I also have an at-100 and I notice that I get alot of hum and buzz. I'm not sure if it's from my furnace(the amp in near the furnace in the basement) or other electrical interference. I have read that it's usually the tubes. Some say it's the pre-amp tubes and some say it's the rectifier tube. I have a power codtioner and a noise gate, but I still get a low hum. I think my plug is grounded, so I don't think it's that. I'll have to take my amp to a different part of the house to test that. I'll think I'll start swaping in new tubes as my first trouble shooting step. Oh and bypassing the pre-amp via the effects loop return eliminated a lot of the hum. I'll let you know.