Sound abruptly stopping after 5 minutes.

[Angst]

My crack has been running perfectly for 2 years.  However when I plugged in today, it would amplify fine for about 5 minutes before surrendering to silence.  I changed both tubes and experienced the same problem.  If I jiggle the tubes it will produce limited static before returning to its mute conditions.

Any thoughts about where to start troubleshooting?

[Doc B.]

Flip it over as if you were going to take voltage measurements and see if you can reproduce the problem running it that way. If you can, then take voltage measurements when it is in the failed state. Also note if any LEDs are not lit.