809's in Class B P-P w/feedback

[rsumperl]

Thank you everyone for the inputs. This experiment/project is probably not worth pursuing. Perhaps transmitting tubes should remain just that, transmitting tubes.

Ray

"I had such a glitch that caused nice ringing on 30 KHz, couple of years ago. Ringing was of quite high Q, and started on frequencies below 100 Hz approximately; when frequency was higher it did not ring. Most probably, when driving with frequencies closer to saturation (Edcor specified 20 Hz on full 100W power), something in M6 iron caused it to increase leakage inductance."

The Edcor P-Ps that I have measured have very high leakage L on the outer 60% winding on one side. Did the the Zobels help before they burned out? I assume there were two of them, each from B+ to plates. You might want to measure the frequency response of each side (separately) of the primary to the loaded output too, you might be in for a surprise.

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"is this a project worth pursuing? I have 4 pairs of new RCA 809's and not sure what else to do with them. And if I do build, do I transformer it or mosfet it or perhaps sell the tubes cheap and cut my losses."

Well these tube are big enough that driving a Mosfet with them in class AB (- grid bias) would be a little silly really, mainly just for show. Going into positive grid V will require some Mosfet followers to drive the grids, high drive signals from the driver stage, and looks like an expensive HV B+ setup and OT. Curing the glitches and crossover problems is somewhat of a research project, although I'm sure they can be tamed, it's mainly a question of which way is best to proceed. Not a beginner type project in any case, will need some decent test equipment. I myself refuse to work with any Xmit tubes over 500 V B+ simply for safety reasons. Lots of cheap Sweep tubes around that will put out good power at low B+, high current.

Solving the glitch problem though is still open territory in DIY, since Mac type OTs are not abundant. And so many OTs have obvious problems with this for at least one side of the primary.

 

[Michael Koster]

Thank you everyone for the inputs. This experiment/project is probably not worth pursuing. Perhaps transmitting tubes should remain just that, transmitting tubes.

Ray

Transmitting tubes can make very nice audio amps; I've built some. But they do require circuits to deal with high current DC filament power, power grid driving, and high plate impedance. The results can be great sound and high power, but it's nothing that can't be achieved in other ways.

For me, it was a learning experience in doing some of these things but to continue to refine bright filament amps would be I think a distraction from my main goals; a sort of hobby activity that I can't sustain right now.

There has to be a reason to go to the extra trouble, and certainly there are plenty. Esthetics, technical challenge, high power (esp. in SE), because you can.

But my conclusion is there is no unique benefit for me apart from the above.

Another consideration I have is that I need to run my amps for hours at a time, recording and mixing, and have limited off-grid power resources. So efficiency is a big deal for me. I can't run room-heater amps.

Everyone's wants and needs are different, though.

cheers,

Michael

 

[multi]

Hi
I have built lots of 809-805-811 amps. Nothing even comes close!
Don't forget most record cutting machines use such tubes.
You run them low voltage about 370v for 809 and 70 ma. Using 6bx7's as the Cathode driver.