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In Reply to: RE: in most usage where the dissipation is low 6CG7s should sound just like a 6SN7 posted by Timbo in Oz on October 16, 2015 at 15:53:06
"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.
Edits: 10/27/15Follow Ups:
If you have read the data sheets you ought to have noticed that. Look at the transfer curves?
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.
Skeptical Measurer & Audio Scrounger
-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.
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.
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.
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.
Skeptical Measurer & Audio Scrounger
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