Re: Puzzling audio circuitry
It is complicated, but that is largely because it uses pentodes with the complication of a screen-grid supply, suitable decoupled, AC isolated, etc.
If you replace the pentodes with triodes, it simplifies. Then consider removing the upper triode, and have just the lower one. For ease, think of the load as connected to HT+. Then load current flows through the cathode resistor of the (removed) upper valve, creating a voltage drop across it as in the sketch:
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