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One possibility is silver migration making leakage paths inside the IF transformers. Many of these used silver mica capacitors without any insulation. Silver plus moisture plus high voltage gives tiny slivers of silver "growth" across the insulator surface... often giving a "thunderstorm" static among other symptoms. If there's a short from B+ to ground, it's not in the siganl path per se, but of course will couple into audio through power supply... "K-Tran" IF transformers are bad for this.
But noisy resistors are a possibility too...
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