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What difference would 12AX7s make? I have a bunch of those laying around.
Suck bad. Overload on a whisper. This ISN'T a straight amplifier: many things that are "good" in plain amplifiers are BAD for vari-gain operation.
12AU7 isn't expensive; that's why I picked it. It isn't an ideal tube for the chore, but is VERY available, both under that number, the older 6SN7, military numbers, and in larger derivatives like 12BH7.
If you must experiment: 6BQ7/6BC8/6BS8 is similar but -maybe- better, being intended for vari-gain duty in TV tuners. It isn't as dirt-common as 12AU7, but hardly rare, and 6BC8 is not at all fashionable. $5 at thetubestore.com or tubedepot.com The voltages are similar enough to 12AU7 that it should be possible to get it to work without radical change. And it may give less grunge in deep GR.
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the +140V supply to the tube output stage
Sorry. PS plan modified. Don't worry if it isn't exactly 140V, it isn't fussy.
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My 12AU7 Limiter page ***
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bias pot as a front panel control provide any interesting sound effect
If you mean the balance pot between the cathodes: offsetting it will give a little even-order distortion and a LOT of thump. Unless you want thump (and it is usually a very bad sound), set R32 for minimum thump when going in and out of limiting, then leave it alone (unless you change the tube).
If you mean the trimmer near the output: that really needs to be set so with heavy gain reduction the output level is 2V RMS (2.8V peak). What you are really doing is setting the signal level in the 12AU7 plate circuit: too high will distort bad, and too low puts you in the noise. It does not have to be 2VRMS exact, but within say 3dB or 1.4V-2.8V RMS. And for Stereo, equal both channels.