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In Reply to: RE: Some might have a different opinion posted by 91derlust on October 18, 2016 at 12:31:30
For some reason I can't post pictures right now but a 2a3 run at 46ma, 250v into 2500 ohms (as far as i know that's how Dennis runs them) produces 4.471% THD at 1 watt.A 2a3 running at 60ma, 250v into 3800 ohms produces 2.146% THD at 1 watt.
Higher current, more plate dissipation, different load imp., less HD.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/18/16 10/18/16Follow Ups:
Roughly 10% higher than the optimal load suggested by PJ's rule of thumb.
Cheers,
91.
P.S. I will be retaining a lot of these posts for future reference. Thanks.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Yeah, if you're down around 45ma at 250 volts it calls for about 3.5k.
4.2k will give a little less max power but the power that it will have will be clean power.
I used to run my 300b (375v, 64ma.) into 6k. Not much power but really clean. Calls for about 4200 ohms.
Now I run at 4k (same voltage and current). More power and not as clean but still sounds good.
One can use whatever OP they want....It's just nice understanding what one is doing and what it does to the HD.
Dennis running at 46ma 250 into 2.5k is not what I would call a "good OP".
Especially when he says his amps are "the best ever......".......but I really don't what to get back into all that. :-(
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I made a feeble attempt at SE 7B4 DC 2A3 using the reduced current biasing of finals some time ago.
That was with cheap Chinese 2A3B monoplates and ESE125 OPT's.
Tubes and OPT's seem to affect the measurements a lot.
With the quality magnetics and EML tubes DF uses, his circuit probably bench tests pretty good.
Of course, I don't ever expect to see schematics or any test results from DF about his amps.
I remember that everything about SS has to be a top secret project.
DT 667
Probably some distortion cancelling between driver and output tube.
My numbers are just the output tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I doubt it from the measurements DDT posted... distortion cancellation typically results in 2nd and third distortions equalling each other because only the evens "cancel"
when you see a fft of a SE amp that doesn't have the typical dropping in distortion as harmonics go up then you have either a saturating output or driver / output even order cancellation.
dave
FWIW, Rage had a similar SE 2A3 circuit using Magnequest OPT + JJ 2A3's (not sure of the plate load or biasing) that he reported getting 1.5% THD @ 1W.
He was probably trying to copy what DF/JM did.
No matter what anyone would assert, JM will say DF is the greatest tube amp builder in the world.
The numbers I showed from the on line sim were just for comparison to each other, not for comparison to what others have done.
My numbers show that when the current is increased, increasing the plate dissipation, and the load impedance is increased from Dennis' operating point, the distortion goes down.
That flies in the face of Dennis' assertion of the idea of "thermaly stressed", what ever that means.
"...Rage had a similar SE 2A3 circuit using Magnequest OPT + JJ 2A3's (not sure of the plate load or biasing) that he reported getting 1.5% THD @ 1W."
I guess my assertion would be, if Rage was using Dennis' plate load and biasing and then increased the current and increased the load impedance his number would get even lower.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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