This is the first time I've had issues with a tube amp, so I'm not quite sure how to go about diagnosing or fixing this issue. After playing for about 20 minutes, the amp suddenly dropped in volume and makes a lot of crackling noises, but is silent when a cable isn't plugged in. I tried a few different cables and guitars but it didn't change anything.

Please suggest some ways I can try to fix this issue. I have no idea what to do, nor could I take it to a technician any time soon, so any advice would be appreciated.

EDIT: The emulated output works and sounds fine, though.
Last edited by Fatlumpthe1st at Sep 3, 2016,
Bad tube, dirty tube socket, or bad solder joint at a tube sockets.

Buy a new set of tubes for it.