I don't know an awful lot about dirty power, or voltage regulators, or even power conditioners, but last night at a gig I think I came face to face with some pretty gnarly power... and it sucked. Actually, I survived the gig completely intact, but my band got nearly wiped out. My lead guitarist has a relatively new Valvetech Hayseed 15 (really nice sounding boutique AC-15) and as soon as he powered up for a sound check it was bad news. Squealing noises shot through the amp. He immediately unplugged his guitar and pedalboard, but the noises continued... it'd basically wail like a tortured moo-cow, so he powered down. Tried it again, same. A few times he'd power up and it'd be fine for a whole minute, but then the wailing would come back. At first we assumed one of his tubes must have blown. And then... our keyboarist attempts his own soundcheck. We had minimal PA support, so he was running through a very well-maintained (and TUBE-FREE) Roland JC-120. About 30 seconds in, his volume starts to cut drastically. We were able to borrow another band's Mesa combo for my lead guitarist, and we borrowed another band's bass amp for the keyboards, but it resulted in us cutting a good 15 minutes out of our 35 minute set. Miraculously, my Ashdown ABM 500 evo II survived completely unscathed, as did our lead singer's Peavey Classic 50. Does that sound like we were dealing with dirty power or is there something else we should consider? Are some amps more susceptible to fluctuations in voltage than others? And how do you deal with it, short of dropping a lot of cash on a voltage regulator?