On this last pic youíll see a heavy twisted black and white cable. Thereís a good reason for this, but you can improve matter just by tightening down bolts. The transformer is bolted to the main chassis and the main earth (ground) is connected to that. As the heaters have a centre tap to ground (via hum balance ) any problems with the chassis connection between the transformer (thatís the big brown thing on the left of the pic, with wires all over it) is big bad news for hum and background noise. The heavy black/white cable actually straps the main and the sub chasisís together, so the electrical connection of the frames isnít really important. If thatís not done, slacken the screws off to the transformer chasis, and then tighten them down again. It will shift any claggy accumulations and refresh the ground connection.