[Alex]

Restoring an old 1953 radio I came across a funny way of applying negative
feedback in an audio 2 stage amplifier (EBF80 + EL80). First stage was using
a EBF80 pentode. Input signal was applied to the control grid, while the
feedback from the speaker went to the screen grid via a 0.1uF capacitor of
course.

Thus the gain of the amplifier was determined by the internal mu (mu g1-g2)
of the EBF80, which is in this case is about 18.

I am wondering what order of THD one can expect from such mu based feedback
"divider"? Also the only way to reduce gain is to find a pentode with a
lower internal mu. I am wondering if a pantode exists with the internal mu
of say 10?

I tried to expand on the mu feedback concept. What about arranging a cascode
(series connection) of two triodes and apply input signal to grid of the
bottom one, and the feedback -- to the grid of the top one? Then the mu of
the bottom one will solely determine the gain of the amp. Advantage of this
topology -- very high impedance on the feedback input (grid of the top
triode). It is convenient to put a high impedance tone control circuit in
the feedback path.

I tried to find any low mu low power dual triodes. The lowest mu i found was
from 12AU7 (17), closely followed by 6CG7 (20). Again what order of THD can
be expected from such a trick?

Regards,
Alex