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In Reply to: thank you posted by eedork on March 15, 2007 at 18:09:44:
Matt,Many console amps provided power to a separate tuner. If that was the situation in the amp you acquired, there is "juice" to spare in the power trafo. If extra "juice" is available and you spaced things out well, you can enlarge the EZ81 socket hole to take an Octal socket and use a 6BY5 as the rectifier. The forward drop in the 6BY5 is lower. Check TDSL for tube data.
The instantaneous plate voltage in a pentode varies a LOT. When the screen grid is connected to the "same" B+ as the plate, the instantaneous plate voltage fall below the screen voltage and screen current is higher than plate current. Therefore, NON-LINEARITY occurs.
NFB works BEST when the open loop circuit is reasonably linear to begin with. Regulate the screen grids at 250 V.! The sound is better when NFB is working primarily to lower O/P impedance, not correct distortion. EVERTHING hangs together well when the NFB error correction signal is small.
Eli D.
Follow Ups:
Thank you for the excellent advice guys. I'm still reading about this, but I have one additional question. If I simply regulate the B+ supply, then I can just use a resistor to drop a few volts to the screen supply, correct? I guess my point is, why not just regulate the whole HV supply instead of only the screen supply?I think I have some 1W zeners around somewhere . . . . . just have to find them.
This is probably in the tube data sheets, but what's a good design point for the screen if the plate is running at 275V?
> > This is probably in the tube data sheets, but what's a good design point for the screen if the plate is running at 275V? < <Matt,
Zobsky gave you a good hint in mentioning the 6p15p/SV83/EL84N's requirements. 150 V. will do nicely. That puts a 0A2 gas discharge regulator "on the map". If you wire your sockets for the 6p15p, 3 decent and affordable tubes are available for "rolling": the 6p15p, the 6p14p-ev, and the JJ EL84.
In any event, use "fixed", as opposed to cathode, bias on your O/P tubes. IMO, you can't afford the voltage drop across a RC bias network.
BTW, you've yet to indicate the heater current capability of your power trafo. Did your console amp support an external tuner?
Eli D.
Eli,Yes, I believe the amp also supported a tuner (it's been a while, but I think it also had a tuner).
That being said, I probably do have enough extra Watts in the power transformer to play around with some of these things.
I think I'm going to start with one of the simple circuits. I'll build it bone stock, and then swap in different elements as recommended by you and Zobsky. I think I'll learn the most this way.
I just answered one of my own questions - just regulating the B+ supply won't do much for the screen supply, as the current through will still vary thus varying the voltage across the screen resistor.
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