Long story/question ahead: I recently had some issues with my Peavey 5150 where the volume would sometimes severely drop out. It was an intermittent problem -- sometimes it wouldn't happen at all and the amp would be normal. Other times, the amp's volume would drop significantly after just 5 minutes of playing. I decided to swap the power tubes to see if that was the issue, but doing so had no affect on the problem. I was able to replicate the issue despite switching to new power tubes. So, I decided to take the amp to an amp tech here in town. The guy looked at the amp, did some cleaning/"preventative care/maintenance," and then told me that my issue was caused by my power tubes (which I knew wasn't accurate). I told him that couldn't be the problem, but he insisted and said my old chinese tubes are bad, and probably overheating, causing them to drop in volume. He said the 5150 "prefers cold tubes," and that he tried some Mesa 6L6 power tubes, and had no problems with the amp. His explanation didn't make sense to me, since I had swapped the old chinese power tubes with new Sovteks, and still had the issue. Anyway, I figured either his cleaning fixed the amp, or he simply got lucky and didn't hear the intermittent issue just by chance once he swapped to the Mesas. Either way, he wouldn't "warranty" his repair unless I bought new power tubes from him or brought in my Sovteks and TADs for him to check in my amp and make sure they were suitably biased. So, I gave him my power tubes, and he tells me that both sets bias "too cold" for my amp (17ma for the TADs, and even less for the Sovteks), and now he claims the 5150 likes/needs 30ma bias to work correctly (even though it's a fixed bias amp and I liked how it sounded before). Plus, all of a sudden he is now agreeing with me that it was probably the cleaning he did that fixed my amp. Anyway, he told me I should do some research and see if I can find some tubes that will naturally bias around 30ma in a 5150. He then added a resistor to my amp so that the TADs would bias closer to 30ma (a mod I didn't authorize, and I will ask him to remove that resistor and leave my amp stock). Does the stuff this guy is saying even make sense? Everything I've ever read/heard is that 5150s are fixed bias very cold.