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In Reply to: RE: Does it make a difference? PPP cathode wiring posted by DAK on July 12, 2015 at 10:53:36
for each tube can't hurt. ?
Some people like PIOs for cathode by-pass caps around these R's. Any other suggestions?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Follow Ups:
Hi Tim, have you read David Gillespie's article on "EFB", in Tronola.com . In combination with matched output tubes the EFB results in much lower distortion when compared to single cathode bias resistors on each output tube or a stock single cathode resistor for the 4 output tubes.
My original post was referring to how to wire the cathodes in a PPP amp as I wanted to have a simpler bias circuit than 8 resistors and 8 bypass capacitors. I am still debugging my EFB circuit but the layout is much better and the bias current is rock steady. cheers, Dak
for each PP pair.
With the SR SS diodes, big caps, 6CG7 LTPs with FET CCS, EL84Ms, and far less total loop NFB, I get 20W RMS 20-20k Pentode and 16W RMS 20-20k in UL, with very low TND. This is a substantial advance on the power bandwidth, distortion and noise of the OEM circuit.
I also think the bi-amped system has a lot more headroom than the STAX DA80 a 45/8ohm pure class-A amp had.
Nevertheless this EFB circuit may be a relatively simple way to get even more continuous clean power, though it may need to deliver higher bias to do that.
But probably not any more headroom.
If you can give me the precise URL for the original article (and any other subsequent URLS) I will point them out to my techie, and see what he thinks. ~ 20 yrs back I had another HV incident and TMFW&F insisted that I use a techie in future. I can solder leads and plugs, but no high voltages.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
.
"Nevertheless this EFB circuit may be a relatively simple way to get even more continuous clean power, though it may need to deliver higher bias to do that."
It doesn't need to deliver more. It only has to hold it steady if long-term average current through the tubes increases. That's what the simple cathode R/C cannot do.
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Buy Chinese. Bury freedom.
I get that the power for the regulator comes from the B+, which is neat.
No need for a fixed bias traffo.I will run the articles past Bill and set his quote against the other plans I have for the system.
I'd add that I don't think the stability of bias or longer life benefit is a really big plus to me. EL84Ms are not expensive.Both LEAK St20 amps currently use 4 single Cathode Rs ie one per tube. Surely this makes a difference? yes? no?
More power might be a plus, though I have enough as it is.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 07/20/15
"Both LEAK St20 amps currently use 4 single Cathode Rs ie one per tube."
I personally think that's the worst possible way to bias the tubes. If you don't want to build a bias supply - which would provide the ultimate in headroom - a separate EFB for each channel (pair of tubes) seems the best way to do this.
--------------------------
Buy Chinese. Bury freedom.
IIRC mine are giving about 9dbw in A IE about 8 watts.
I will nevertheless run the idea past my techie.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
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