My old Ampeg V4B hums like an a old Kelvinator fridge when I turn the dial anywhere past 1/4 of the way up. Both channels. The hum can be diminished somewhat by playing with the boost switches, particularly the bass one, but still, this isn't normal. And it is not just the ground in my house this happened at the practice space too. It actually over head and quit on me once too the last time i I had it on and played for a couple hours.