My '57 CS Champ started make occasional loud snaps and pops . . . but then it went 'pop' and silent. Nothing. No sound at all. But the light stays on, so I'm guessing it's not a fuse. I swapped out each tube one at a time, and with each swap, the same thing would happen: the amp would work, then within a few minutes make a few loud snaps and pops followed by silence. Okay, so it's not a tube. So what do I check next?