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Using the UTC LS-21

Just found this post--hope you're still watching, Kent.

Anyway, the way to use the LS21 is parafeed, which is what '0 ma DC' implies it was designed for. That is, you don't run the plate current of the driver tube through the primary of the transformer, you run it through a resistor (like any normal plate-load resistor) and block the DC from the primary with a coupling capacitor. That way, no DC gets into the transformer primary. There are many advantages to doing it this way. It's explained in some detail in the Radiotron Designer's Handbook. If you don't have one, most libraries do, and it's a good read.

For examples of the circuit, see the schematic for the Western Electric 86C (should be able to find it online). Or check the link below for Jeff's simpler but similar approach. The schematic in the link has a lot of notes on it, and Jeff favors multiple capacitors where one will do, but it's really a very simple circuit, and should illustrate how parafeed is done.

I've been up to my ass in alligators for a couple of years now (i.e. really, really busy), so, no, I haven't finished the PP6A5G amps yet. I think I'll dig them out and try and work on them some, though. Seems like a good way to blow a long weekend. :-)





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