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I've seen some radios use a special output transformer with an extra winding appended to the plate winding. This extra winding was about 15% of the turns of the plate to B+ winding. It's tube plate to B+ and then beyond that the 15% winding. This extra winding they passed the B+ current for the rest of the set. More importantly, the usually 1K resistor usually between the two filter caps is now fed by the 15% winding, and the other end of the 1K resistor has the 2nd filter cap. The idea is to balance the 1K resistor against the plate resistance of the 50C5 (10K) to get the hum ripple of the power supply to cancel, this cancellation happening inside the transformer, and thus little hum getting to the speaker. The far end of the 1K resistor having a cap to ground makes it look like a 1K load with respect to AC voltages.
More directly related to your radio project, you'd have to find a junker radio that used this special kind of transformer.
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