I recently got myself a Marshall Valvestate 8100 head, which sounds amazing for a hybrid amplifier. Now there is one problem with the amp, and that is that there is a low buzzing sound when I turn it on. It does not get louder when I turn up the volume or gain, and it doesn't disappear when I switch channels, which means that it can't be the pre amp tube that causes the problem. I don't use a grounded power socket, but my other amp (an Orange Tiny Terror) doesn't hum so I don't think there is a problem power wise. My guess is a bad soldering joint somewhere in the amp, but I have no idea where to start looking. Is there someone who can tell me how to get rid of the buzzing? And how do I identify a bad soldering joint?