Quote Originally Posted by Moshe
I would swap out the tubes one at a time until the feedback goes away.
Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?