Is this a tube issue? Or is there another component that would cause this? I bought the amp with the caveat that after a few minutes playing it would die. The seller said it had something to do with the volume. I bought the amp, used contact cleaner (as best I could; the pot is rather well sealed), then tested for a few minutes with no problems. I thought I'd fixed it. Then I sold the amp a week ago, never having really used it. The buyer just called and described the same symptoms the the person who sold it to me described. I'm now offering my buyer his money back, but need to fix this issue in order to sell the thing. Any ideas? Bad tube? Do bad tubes cause problems like this? It's an on and off thing in that it will power up and play, but will, consistently, apparently, die after a few minutes use each time. Capacitor? Diode? (I'm grasping at straws here, I know). Thanks.