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In Reply to: Re: Re: I did that too !!! posted by drlowmu on February 23, 2007 at 21:59:54:
Thanks again Jeff. It all makes sense, except I would have guessed a fixed bias. I have always used self bias for SE DHT's as well, but I thought a fixed one "might" "compliment" the low everything situation,
have you tried it together with an "optimized" supply?
I prefer a 45 or a 2A3 mainly because I can use AC filaments, and with my (guessing) 98db speakers, that works out fine. Poor 300B's have such a mediocre reputation, yet I wonder how much of this is truly deserved,
and how much is anti-fashion nose picking. I honestly don't know. I have a couple NAVY marked 300B's, that test good, they seem like a big 2A3 to me. Which brings up the starved filament concept. Perhaps you read Steve Bench's article/research. 385VDC minus a bias, would be a relatively low OP point for a 300B, maybe 60% of max dissipation, which then "might" allow a reduced filament voltage of even 3.8VAC ? which in turn might be quiet enough (lower HD too).
Also, have you applied the low everything concept to a filament supply? Why do we always use "big" cap values in this situation?
I have found that a "shitty" filtered supply (with a hum pot) can be made quiet, Maybe a 300B just needs "slighlty" rectified and filtered
(and lowered) filament supply? maybe just getting 5VDC to have maximum 2.5VAC ripple, is "enough" for quiet operation? Maybe a 300B, if surrounded by an "ideal" PS would sound fine? With basically the same Rp and mu as a 2A3, what could explain the perceived "inferiority"?
I may throw together a test amp, and use the PS parts I have (with bigger cap values), which does spell 300B for the moment, until I can come up with the right power transformer for a 2A3. Bench suggests that you can lower the filament voltage by an amount that is a "similar" percentage value less than whatever % your op point is less than maximum. I have tried this to a 20% degree with other DHT's, and yes it worked fine, and had measurably lower harmonic content. Long term tube life? don't know, yet Bench did "some" tests here, and found no premature death.
And then there are those individuals who believe that "stated" maximum ratings are actually at that 62% "golden ratio" point already, and that you can run a 2A3 at 400V on the plate, 50ma all day....?...?
I know that this has been tried successfully, well, "without failure".....
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Follow Ups
- thanks for the follow up, poor 300b's... - bobbyj 09:27:33 02/24/07 (1)
- Re: thanks for the follow up, poor 300b's... - drlowmu 11:02:07 02/24/07 (0)
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