During practice last night, I noticed a significant loss of power with my YBA-200 (w KT88's). I glanced over at my head and noticed one of the power tubes was glowing about 10 times brighter than the other three. I went back to standby for a few minutes, then flipped it back on. Everything sounded and looked Ok. What happened? I have a gig next weekend and am starting to get a little worried. I let the head warm up for at least 30 minutes last night. I have noticed a popping sound before at home while practicing on another cabinet. Is one of the tubes going bad? They're 2 months old!