1. The 125ESE is not known for extended frequency range; this
is most of your problem. Most transformers will not do very
well with a 10k square wave; consider that it is made of
harmonics (reaching to infinity, in theory) - 10k, 20k,
30k, etc. This TFX has some resonances early on, IIRC.
Years ago when the 125ESE was popular in the 'Darling' amp,
some people measured it; it's a good budget TFX. Don't
expect too much from it, especially with a 10k square wave.
2. You are actually dealing with one of the most complex
situations in a tube amp: feedback compensation. It's a
very involved and complex situation, one could read and study
*many* hours on this. Crowhurst and Bode would be a good start.
A short answer - try different caps and listen. Use sine waves
for measuring frequency response. Don't expect too much from
the square wave response, especially that high (10k).
Would you consider sharing your schematic? It might help
the discussion; this seemingly simple/ very complex subject
could make for a good discussion. Oh, and... have fun!
