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6CG7 instead of 12AU7?

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Posted on October 6, 2015 at 14:55:34
ChasLieb
Audiophile

Posts: 284
Joined: April 7, 2002
Other than the heater voltage, is the 6cg7 a substitute for the 12au7? The amplification factor, voltages and plate resistances are just about the same and the tube curves are much more linear on the 6cg7. Is the 12AU7 more suited to small signal amplification, i.e. less microphonic?

Just putting the question out there for thought.

Chas.

 

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RE: 6CG7 instead of 12AU7?, posted on October 7, 2015 at 10:18:39
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
Yes, other than the filament issues a 6CG7 can be used where a 12AU7 is used. The 6CG7 IMO is a better tube; its the same as a 6SN7 but in a miniature package (the 12AU7 being the same idea but 1/2 the plate area). Of course the 6SN7 is even better...

 

in most usage where the dissipation is low 6CG7s should sound just like a 6SN7, posted on October 16, 2015 at 15:53:06
Timbo in Oz
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Posts: 23221
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
My old stock AWV and AWA examples have the same elements as their mfr's same year 6SN7s.

The only audio role I would use any 12AU7 (even RCA clear tops) is in follower mode when they are blameless.

That is I would never use any 12AU7 for gain.

It's not really an audio tube anyway, but an RF tube.

The preponderance of 12AU7 rolling posts on the web reflects its non-stellar sound which they all have. ;-)! and ;-).



Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger


 

Much better sound and the pin-outs are different, some resoldering required., posted on October 16, 2015 at 15:56:32
Timbo in Oz
Audiophile

Posts: 23221
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
Use of 12AU7s in audio is not a good idea. Cheap for MFRs is the cause.

? See my earlier post further down this thread.


Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger


 

RE: in most usage where the dissipation is low 6CG7s should sound just like a 6SN7, posted on October 17, 2015 at 16:03:09
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
"It's not really an audio tube anyway, but an RF tube."
Actually read the data sheets from GE, RCA and Tung Sol. The 12AU7A is described as a class A audio tube first, and then as an RF oscillator.

"The preponderance of 12AU7 rolling posts on the web reflects its non-stellar sound which they all have. ;-)! and ;-)."
That's a broad and incorrect statement unless you've heard every amp or preamp that uses this tube. While I agree the 6CG7 is a better tube, there are hundreds of examples of excellent performing preamps and amps employing the 12AU7. It just needs to be run correctly. Some of the poor examples try to run them too lean, and distortion rises rapidly when run like that.


Dan Santoni

 

Sorry no, its designed in distortion numbers are quite poor, posted on October 17, 2015 at 20:19:16
Timbo in Oz
Audiophile

Posts: 23221
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
If you have read the data sheets you ought to have noticed that. Look at the transfer curves?

http://www.audioasylum.com/audio/tubes/messages/26/262921.html

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=264422

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=264409

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=260468

http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=259993

When used as a cathode or concertina phase splitter they are all but blameless, but that's with 100% local negative feedback.

Do a bit of digging and I'm reasonably sure you will find that the 12AU7 does have RF antecedents.

When I first got into audio - as an idea - it was via getting to know recording engineers for our national broadcaster - down here in Aus. - who would often record the cathedral choir I sang in, with just one single-capsule stereo-mike. I was a tween at the time.

Valves ruled back then, and DIY too, and I don't recall anyone telling me how wonderful a 12AU7 was as a gain valve. All who commented said what I am saying.

The 12AU7 is/was used by mfrs because its current draw is low and thus the PT doesn't have to be so expensive. It's strange fame with rollers may also contribute to its ongoing use.

Nevertheless this is a perfectionist hobby, and keeping overall/total NFB down is a pretty good idea, no?

But, using a 12AU7 for gain duties means extra NFB.

My current (version 10 ???) pre-amp still (since v3) uses a 12AU7 at the output, it's currently a selected Brimar 13D5 but it's in CF line-driver role. Good economical choice.

If I ever do build with a balanced output stage I might just double up but add CCS's under each valve.


Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger


 

RE: Sorry no, its designed in distortion numbers are quite poor, posted on October 17, 2015 at 20:41:11
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
Yes the numbers for a 12AU7 are worse than most. However they do have their uses. Low gain stages are one of them. You have fallen into the same trap as your hero Eli D. The 12AU7 is very nonlinear when driven lightly. Ram at least 6-7 ma through and it runs out fairly well. It also works pretty good at low voltages unlike 6CG7 or 6SN7.
Is it a great tube. No, and I never said that. What I took exception to is your generalization.
The links you supplied are all gibberish.
If you want to see how a 12AU7 can be used in a line stage check out
Copland, Conrad-Johnson, Belles, Beard, Cary, and any number of manufacturers. Yes they were once cheap, but so were all the other tubes.


Dan Santoni

 

I don't buy new, I now just upgrade / rethink vintage stuff. , posted on October 17, 2015 at 20:51:31
Timbo in Oz
Audiophile

Posts: 23221
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
Okay?

And I along with many other believe that $$$$$$$$$ audio gear is mostly con-job and conspicuous consumption.

Particularly or especially cables. Closely followed by mega-buck speakers.






Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger


 

RE: Sorry no, its designed in distortion numbers are quite poor, posted on October 19, 2015 at 08:41:33
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
I know it's customary to believe that a 12AU7 used as a cathode follower is benign, because the stage produces no gain, but I recently replaced a 12AU7 with a 12FQ7 (essentially the 12V filament version of a 6CG7) in a vintage Quicksilver preamplifier, and the improvement in sound was easy to detect. One "problem" with the older Quicksilver is that perhaps inadequate current is made available for best performance of either a 12AU7 or a 12FQ7, I admit. I have been thinking about how to pull more current from the PS without stressing it.

 

Never seen a 12AU7 in an RF circuit, posted on October 19, 2015 at 09:22:03
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
-That's a new one on me and I've been in the service industry since 1974.

The primary use of the 12AU7 is actually in audio. The tube is modified from the 6SN7/6CG7. Most of the applications I see are in guitar amps in the driver stage where high gain is not required but current is.

You don't have to run NFB on a 12AU7 either. You just need to run it biased correctly.

 

Cheap?? That's not true at all., posted on October 19, 2015 at 12:45:08
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
They cost the same as most any other tube- 12AX7, 6DJ8, etc.

Your other comments down the thread don't seem to hold much water either.

 

Re:Other than the heater voltage, is the 6cg7 a substitute for the 12au7?, posted on November 12, 2015 at 10:08:28
woffeldjur
Audiophile

Posts: 8
Joined: March 23, 2013



"Other than the heater voltage, is the 6cg7 a substitute for the 12au7?"


And, if you are a bit unsure about the heater voltage, there is always the 8CG7..

 

Almost, posted on November 12, 2015 at 11:11:58
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
The pins on the bottom are different.

All the connections are the same except the filaments- the 6CG7 needs 6.3V between pins 4 and 5, while the 12AU7 needs 12V or you can also run the 12AU7 on 6.3V by tying pins 4 and 5 together and then applying 6.3V between them and pin 9.

The 6CG7 of course can handle more current having twice the plate and cathode area. But otherwise it and the 12AU7 are similar geometry.

 

RE: Almost, posted on November 13, 2015 at 08:42:08
woffeldjur
Audiophile

Posts: 8
Joined: March 23, 2013
Yes, indeed - but are easy to correct.

If your amp is hardwired, just shift the wires.

Or, are the sockets soldered to a printed board, you can cut the print, or the socket on top.

Very easy to work with, no multilayer printed boards, and you dont have to worry about ESD :)

 

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