Thread: mu feedback
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Ian Iveson Ian Iveson is offline
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Default mu feedback

Alex wrote:

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?



Consider putting a triode in the feedback path. That would
tend to cancel out the loose input triode (or g1,g2, k combo
of the pentode).

This idea probably has a name, and a treatise to match,
which may in turn answer your questions.

Ian