Marshall valvestate problem

I have a little 10 watt Marshall Valvestate amp which I use for playing at home. When I plug in my guitar, it starts to feedback on each volume level, and you can't here anything of my guitar. So it must come from the inside, not from my guitar. Does anyone know what's wrong with my amp? Could it be dust?

[Edited by Flying V on 08-27-2001 at 10:10 AM]

Your best bet is to take it to a guitar shop that works with amps. It's really too hard to tell what could be wrong with it just by a description.