the
>> internet I found more examples of its supposed equivalent the 6FQ7 than
>> of the 6CG7 itself. Looking at the data sheets the two seem extremely
>> similar and the 6FQ7 even has slightly less plate to grid capacitance.
>>
>> So, is the 6FQ7 a worthy alternative to the 6CG7? Are there any real
>> differences between the two? Presumably there must be some difference
>> else why have a different number.
>
> I see in all my substitution books that the 6CG7/FQ7/GU7 are
> functionally equivalent and may be substituted for each other.
>
> *Purely* speculative SWAG here - it is entirely possible (and based on
> other similar tubes) that the original model was developed and
> patented for a specific use. Other makers wanted "in" on the
> substitution market, or created a similar device requiring that sort
> of tube. So, several iterations were developed ultimately combined in
> the data-base after the 'separate need' went away.
So, so you happen to know if the 6CG7 preceded the 6FQ7?
>
> Or, the original tube was refined over the years by different makers -
> therefore they assigned different numbers to it.
>
Yes, quite possible.
> On the evolutionary side: A 6GU7 shows a 6FQ7 and a 6EV7 as possible
> subs. But *NOT* a 6CG7. 6EV7 reverses to only those two as well. A
> 6CG7 only goes to 6FQ and 6GU....
>
So, do you have any experience of the 6FQ7 in audio applications?
Cheers
Ian
PhattyMo
I have some tubes that are marked as both types on the glass.
They show 6CG7 with 6FQ7 right under it on the next 'line'.
Ian Bell
Interesting. Who made them?
Cheers
Ian
Ian Bell
>
> I doubt it matters.
>
> The GE plate curves are identical for the two with pin 9 shield the
> only different I see.
>
> The 'normal' plate curves look the same as the 6SN7 but the +ve curves
> run slightly higher... but that may be measurement error.
So, on balance it looks like they are identical. Have you noticed any
differences in real applications?
Cheers
Ian
Iain Churches
"Ian Bell" <ruffr...@yahoo.com> wrote in message
news:gggg8k$182$1...@localhost.localdomain...
6CG7 tubes are available from a number of makers.
I have found Westinghouse to be reasonably priced,
consistent, and good sounding.
Iain
Peter Wieck
> > I see in all my substitution books that the 6CG7/FQ7/GU7 are
> > functionally equivalent and may be substituted for each other.
>
> > *Purely* speculative SWAG here - it is entirely possible (and based on
> > other similar tubes) that the original model was developed and
> > patented for a specific use. Other makers wanted "in" on the
> > substitution market, or created a similar device requiring that sort
> > of tube. So, several iterations were developed ultimately combined in
> > the data-base after the 'separate need' went away.
>
> So, so you happen to know if the 6CG7 preceded the 6FQ7?
From what little evidence I have (vintage RCA tube manual & several
substitution/characteristic books by Sams, RCA & Sylvania) yes. But
not by much.
> > Or, the original tube was refined over the years by different makers -
> > therefore they assigned different numbers to it.
>
> Yes, quite possible.
>
> > On the evolutionary side: A 6GU7 shows a 6FQ7 and a 6EV7 as possible
> > subs. But *NOT* a 6CG7. 6EV7 reverses to only those two as well. A
> > 6CG7 only goes to 6FQ and 6GU....
>
> So, do you have any experience of the 6FQ7 in audio applications?
Same books list it as a "Medium Mu Dual Triode" for "General Use". I
have never seen these tubes in an audio application, but "general use"
means just that.
Ian Bell
Thanks for the info Peter. Looks like I'll just have a to buy a few and
try them out.
Cheers
Ian
tubegarden
Try the ST 6F8-G. Top cap is one Grid, built for specific application.
The tube is nice and plentiful ... the rolling is more fun cheap ...
Happy Ears!
Al
Ian Bell
I just found a web site called appropriately enough www.6SN7.com which
has loads of info on this tube. In the history section it states:
"In 1954, RCA came out with the 6CG7, which was pitched as a direct
equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT
typically sounded better, this was the beginning of the end for
widespread usage of the 6SN7GT. The 6CG7 was unique in having a shield
between the two sections. This was removed as a cost-cutting measure in
the 6FQ7."
It also states that:
"According to Ludwell Sibley's Tube Lore, the 6SN7GT was registered with
the RMA on March 3rd, 1941, with data from RCA and Sylvania. RCA was
likely the developer, since it had assigned a development number of
"A4273B". The immediate predecessor of the 6SN7GT was the 6F8G,
developed by RCA in 1937."
Lots of other interesting stuff about this tube on this site too.
Cheers
Ian
PhattyMo
You could be right,I'm going from memory. I do know that they are marked
with both numbers however.
PhattyMo
> On Tue, 25 Nov 2008 17:42:16 -0800, PhattyMo <phat...@not.net> wrote:
>
>> flipper wrote:
>>> On Mon, 24 Nov 2008 16:20:52 -0800, PhattyMo <phat...@not.net> wrote:
>>>
>>>> Ian Bell wrote:
>>>>> The 6SN7 is said to be a good tube for audio and the 6CG7 is apparently
>>>>> its B9A direct equivalent. Looking for sources of the 6CG7 on the
>>>>> internet I found more examples of its supposed equivalent the 6FQ7 than
>>>>> of the 6CG7 itself. Looking at the data sheets the two seem extremely
>>>>> similar and the 6FQ7 even has slightly less plate to grid capacitance.
>>>>>
>>>>> So, is the 6FQ7 a worthy alternative to the 6CG7? Are there any real
>>>>> differences between the two? Presumably there must be some difference
>>>>> else why have a different number.
>>>>>
>>>>> Cheers
>>>>>
>>>>> Ian
>>>> I have some tubes that are marked as both types on the glass.
>>>> They show 6CG7 with 6FQ7 right under it on the next 'line'.
>>> My double marked tubes have 6FQ7 first.
>>
>> You could be right,I'm going from memory. I do know that they are marked
>> with both numbers however.
>
> Maybe it means no shield.
Now you've got me curious,so I grabbed one.
Marked as:
USA
6FQ7/
6CG7
It does not have a shield. *shrug*
Ian Bell
> that defines the feature set first.
>
> Doesn't mean that's what they were thinking but it fits the few we've
> looked at.
>
According to www.6sn7.com the 6CG7 with the screen came out first and
the 6FQ7 was a later cost reduced version without the screen.
Cheers
Ian
Patrick Turner
The 3 tubes have virtually the same gm, ต, and Ra wherever you find
them.
Many varieties exist, with Siemans NOS probably the best. The CG7/FQ7
have slightly lower Pda ratings than the 6SN7.
I found the Siemans 6CG7 to have a slightly higher ต.
Anode and grid size varies with varieties, but as long as you keep the
relative distances inside the tube about the same the
gm, Ra and ต stay about constant.
Patrick Turner.
Ian Bell
I simulated a 6sn7 in mu follower circuit and it gave very good
distortion figures - best I've seen yet - this was at 8mA Ia. So I
ordered some from Colomor valves in the UK so I can try them out. They
are the 6FQ7 version made by Matsushita - didn't know they made tubes.
Does Matsushita have a good or bad reputation for its tubes??
Cheers
Ian
Patrick Turner
Japanese. I doubt the japs made worse sounding tubes than anyone else.
Its not going to cost you much to find out for yourself.
The secret to the ต-follower is that you have a large enough R between
top cathode and bottom anode.
If the Ia = 8mA, and that would be only good if output voltage swing is
smallish, then the Ea can be say 100V for top and bottom tube
and if the B+ = 300V, then you can have 100V across the R between the
two tubes, so R = 12k.
If the top tube internal gain is 18, then the bottom tube has an anode
load = 18 x 12k, plus any biasing R.
Its still only 216k effectively, or about 25 x Ra. But you should get
less than 0.1% THD at 10Vrms output,
which means less than 0.01% at 1V output, which isn't much, eh.
Patrick Turner.
>
> Cheers
>
> Ian
Ian Bell
The design I simulated is very similar to that but with a 285V supply
and R = 10K resistor and 100V across each triode. I did not want to up
the supply much beyond 300V because at present the top cathode sits at
over 180V so I'll need to raise the heaters to about 100V because the
max dc heater positive to cathode voltage is only 100V according to the
data sheet.
From what I can gather there are three factors that govern the
distortion in mu followers. The first is anode current - higher anode
currents give lower measured distortion. The second, as you say is the
effective Rp determined by the top tube mu and the resistor between.
This essentially reduces the effect of varying ra on distortion. The
last factor is the tube type and from the tests I have done so far this
seems to make the largest difference. You can try almost any of the
regular modest mu double triodes (6DJ6, 12AU7, 6N1P) and you will get
between 0.5% and 1% THD at 20V rms and about a tenth that at 2V rms.
However, use a 6SN7 (or one of its derivatives) at the same current and
the 20V rms THD drops to little more than 0.1%.
I was curious why this should be and then I came across an interesting
web page about the internal construction of tubes and its effect on
linearity. You can see for yourself here:
http://myweb.tiscali.co.uk/g8hqp/audio/valvedist.html
I have no idea if what he says is true but it would explain why the 6SN7
is so well liked in audio circles.
Cheers
Ian
> Patrick Turner.
>
>> Cheers
>>
>> Ian
Patrick Turner
The idea is to have the heaters biased at Vdc about 1/2 way between the
two cathodes.
>
> From what I can gather there are three factors that govern the
> distortion in mu followers. The first is anode current - higher anode
> currents give lower measured distortion.
This is true with resistance loading also. But the higher Ia, and lower
the Ea, the lower the range of Vo because of grid current.
> The second, as you say is the
> effective Rp determined by the top tube mu and the resistor between.
> This essentially reduces the effect of varying ra on distortion.
The higher the triode RL becomes, the flatter the load line becomes when
you plot it across the Ra curves.
The Vo swings don't intersect Ra which changes value. If you have an RL
> 20Ra, then for 6SN7 that'd mean about 200k,
and if the maximum Vo = +/- 33pk then load current change is only +/-
0.16mA if the 200k is the only loading present.
So you can have the Ia lower than with a resistance load and still get a
linear outcome. Put it this way, if the load approaches a CCS, it
becomes pointless to have 6SN7 Ia higher than 4 mA providing 4mA is
adequate for the wanted Vo into whatever other load you may have, or
Miller C.
> The
> last factor is the tube type and from the tests I have done so far this
> seems to make the largest difference. You can try almost any of the
> regular modest mu double triodes (6DJ6, 12AU7, 6N1P) and you will get
> between 0.5% and 1% THD at 20V rms and about a tenth that at 2V rms.
> However, use a 6SN7 (or one of its derivatives) at the same current and
> the 20V rms THD drops to little more than 0.1%.
>
> I was curious why this should be and then I came across an interesting
> web page about the internal construction of tubes and its effect on
> linearity. You can see for yourself here:
>
> http://myweb.tiscali.co.uk/g8hqp/audio/valvedist.html
>
> I have no idea if what he says is true but it would explain why the 6SN7
> is so well liked in audio circles.
There are many varieties of 6SN7 and the ECCxx range of double triodes.
ECC32 is a particularly linear twin triode and the 6SN7 is of the same
evolutionary "family".
Indeed the 6SN7 is a more linear tube than 12AU7, or 12AT7.
The 6SN7 often had tubular shaped anodes, grids and cathodes, and to my
mind is a more perfect way to make a tube.
But take a look at the range of 6CG7 around. Many had the 25mm tall box
section anodes of the 6SN7 crammed into the smaller bottle.
Damned linear though. Many 6CG7 had overall smaller anode/grid/cathode
structures, but the internal part of the anode was a square section.
The Siemens 6CG7 are like that and are regarded as the gold standard in
twin triodes. But then take a look at a 300B. OK, its a power tube, but
its amoung the most linear of all.
What I do know is that any variety of 6CG7 and many other medium ต twin
triodes are amoung the most linear devices ever invented and which don't
need to depend on external loops of NFB to reduce the distortion.
The lowest possible THD in a triode occurs where you have a CCS anode
load, and zero Ia change when Va changes.
The only way to exploit this fully is to have a CCS loaded gain triode
direct coupled to a cathode follower buffer.
Then you have a CCS cathode load for the CF, and the only load might be
a 100k gain pot. The majority of the THD generation is then in the CF
but its reduced by the series voltage NFB inherent in the follower
connection; ie, if you had 0.1% THD at 10Vrms output with a load = 100k,
then the follower connection with 6SN7 would reduce that 0.1% by 1/18
approx, or down to 0.0055%.
Even with a horizontal load line, ie CCS load only on the 6SN7, the
spacing of the Ra lines for various Eg1 bias values does show that
you'll get some THD; the spacing is *not* even between Vg1=0 and Vg1=-5,
when the Ea change would have a range of +/-50Vpk.
Often when you test a given triode carefully by measuring the THD you
will conclude that the people who drew the Ra lines for a given tube
were obviously using gear with distortion in the test gear which was of
course all tube based and not necessarily more linear than the tubes
being measured. The curves drawn so impressively in the best data books
such as those produced by STC back in the 1950s are a cause for me to
smile, because the nice large pages so well bound and produced are full
of what are only approximate Ra curves, and curves which look simply
impossible, or implausible, because of their irregular spacing that if
true would produce many more artifacts than the usual bit of mainly 2H
with all the other harmonic garbage -20dB below the 2H.
So hence the breadboard test with accurate measuring gear using test
signals with THD < 0.002% is essential if you want to bathe your mind
with the purity of the truth on vacuum tubes.
You cannot simulate the taste of a fine chocolate cake.
So don't depend too heavily on a simulator; its only a guesser's tool.
Patrick Turner.
Ian Bell
I won't argue with that!
The Matsushita 6cg7 tubes have arrived and I have tested them in the mu
follower circuit at 8mA. The results were disappointing. All four gave
about 0.7% THD at 20V rms and 0.07% at 2V rms which is no better than I
got with an ECC88 or a 6N1P at the same 8mA current.
Looking at Morgan Jones test circuit I notice he uses two LEDs in the
bottom cathode leg for biassing which sets Vg at about -3.4V and from
the curves means the plate voltage is about 140V. He uses a 10K
intermediate resistor as I do but his pentode CCS has a 390V supply. My
circuit uses a cathode resistor to set Vg to about -1.34V which gives Ea
about 117V (measured) and I have a 320V supply. So I thought I might try
moving the operating point closer to Jones's, because the curves look
less bunched in that region but if I operate both triodes at the same
point they have 140V each across them plus 80V across the 10K meaning I
need a 360V supply which I don't have. I must admit I assumed both
triode halves should be operated at the same point but I am not sure
that is necessary assuming the bottom triode is the principal source of
distortion. So an alternative would be to run the top CF at the same
current but a Vg of -1.2V so it drops just 100V which would fit with my
320V supply. It's an easy mod to make so I think I'll try it.
You also mentioned the 6SN7 has 25mm boxed anodes and some 6CG7s cram
these into a 9 pin bottle. The Matsushita ones I have measure about 19mm
so there is clearly a difference there but the ECC88 and 6N1P I have
measure only 9 and 10mm respectively so the 6CG7 are twice as tall and
the box is definitely fatter than the ECC88 etal.
Cheers
Ian
> Patrick Turner.
>
>
>
>> Cheers
>>
>> Ian
>>> Patrick Turner.
>>>
>>>> Cheers
>>>>
>>>> Ian
Ian Bell
I have now made the mods described above and retested the four
Matsushita 6CG7 tubes. I am pleased to report they all now achieve 0.4%
THD at 20V rms. Bearing in mind that Morgan Jones' 6SN7 tests had a 2nd
harmonic at -50dB when tested at 19.5V rms which is equivalent to 0.316%
and he was using a pentode CCS then I think 0.4% from the 6CG7 mu
follower is pretty good. Distortion at 2V rms was 0.04% (all at 1KHz).
Cheers
Ian
Patrick Turner
Ian Bell wrote:
>
> Patrick Turner wrote:
> >
> > Ian Bell wrote:
> >> Patrick Turner wrote:
> >>> Ian Bell wrote:
> >>>> Patrick Turner wrote:
> >>>>> Ian Bell wrote:
snip
Nearly all small triodes produce 1% at 10V into RL = 4Ra approx.
So at 50 Vo, maybe you get 5% 2H at onset of clipping, and with
considerable 3H.
But at 1V, you get 0.1%, and very much mostly 2H.
You are getting 0.35% at 10V and 0.035% at 1V, a 9dB betterment from
what was regarded as normal industry standard.
Try using a CCS at the gain anode and have a direct CF to buffer the
output to your measuring gear.
This will tell just how low THD can ever go.
I've seen 1% at 100Vrms from a single 6550 as a triode with CCS.
Once over 5mA, and RL > 10Ra, extra Ia should not make much difference.
Then there are balanced circuits. If you really want to see 2H plunge,
try a balanced LTP with CCS cathode sink driving a CF buffers hooked to
an OPT.
The THD at 10V might be 0.02%, but 3H, not 2H.
Patrick Turner.
Engineer
> Peter Wieck wrote:
> > On Nov 24, 6:40 pm, Ian Bell <ruffreco...@yahoo.com> wrote:
(Snip
> I just found a web site called appropriately enoughwww.6SN7.comwhich
> has loads of info on this tube. In the history section it states:
(snip)
> Cheers
>
> Ian
Ian, a great story... thanks, bookmarked. Glad I've got a small
boxful... I must now sort them into variants! <g>
Cheers,
Roger
Ian Bell
Are you in the UK? I would like to buy a couple of genuine 6SN7 tubes so
I can compare their performance with the 6CG7.
Cheers
Ian
bigwig
>
> - Show quoted text -
Ian,
I am in the UK, I have some Sylvania 7N7s that are for all intents
and purposes 6SN7s. I only have six and I am using four in my curent
project so I would like them back but you can borrow them if you like.
I have sockets aswell if you need them. The only difference I see in
the data sheets is the disipation is the same as the basic 6SN7 2.5W
(3.5W?) and the capacitances are way lower. I say 3.5W because a
couple of sheets say max anode dis. per envelope as 7-7.5W and the
anodes are a bit large for just 2.5W, I may be wrong.
I also have quite a few of the Pinnacle 6J5s that Allan Morgan
likes. These are clearly re-branded Russian tubes but as Allan clearly
demonstrates they measure very well. They are eactly the same as half
a 6SN7.
Matt.
Ian Bell
That's a really generous offer, Matt. Email me at
ianbellATukfsnDOTcoDOTuk so we can sort out details. No rush as it is
nearly Christmas and anyway my trusty Ferrograph RTS2 has just died -
again - now needs a new FET in the millivoltmeter circuit.
Cheers
Ian
bigwig
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
OK Ian,
No probs I will mail you tomorrow, getting late and have to be the
other side of Oxford by 8:30am in the morning.
Matt.
bigwig
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
Sorry, what is the FET in your Ferrograph I may have some?.
Ian Bell
It's a BF256S. Can't seem to find them anywhere. I discovered the BF254C
is very similar so I ordered a few from Maplin.
Cheers
Ian
bigwig
Ian,
I have tried to emil you but it was sent back I guess I didnt decode
your address properly. I cant see how it looks alright to me. Maybe it
was sent back by UKFSN, it says delay in the message title I got back.
Matt
Peter Wieck
>
> - Show quoted text -
Sheesh, guy!
How many do you need? I have at least a dozen-or-more and would be
glad to drop a few in a box for you, gratis. Even shipping to your
side of the pond can't be all that much.
I am not going to take the trouble of matching dual-triodes for you,
but I can mark their GM on each side & plate currents for you at least
as based on a Hickok tester.
Sorry it took me three days to notice this.
Petrer Wieck
Melrose Park, PA
Ian Bell
No, it's my fault, I got my owm email address wrong!! It should be:
ianbellATukfsnDOTorg
Don't know what made me put.co.uk
Cheers
Ian
Ian Bell
Wow, there are times when this is an incredibly friendly group. I am
overwhelmed by your generosity. Yes please, just a couple are all I
need. Matching is not an issue, I want to compare their performance in a
mu follower with that achieved by the 6CG7 tubes I have.
Drop me an email at ianbellATukfsnDOTorg and I'll send you my snail mail
address.
If you have a paypal account I would be happy to cover shipping costs.
Once again, many thanks for your generosity.
Cheers
Ian