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In Reply to: RE: Small IDHT tube cathode bypassing posted by twystd on July 06, 2015 at 14:14:19
I spent a whole day at one point with a huge stack of capacitors, sitting on the carpet linking them up in various combinations. All possible combinations sounded worse than none, even though there was clearly an impedance mismatch which was far from ideal. At that point I gave up.
Soon after I discovered filament bias with the 26 tube I was using, and life just got better and better from that point, which is now quite a few years ago just after Thomas Meyer started telling us about his filament bias experiments. I built a 26 preamp with a LL1660 and filament bias, then the Coleman regs came out in their first version and it got better still, then several updates of the Coleman regs came out and here we are today. I now use the 4P1L in place of the 26.
Going back to cathode bypasses for me would be like living in a mud hut.
Follow Ups:
Andy,
From my experiences, and a fairly good understanding of this subject, you made at least three or four mistakes in this experimenting you reported above to us.
(1) One day is nothing !!!
(2) Do you know, have a good handle, on what values play what part of the music's spectrum? Likely you don't know what values to use.
(3) Only the very best film capacitors can possibly work on Rk Bypassing.
For example, Mundorf Silver and Oil would be a good minimum single brand suggestion.
Q : Were any and all of the film caps you used equal or better than that?
I would guess not.
It takes lots of listening, thought, insight, and money to do this right.
The above fact, (and the three points I outline above ), is where most DIYers will certainly fail.
Dennis Fraker is the only person "I" know of in tube audio, who knows how to do this really well, and since about 1989 BTW !!
Jeff Medwin
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
I always wanted my output stage in filament bias, and didn't achieve this until I hit on PSE 4P1Ls. The thing is that you need quite a low bias voltage. Maybe 10Y is another possibility. There's not much else I can think of. Bias voltage on 2a3 and 300b is too high - the demands on filament bias would be ridiculous.
But 4P1Ls work like a charm. I have 2 of them in parallel, but 3 in parallel is quite do-able.
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