I picked up a SD18 head off ebay a couple weeks ago and I think I need new power tubes/rectifier. While I was jamming last night I started get a real nasty crackly sound, with volume drop. I removed everything from my chain and went guitar -> amp, and it was still doing it. What do you guys recommend? I'm probably going to call dougstubes today, and hope I have them by Friday for my weekend gigs.