Thread: mu feedback
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Alex Alex is offline
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Default mu feedback


"flipper" wrote in message
...
Funny you should mention it because I'm playing around with some dry
battery tubes and considered doing just that: screen feedback. I ended
up not including the pentode preamp in the loop, though.

Basically a toy at this stage but kinda cute, I think anyway.

http://flipperhome.dyndns.org/Batman.htm


Nice crcuit. I would remove (short out) redundant R11 (and reduce R10 to
maintain the overall gain). This would reduce AF voltage on the first
pentode 1N5GT plate.

Also if you are concerned about a phase shift (mismatch) in the fhase
splitter at high frequencies, why not throw a small (trimmer) cap, probably
1...3pF, across R1 to cancel Miller effect in V2AA?


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"?


I can't help you there as I didn't breadboard it and, so, made no
measurements but I find it hard to believe it would be as good as grid
or cathode resistive feedback because you have, essentially, an
'active element' in the feedback path coloring the feedback signal.

I'm just guessing, though, and suppose serendipity could make it
coincidentally 'just right'.

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?


You might try looking for some real old types. I seem to remember some
down around 9 or 10 but they weren't duals.

Out of curiosity, why are you looking for practically no gain?


Just to be able to have lower minimum gain. One can always increase the gain
by placing a divider in the feedback path, but it is impossible to get the
gain below the mu value.

Alex