Major diff between 12AX7 and 12AU7

[24kilo]

There both Preamp and I know the 12ax7 has a little more gain but what eles is there. I see some amps use one or the other and some amp use both. Can someone enilghten me?

[Giaime]

They're very very different tubes, with different purposes in mind. Only the pinout is identical, heater is the same and the power rating is similar. But mu, rp, gm, noise figure, distortion, microphonics... all different.

(the difference is that ECC83 sounds good where is needed, ECC82 cannot sound good because it's not an audio tube )

[cogsncogs]

Yup, A 12AX7 is a high mu/gain (100), high rp-low gm tube used as an AF low-level/hi-gain twin triode.
A 12AU7 on the other hand is a medium mu (17), medium everthing, designed for vertical oscillator/vertical amplifier in B&W Television sets! Not considered good for quality audio by many because of not-so good linearity when used at the operating points used for AF voltage amplifiers. Seen many times used as a concerta phase splitters in power amps tho and cathode follower circuits because of the resulting low(er) output impedance when used in this manner. And because of the indentical filament requirements as the 12AX7 and permissable maximum voltage specs. A 12BH7 would've been a little better!
With different bases and filament requirements the 6FQ7/6CG7 or the octal 6SN7 are better alternatives w/approx. the same gain.
Maybe too much info but you asked for it!

Wayne

[kathodyne]

i've heard it's a common misconception that an ecc82 would not sound good....

the curves don't look bad......

* kathodyne hides behind an old unused chassis *

[cogsncogs]

Speaking of Kathodynes, this led to the advent of the 7247. Section 1 (6-7-8) being a 12AX7, section 2 (1-2-3) a 12AU7. Too bad we don't have a 12ax->12at(v,z)7 like a 7248 or sumthin'!
Any takers?

Wayne

[ray_moth]

Too bad we don't have a 12ax->12at(v,z)7 like a 7248 or sumthin'!

Good idea. Or how about a 1/2 6SL7 + 1/2 6SN7 to do the same job?

[cogsncogs]

Good one! Wait! I think I might have a couple of those lying around! Where did those come from?

Wayne

[lndm]

There appears to be a cult following around the 12au7. I have noticed some asking good money for some select 12au7's. They seem to be known for being a little coloured but it is pronounced 'rich mids'

I have tried them, they seem to work a little better at more than 8mA or so. They are capable of giving a fine performance if you happen to like the flavour (and if you get a good one).

[lndm]

Norman Crowhurst used them both in an amp from Audiocraft of 1956.

12ax7's as gain stages, 12au7's as drivers.

[Brian Beck]

In his excellent book “Valve Amplifiers”, Third Edition, Morgan Jones presents a useful analysis of the distortion spectra of several tube types using a common test bed. The big winner was the good old 6SN7, not surprisingly. One the worst of the common tube types was the 12AU7, which he labeled “particularly ghastly”. It produced significantly more odd and even distortion that many of the other tube types. Of course he didn’t test every brand and style of 12AU7 / ECC82, nor every possible bias / operating point, but the trend is clear among the 28 samples that he did test. IMO, there appears to be crowding of the plate curves in the high voltage region which would explain this problem.