Whut Eli sez... The 6P3S-E is probably the most similar to the 7591 in plate dissipation ratings (they even look rather similar to the EH coin base 7591s), but the transconductance is lower for the 6P3S-E. The 6P3S-E will need more negative bias at G1 to establish a given bias current than the 7591A, and more drive voltage for the same output power. With that in mind, though, there's no reason why the 6P3S-E couldn't be used with the Sherwood transformers. I'm going to be using some with Fisher 400 iron, but with a radically different circuit than the one Fisher used with the 7591As originally employed.
Eli,
I've been looking at the Tung-Sol data page for the 7591. I've usually used the Duncan amps tube data, but the Tung-Sol page is easier to read. From that page it seems the best use of the 7591 is AB1 with 400 and 350 volts with fixed bias on G1 of -16 volts.
You're right the Maida regulator looks to be more complicated than I would like for this amp.
I'm assuming that the triode connection (100 ohm resistor connecting grid to anode) would limit grid current, and thereby output. From the data page I see that the grid voltage is always 50 volts less than the plate voltage. So am I right in assuming that the regulator will give the necessary 50 volt difference, without dropping the current as much as the triode resistor connection?
Maybe I'm reading between the lines but is there a way to do full pentode without the complicated regulator? Might a 51 volt 5 watt Zener diode work? Or is that wishful thinking on the part of an amateur?
Also, if maximum grid current is 8ma is that what you would want to shoot for?
Kevin
I've been looking at the Tung-Sol data page for the 7591. I've usually used the Duncan amps tube data, but the Tung-Sol page is easier to read. From that page it seems the best use of the 7591 is AB1 with 400 and 350 volts with fixed bias on G1 of -16 volts.
You're right the Maida regulator looks to be more complicated than I would like for this amp.
I'm assuming that the triode connection (100 ohm resistor connecting grid to anode) would limit grid current, and thereby output. From the data page I see that the grid voltage is always 50 volts less than the plate voltage. So am I right in assuming that the regulator will give the necessary 50 volt difference, without dropping the current as much as the triode resistor connection?
Maybe I'm reading between the lines but is there a way to do full pentode without the complicated regulator? Might a 51 volt 5 watt Zener diode work? Or is that wishful thinking on the part of an amateur?
Also, if maximum grid current is 8ma is that what you would want to shoot for?
Kevin
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The mass market guys like Scott and Fisher (and no doubt, Sherwood as well) pulled the screen bias off a dropper resistor string with filter caps festooned along it. This is not the most optimal solution, but it works. Check the schematic for any Scott or Fisher amp/receiver (readily available on the web - Fisher 500B is a good start) and look at the power supply circuit - you'll see exactly what I mean. If you want nicer performance, then you can opt for a real screen regulator.
Hey Guys,
The first thing I want to say is DUH. I took WO's advice and looked back at the S5000 schematic. I didn't realize the grid voltage could be supplied from a source other than the output transformer. Now its a simple matter of sizing the cap/resistor combination to get the correct voltage. I also noticed that the Sherwood is a fixed bias amp with -17 volts at the grid. So now I have a way to simply design the amp that doesn't include complicated regulators or circuit boards.
There is still the a question that hasn't been answered though. And If I could impose. The question is this. In "full pentode" mode is the grid voltage meant to be fixed at a particular voltage? Or must it be fixed in relation to the plate voltage, i.e. 50 volts less than the plate voltage but following any change in voltage at the plate?
And lastly, would a 50 volt Zener not work because it would be noisy?
Thanks again, Kevin
The first thing I want to say is DUH. I took WO's advice and looked back at the S5000 schematic. I didn't realize the grid voltage could be supplied from a source other than the output transformer. Now its a simple matter of sizing the cap/resistor combination to get the correct voltage. I also noticed that the Sherwood is a fixed bias amp with -17 volts at the grid. So now I have a way to simply design the amp that doesn't include complicated regulators or circuit boards.
There is still the a question that hasn't been answered though. And If I could impose. The question is this. In "full pentode" mode is the grid voltage meant to be fixed at a particular voltage? Or must it be fixed in relation to the plate voltage, i.e. 50 volts less than the plate voltage but following any change in voltage at the plate?
And lastly, would a 50 volt Zener not work because it would be noisy?
Thanks again, Kevin
Kevin,
Contact Jim McShane about the choke mod. he uses in H/K Cit. 5s. Like that which you are looking into, the Cit. 5 is a pure pentode setup. While active regulation is "best", Jim's choke tweak is BETTER than a RC section and not especially complex.
The Zener diode idea is totally off base.
Contact Jim McShane about the choke mod. he uses in H/K Cit. 5s. Like that which you are looking into, the Cit. 5 is a pure pentode setup. While active regulation is "best", Jim's choke tweak is BETTER than a RC section and not especially complex.
The Zener diode idea is totally off base.
Hey Eli ET-AL,
I've looked at some regulator schematics and they all seem to have one input voltage and one output high voltage. For a pure pentode amp which is more important to regulate? The plate voltage or the screen voltage? Or both? Is the point to keep the difference in voltage constant?
Or alternatively, is there a good article that explains WHY tight regulation of tube amp power supplies helps?
Kevin
I've looked at some regulator schematics and they all seem to have one input voltage and one output high voltage. For a pure pentode amp which is more important to regulate? The plate voltage or the screen voltage? Or both? Is the point to keep the difference in voltage constant?
Or alternatively, is there a good article that explains WHY tight regulation of tube amp power supplies helps?
Kevin
Eli,
I'm looking at Jan Didden's regulator at Linear.
How's this for a strategy? Use an LM317 to regulate the bias voltage (that is if an LM317 can regulate negative voltage?). I already have one with a heat sink. Then two completely separate power supplies for the screen and the plate. Separate transformers and rectifiers. Both using choke inputs. In this way the screen supply will be completely isolated from the plate supply. What do you think? Is this a stupid idea?
Kevin
I'm looking at Jan Didden's regulator at Linear.
How's this for a strategy? Use an LM317 to regulate the bias voltage (that is if an LM317 can regulate negative voltage?). I already have one with a heat sink. Then two completely separate power supplies for the screen and the plate. Separate transformers and rectifiers. Both using choke inputs. In this way the screen supply will be completely isolated from the plate supply. What do you think? Is this a stupid idea?
Kevin
Wrenchone, Et Al,
Thanks again for helping. I've been talking with Jim McShane about this problem also. He pointed out that if the bias supply is regulated because of mains variations the screen and plate supplies would also have to regulated.
But I thought of another possible strategy.
We have two causes for variation in the voltages. The first is variation in the mains voltage. And the second is the variation produced by the constantly changing signal, the music. Is that correct?
If that is correct then using discrete unregulated power supplies would allow variation from the mains voltage. But would isolate the screens variation from the signal loads?
I would use two separate transformers and rectifiers. The first would be a PSU capable of 350 volts @ 30 ma for the screens. Using its 6.3 volt heater winding through a Greinacher for the bias voltage. (Assuming I can find a low amperage 6.3 winding?). And the second, a 350volt @ 300ma for the plates using it's 6.3 volt heater supply? Putting a thermistor on the plate supply so the bias voltage would always precede the B+.
I know that a high voltage regulator would be the best way to go. But I am curious to know if the above solution might work. So much so that unless there is an obvious reason, to you that is, that it wouldn't, I would be willing to build the power supplies just to find out.
Kevin
Thanks again for helping. I've been talking with Jim McShane about this problem also. He pointed out that if the bias supply is regulated because of mains variations the screen and plate supplies would also have to regulated.
But I thought of another possible strategy.
We have two causes for variation in the voltages. The first is variation in the mains voltage. And the second is the variation produced by the constantly changing signal, the music. Is that correct?
If that is correct then using discrete unregulated power supplies would allow variation from the mains voltage. But would isolate the screens variation from the signal loads?
I would use two separate transformers and rectifiers. The first would be a PSU capable of 350 volts @ 30 ma for the screens. Using its 6.3 volt heater winding through a Greinacher for the bias voltage. (Assuming I can find a low amperage 6.3 winding?). And the second, a 350volt @ 300ma for the plates using it's 6.3 volt heater supply? Putting a thermistor on the plate supply so the bias voltage would always precede the B+.
I know that a high voltage regulator would be the best way to go. But I am curious to know if the above solution might work. So much so that unless there is an obvious reason, to you that is, that it wouldn't, I would be willing to build the power supplies just to find out.
Kevin
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