NewYorkDave wrote:

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.

I have seen the very article in Wireless World to which you refer.
What Mr Baxandall said was all too true.
But the thd without FB has to be quite high before you'd worry about
the increase in high order artifacts after applying a modest, ie, less
than 14 dB of NFB.
Where you have only 1% of thd you won't have a problem with thd increase
after
FB application.




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).

The earlier clipping isn't caused by the FB, its due to the
load imposed on the second 1/2 of the 12AX7.
The 12AX7 operates ideally with only 0.7mA of anode current,
and if it clips at 27 vrms, then the load including
the DC supply resistor, external load on the amp, plus the
feedback resistor will total 54k.
This is a low load for 12AX7, and they like loads up around 150k.
But you could have a 12AU7 cathode follower, and take the FB from the
follower cathode to the cathode of the V1, and then you get
an extremely low output resistance, and a large output voltage capability,

and very low thd.

On the other hand, why use any loop FB at all?
triodes like 6CG7/6SN7 are very linear at 1 v output,
which may be all you use in a mixer.
It does depend on the gain you want, and the circuit you propose to
use, and its use.
12AU7 isn't quite as linear, but has a warmer sound, imho,
and your use of moderate, and trimable FB to establish a fixed known gain
shouldn't affect the sound much.

Patrick Turner.