Larry is absolutely right about making sure that the 220Ω heater balancing resistors remain grounded. When pin 5 of the output tubes were grounded in the original design, it became a convenient point at some of the tubes to also connect the end of the 220Ω balancing resistors that required grounding as well. Now if the ground lead to pin 5 is broken and a 10Ω cathode resistor is inserted in its place, the cathode no longer grounded, and it becomes a non-inverting input to the tube. If the 220Ω heater balancing resistor is still connected to pin 5 -- now acting as an input to the tube -- then the heater voltage that the other end of the 220Ω resistor is connected to becomes a source; a 60 Hz signal source that is now applied to the non-inverting input of the output tube. The tube then simply takes this signal, amplifies it as it would any other signal applied to any of its inputs, and reproduces it in the speaker as 60 Hz hum. By removing the 220Ω resistor from pin 5 and reconnecting it (or them) directly to ground when cathode resistors are installed, it prevents the heater voltage applied to the balancing resistors from becoming an annoying source of hum in output of the receiver.
I hope this helps!
Dave
I hope this helps!
Dave