What is needed is a three position rotary or toggle switch, two power resistors, two 1 meg resistors, and two large valued capacitors. By selecting the connections between these components, all three output stage configurations can be realized. In the example shown, when the toggle switch is at the middle position, the only effective path the to ground for the cathodes is through the 470 ohm resistors; thus, the circuit is like configuration A. When the toggle switch is at the top position, the two 470 resistors are wired in AC parallel through the capacitors; thus, the circuit is like configuration B. When the toggle switch is at the bottom position, the cathodes connect to ground through the capacitors; thus, the circuit is like configuration C.
                                        // JRB

Push-Pull Cathode Biased Output Stages
  Push-pull amplifiers that use cathode biasing to set the idle current of the output stage can be configured in at least three different ways. One way (A) is to use a separate cathode resistor (not capacitor bypassed) per output tube, which allows for each tube to find its own bias voltage. The second way (B) is to use a separate unbypassed cathode resistor per output tube, which still allows for each tube to find its own bias voltage and allows the cathode degeneration across the cathode resistor both to increase the overload voltage and to lessen the distortion of the output tube, at the cost of decreased sensitivity and a higher output impedance. The third way (C) is to use a single unbypassed cathode resistor for both output tubes, which allows the output stage to function like a differential amplifier and suppress common mode signals.

   Each configuration has its own sound as each has its own distortion signature. No one configuration is best as the listener's taste must be the arbitrator. If fact, one con-figuration may sound great with string quartets, but not with reggae. In such instances, the decision we usually make  is to go with what sounds best in general. What we need the option to choose, to be able to flick a switch to change the sound much in the same way as we can in our car choose the best gear out of five for a given speed and terrain. Such control is possible by switching through these three output stage configurations.

3 output stage configurations
in one amplifier

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