JeffS65,

Today when I played the problem was almost disapeared!! I am pretty sure that the problem is related to the devices plugged in the outlet with my guitar amplifier. There I have a power bar for the computer with computer, monitor, computer speakers, a lamp, etc. all plug in. So I think it can cause so noise that the amplifier amplify... and the power outlet is at 60Hz then it's low frequencies. I remember that when I notice the problem the computer monitor was on. But well you know sometime strange things happen! To conclude when I'll have time I'll try my Randall amp plugged alone.

Thanks for your time dude