I had to plug into the PA last week and it sucked.. Not as bad as you'd think but it still sucked. A bad crackle started and got louder until a few seconds later there were all lights on, but no sound. I thought I fixed the problem and I may have, could be something different but as I was playing at home tonight it started crackling again.. I still had signal though, had the back off as it happened so I tapped tubes, no difference. Then I moved the power tubes in a circular motion and it went away. Is it possible for power tubes to do this? Thank you.