John,
I had similar issues with a stereo 6L6 amp I built many years ago. The
oscillation would come and go, and it eventually killed the tweeters on my old
Utah 12" 3 ways. The following article describes the problem well in a similar
amplifier:
http://www.pmillett.com/file_downloads/chicago_100W_amp.pdf
See R1, and C1, and description on paragraph 1 on page 63.
I know you are experiencing the issue in only one channel. I bet if you swap
transformers, the problem stays in that same channel. Adding the R1, C1 like
the article might completely eliminate the problem.
Here is a more updated circuit designed by Patrick Turner. He uses a transistor
current source in the phase inverter such that he gets perfect balance:
http://www.turneraudio.com.au/Integrated5050.htm
Patrick talks a bit about stability, and ultrasonic oscillations. A good read,
as is his whole website.
Regards,
Jim
WD5JKO
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