Buzzing all-tube amp... How to make it stop?
I damaged my Peavey Valveking 112 this morning when I plugged my Boss RC-20XL into the effects loop while the amp was turned fully on (not on standby). I plugged the last cable of the loop into the "Return" plug of the amp's effects loop, and as I did so, there was a LOUD gunshot-like crack/pop. The amp was set at a bedroom volume, but this crack/pop sound was much louder than the volume the amp was set to. I'm sure this was not good for the amp, and I already knew that I should always insert/remove patch cords (from inputs / effects loops) when the amp turned off or at least on standby, but I guess I was in an impatient mood (been a bit grumpy about my RG350's relatively thin sound lately). Shortly thereafter, there was a constant passive buzzing noise from the amp. Plugging in / removing the guitar and the effects loop doesn't help, the amp is buzzing even if nothing is plugged in.
Later, I was dumb enough to repeat the mistake - I flicked the amp on standby but didn't wait long enough for the tubes to fully go out, and I yanked the cable from the "Return" plug in the effects loop, yielding another loud gunshot/crack/pop sound. Worried that I may have further damaged the amp, I removed every cable from every jack in the amp, flicked the standby switch back on, and, yep, the buzzing was there, stronger than before.
Now, question is - what kind of damage have I wreaked upon my amp to cause the buzzing? I'd expect it's a circuitry / tube issue, maybe a ground wire gone loose? Advice please. I know it's an amp issue, not a guitar or stomp box problem...