Get lower hum?
• Replace the defective humming tube • Replace or improve the power supply filter capacitors • Fix the defective internal ground wiring, as on a reverb tank, or previous “improvements” and modifications • Run the preamp filaments on regulated DC, not AC, starting with the input tubes • Rewire the grounding so the amp is star grounded, and does not use the chassis as a ground bus • Move the signal wires around, nearer/farther from the chassis or 60 Hz AC carrying wires • Use coax cable in the signal path, at least in the early sections where noise counts the most. Tie one end of the shield to ground and terminate the other end with some shrink tube so it cannot touch anything. This way the coax shield acts as an antenna and conducts the RF to ground (as well as Faraday shielding hum out). If you tie both ends to ground you set up some capacitance (and the possibility of ground loops) you’re better off without. The shield should be tied to the star ground point individually, and bypassed
