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In Reply to: Old design detail on PPP tubes....... posted by Jimmy on February 2, 2005 at 08:35:54:
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
Follow Ups:
Oh, I have built DIY linear amps. I wonder why new designs do not use the inductors.
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.I have plenty of 5965's btw.
regards,
Douglas
Show me the Math...there's no such thing as magic!
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