Tube Filaments – Wiring to Reduce Hum
Here is a great tip for DIY tube amp builders. When wiring the filaments of the output tubes in a push-pull power amp always remember to keep the wires from your tightly twisted pair in phase. For example one filament wire from the twisted pair going from pin 2 on the first output tube socket for a 6L6 should also connect to pin 2 of the next output tube socket and so on. The same procedure goes for pin 7 on the sockets as well. Doing this cancels some of the hum at the output transformer resulting in a quieter amp. Using two different colors in the twisted pair makes it easy to observe correct polarity throughout the output tube filament string.