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I noticed organ amps from the '40s & early '50s using plate inductors on parallel PP tubes. Both Hammond Jensen & Baldwin use a plate inductor of 1/4" diameter x 3/4" long with 20 turns on just one parallel tube plate on each side.
They are there to supress parasitic oscillations in the VHF/UHF range. The tubes have plenty of gain there, and the connecting wires have a resonant frequency in that range... so a parallel audio circuit becomes a push-pull UHF oscillator! Sometime a low value resistor is enough, sometimes it's a combination, often a choke wound on a 47 Ohm resistor.
I'm currently experimenting with a pair of old organ amps like you describe. Thw are Webster, made for Hammond organs, with 4-2A3 in PPP. They do have the plate chokes you describe. I assume they are there to stop HF (and RF)oscillations. I haven't measured the inductance of the chokes, but because they are air core, inductance must be very low. These would have no effect at audio frequencies, but would block ultrasonic and RF frequencies. I've also seen RF linear amps using a similar arrangement, sometimes 10-20 turns wound around a 47ohm/2W resistor. I've been told these block higher harmonics, out of the intended operating frequency range. Hope this helps! Al
Perhaps if all linear amps had them, we would experience less interference in out sound systems, TV's and telephones from renegade CB operators.
McIntosh used a 2,2 uHy choke on the anodes of the 75 and 275. I thinnk the rest got something similar. My Baldwin PPP 6L6G amps all had inductors. Looked like they wre ~5/8" long and wound on a pencil-sized spool. Probably less than 15 uHy.
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