Just as the title says.... when I turn up the master volume on my mark IV combo the amplifier does not get any louder. At all. It stays at a fixed volume (not very loud) but still sounds strong. No cutting out etc.. I talked to a tech who said it could be the volume pot but that seems highly unlikely to me. I have another amp from the 40's and that one is doing the same friggin thing now! What? did both master volume pots go out? Hmmmm.... Could this be a tube issue? Any help is appreciated.