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In Reply to: Re: single ended EL84 in pentode mode posted by Eli Duttman on March 15, 2007 at 15:52:37:
Eli/Zobosky -Thank you for the excellent information!
My original goal was to keep the circuit as close to the original (at least using the original components) as possible. As a result, I already punched and assembled the chassis assuming tube rectification (which I've never used before - another learning experiment). I thought about switching to SS rectification, but wanted to try tube since I've done it. Perhaps I should have gone with SS . . . . (maybe on the next rev).
I still have some learning to do regarding pentodes as well. I was not aware that the screen grid was as important as it is. My understanding was that the screen grid in a typical pentode could further increase or decrease the current through the tube, but that it had a relatively small impact compared to the control grid. I need to do some reading.
Regardless, many of the simple SEP circuits that I've been looking at (like the two in the original post) just use a resistor from B+ to provide the screen grid voltage, which is usually a few volts less than the plate. I don't quite understand why this approach won't also work? The voltage won't be as solid as it will be using zeners, but it should still work? I need to research this more too . . . .
Lastly, I have not thought much about the amount of feedback to be used. I was hoping to find and build a simple circuit and learn about feedback in the process. At this point it is a totally foreign concept to me. One of my co-workers (who is much more knowledgeable about these things) tried to explain in to me, but I still don't quite get it.
I need to better understand how much feedback is needed for optimal fidelity and how to design that feedback into the circuit. And just when I had it all figured out for SET circuits without feedback . . . .
Follow Ups:
Hello,Never mind the tube rectifier! It always adds a natural warmth and a balanced tone to the sound, insted of the edgyness of solid state. When solid state rectification is done right, it does sound really good, but a tube can still help it. Simply parallel the tube rectifier with solid state diodes! That way, you will get instant high voltage, and you can put no matter what size cap for the first position, as the tube will not be strained: the SS conducts the bulk of the current (due to its much lower internal resistance), and when the SS diode gets cornered (around the recovery point) the tube takes over.
The voltage drop of paralleled tube / SS diodes is the same as of SS only. In addition, you can toy around a LOT with rectifier tube rolling! When the amp is on, simply unplug the rectifier. You won't even hear a pop, the amp is not affected, but you'll be able to hear the sonic difference. I've made a preamp with such rectification, and the combination of tube + solid is better than any of the two alone.Good luck!
Janos
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.
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.
Regardless, many of the simple SEP circuits that I've been looking at (like the two in the original post) just use a resistor from B+ to provide the screen grid voltage, which is usually a few volts less than the plate. I don't quite understand why this approach won't also work? The voltage won't be as solid as it will be using zeners, but it should still work? I need to research this more too . . . .
For one, the B+ can sag too, depending on the load conditions, which would cause the screen voltage to vary too.
You could go overboard and implement a separate screen supply ( a la drlowmu) but I suspect that's more than what you want to do for a beginner project.
Do try a regulated screen supply. Use a cheap easy series string of low voltage zeners (and not a high voltage zener) or look up Maida regulators on diyaudio.
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