Zenith HiFi amp making "angry tweets"

[DavGoodlin]

Originally Posted by Electronic M

Try un reversing what you may have reversed before and see what it does.

Another thought is microphonic tubes...If they like to resonate at that freq and volume is high enough to stimulate them that could be the source of the oscillation.

It was exactly that Tom. As I wrote this thread a week ago at VintageHiFi, I had no idea.
When I wrote it here, it became obvious what I thought I did. There is a 8+lug terminal strip between the two output tubes. This is where I replaced two .022 grid signal coupling caps. the two lugs toward the back of the chassis had both the caps on feeding the opposite tube's grid