I'm not sure that it is defective, let me refrase my previous post. if i plug the amp into the outlet closest to my equipment the hum begins as soon as the amp is switched on. if the amp is plugged into another outlet on a differant wall the hum begins after the amp is running for 20-30 min's. i will try rotating the plugs a little later today. thanks for all the responses, i owe every one a beer.