Problem with Single Rec 50 Solo

[meegs]

When I play through my single rec head at rehearsal it sounds fine but after about an hour, the sound fades in and out. Does this sound to you guys like a tube is bad? I don't want to start chasing my tail until I have a better idea. Thanks for the help!

 

[kekam35]

Most likely cause would be worn power tubes. If you swap them out and still have the same problem
try swapping out the phase inverter 12ax7.