Http://www tomshardware com/forum/48404-6-1953-supro-tube-valco

When you move the volume knob is it accompanied by a crackling sound ?

Volume potentiometer probably needs replacing. You could try using a switch cleaner spray but on something so old it's likely to be more than a dirt problem. Not very expensive for a service guy -- or you could just unsolder the whole volume control unit yourself and replace with one of the same value (that should be printed on it).