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Original Message

RE: I agree - avoid bypasses like the plague

Posted by twystd on July 7, 2015 at 18:04:16:

Andy, filament bias certainly works with the 4P1L, that tube is a natural for it. I'm sure the Coleman reg with filament bias is the hot setup for DHTs. However, implementing that on a tube like a 300B seems to raise some serious heat sink issues, that I don't know how to solve, as I'm pretty much a dolt when it comes to solid state.

I've been getting into some Russian tetrodes, when run in triode mode. They are just as, if not more, linear than DHTs, plus they are indirectly heated, so you don't have to mess with a filament supply.

With a driver tube, like the 5e5p or 6e6p-dr, you can diode or LED bias them, and avoid the bypass cap. With a power stage, like in my 8 watt 6p36s 2 stage integrated SET, that I'm working on now, the only way I can eliminate the cathode bypass cap on the power stage, is to go fixed bias. Unfortunately that necessitates a negative bias supply, but that's still a lot easier than a DHT with a Coleman reg and filament bias.

BTW, from what I've read, I'd sure like to hear your 4P1L preamp. Bet it sounds mighty fine!

twystd