"NewYorkDave" wrote in
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Hi gang. I'm working on the design of an amplifier to be
used in a tube mixing console. (Please, let's ignore for
the moment the question of WHY someone would want to
design a tube mixing console in 2003!). Anyway, I cooked
up a usable circuit in short order: two triodes in
cascade, with negative feedback from the plate of triode
2 to the cathode of triode 1. I'm using both sections of
a 12AU7. The open-loop gain of the circuit is 123, and
with feedback it's 76. So, there's
4.18dB of feedback, a very modest amount. I didn't like
the idea of using NFB at all at the beginning, but this
application really demands predictable gain that isn't
unduly affected by tube aging or manufacturing spread.

Thing is, I've come across a reference to a study done by
P. J. Baxandall in the '70s, in which he showed that
using less than 15dB or so of feedback can actually
INCREASE the generation of higher-order distortion
products. My distortion analyzer is out of order at the
moment, so I can't look at the distortion products of my
breadboarded circuit. I guess I'm just wondering if
Baxandall's assertion (which apparently arose from
experiments with a FET amplifier) also applies to tubes.
Your comments are very much appreciated.

Why not just use, say, a 12AX7 (with an open-loop gain of
1600 in this particular circuit)? Well, I tried it, but
it clips with a much lower input voltage (0.14VRMS versus
0.67) and has a much lower maximum output swing than the
12AU7 (27 VMRS versus 45).

Perhaps you might try cascading multiple gain stages with global NFB? Maybe
a 12AX7 for the high initial gain and a 'U7 for the output swing?
Just a thought.