Hi, I have a McGregor Bassman 200W 4 Ohm solid state head. I bought it 2nd hand today and it had really scratchy pots, they're all contact sprayed clean and they sound lovely. However I'm picking up French radio still. It increases with the volume and is more pronounced when a cable is plugged into the amp. How do I stop it? I'm willing to tinker a bit (then I'll fix the amp. Sorry.).