Fisher 500C Right Channel Distortion

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
 
Appreciate the additional comments from you all. Will take all the advice seriously and implement all the mods that are suggested. Might as well get it right in the interest of longevity.
 
Plus it negates the look on the wife's face when she runs up to you and asks if it's normal for a Tube stereo to smoke like a COLD Cummins Diesel!!!
 
Like your sense of humor, Larry. Getting back to tech questions, I'm not finding any 220 ohm resistors on 2 of the outputs connecting pins 5 & 7. Those are the ones I'm supposed to disconnect at pin 5 side of resistor & connect to ground. Any idea what gives?
 
In the 500C, they are located over in the power supply area, so the problem is really associated with the 400, that definitely mounted them on the output tube sockets.

Dave
 
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