MarkB wrote:
It did seem better when I put a piece of aluminum foil-covered cardboard over the open top of the chassis. I'm not sure why that would help the squeal. I was working on some buzz, but it seemed to raise the squeal ceiling.
By open top, do you mean circuits and guts exposed, or the top above the tubes and transformers? Maybe you need some additional shielding. Aluminum does a good job of blocking EMF. Copper and steel work well also.
Does the squeal happen when nothing's plugged into it. if not, your guitar's pickups might be magneticly coupling with the transformers of your amp. In which case, the cheap cure is a longer guitar cord, the expensive cure is mu-metal shielding: the first step down a dangerously slippery slope.
I've had the best luck chasing down such things by using a wooden chopstick to nudge and tap components and solder joints while the amp is running and making the offending noise. Rather unscientific, but it works.
Good luck.
-neil