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Re: Old design detail on PPP tubes.......

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


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