Amp buzz

I purchased a Marshall Avt 50 last December and have never turned it up past half way (on overdrive) and even at very low levels it hums loudly. Ive changed the cords, I got a new guitar (thinking it was the cheap pickups, I have an Epi Les Paul now) and it still buzzes. Does anyone have any suggestions on what might be causing this and some suggestions on how to fix it?

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Thanks for the help lord, im gonna try some of those suggestions and if nothing else works, ill get a noisegate.

Maybe your wall outlet is not grounded, you can try to connect the chassis of your amp (ground) with lets say the central heating pipes in my case the humm stopped.

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