High Voltage Power Supply

Mike McLane

Active member
I got your High Voltage PSU and have a couple of questions:

1. Heater leg is 6.3vdc, therefore (+) to pins 4&5 and (-) to 9?
2. You have two 9vdc & Grd eyelets side by side. Is one to provide a 9vdc leg to operate LED's?
3. As far as ON and OFF is concerned would you effect that simply by using a foot switch ground the signal at the Volume control?
 
1. Correct, but the heater is like a resistor, nondirectional, so you could run (-) to 4&5 and (+) to 9, either way.
2. The intention was to have a convenient place to tap off the 9V to feed to something else in a design. You can do whatever you want with it.
3. If you're talking about bypassing the circuit, you can treat a tube preamp like any circuit for true bypass. If you're looking at just turning it on and off then you can of course disconnect input power to the power supply.
 
Gotcha, thx! I believe you said that 22 or 24 gauge hookup wire is sufficient for wiring up your pedals. . . please confirm? BTW, that Echo Foxtrot looks mighty tempting (I call her Voxy Lady ;)).
 
Building the HV PSU. The link to Tayda for the 100u cap is for a 25v unit. The BOM says "Set for desired input voltage" and the PCB itself has a large footprint more in keeping with the 400v 10u cap that sits immediately adjacent. But the schematic indicates it's taking 9v input to ground so I'm thinking 25v is a good value??? What say you Mr. Wizard?
 
Got the HV PSU hooked up to my preamp. Is there a way to replace the trim pot with fixed resistors in order to yield a 250v HV value?
 
Got the HV PSU hooked up to my preamp. Is there a way to replace the trim pot with fixed resistors in order to yield a 250v HV value?
I have no experience with the board, but you probably could adjust it, measure the trim pot and then replace that way right?
 
swyse - Probably no worky getting a good reading from the trimmer while its in the circuit. Also, it is VERY sensitive. A slight twiddle either direction can mov the B+ from 150v to 300v so jostling the board around would probably alter the trimmer setting. I need 250v so knowing that I thought it might be possible to calculate the proper value(s). Also, the trimmer works as a voltage divider, not a variable resistor. As resistance goes up to one pin it goes down to the other so there would have to be a two resistor solution I'm guessing.
 
Got the HV PSU hooked up to my preamp. Is there a way to replace the trim pot with fixed resistors in order to yield a 250v HV value?
I think 560R between pins 1 and 2, then 4k7 between pins 2 and 3 is what I do for my fixed builds to land around 250V. It may not be exact, but should get you pretty close.
 
I think 560R between pins 1 and 2, then 4k7 between pins 2 and 3 is what I do for my fixed builds to land around 250V. It may not be exact, but should get you pretty close.
I figured that from looking at the Space Heater schem & assuming that 250vdc +/- was what you were going for there. As a learning opportunity. . . . 4K7+560 = 5,260 (71% + 29% = 100%). If I wanted to vary the HV is this a linear relationship that I can use to calculate other desired values?

For example: This circuit would appear to have a top end of about 350vdc (250/.71). If I wanted an HV of 275 that would be factor of .79 . . . . 5,260 x .79 = 4,155 (say 4.1K) and derive it's "5,260 compliment" (say 1.1K) and use the closest "fit" I can find to those values as a starting point. No need to get into where the 5,260 total (5K trimmer in the case of the HV PSU) came from. I'll take that on faith at this point. . . another day, another lesson maybe. Thx for the reply!!
 
I figured that from looking at the Space Heater schem & assuming that 250vdc +/- was what you were going for there. As a learning opportunity. . . . 4K7+560 = 5,260 (71% + 29% = 100%). If I wanted to vary the HV is this a linear relationship that I can use to calculate other desired values?

For example: This circuit would appear to have a top end of about 350vdc (250/.71). If I wanted an HV of 275 that would be factor of .79 . . . . 5,260 x .79 = 4,155 (say 4.1K) and derive it's "5,260 compliment" (say 1.1K) and use the closest "fit" I can find to those values as a starting point. No need to get into where the 5,260 total (5K trimmer in the case of the HV PSU) came from. I'll take that on faith at this point. . . another day, another lesson maybe. Thx for the reply!!
If I remember right it's not a linear relationship, when you turn the trimpot (which is linear) it increases slowly from 30V to 80V or so, then once you pass 80V it starts going up much faster, to the point where the line between 250V and 300V is just a teeny tiny trimpot turn.

I know when I'm aiming for 250V I do 4k7 over 560R, and when I'm aiming for 350V I do 4k7 over 359R (1k in parallel with 560R).
 
I know space it at a premium, but the trimmer is UBER sensitive up in the desired range. If you use 4K7 as a fixed value and "bias" it by varying resistance to GRD would there be any value to shoehorning a 4K7 somewhere on the board in series with the 220K (or does somebody make a 225K?) and putting in a 1K trimmer as a variable resistor to GRD?
 
I know space it at a premium, but the trimmer is UBER sensitive up in the desired range. If you use 4K7 as a fixed value and "bias" it by varying resistance to GRD would there be any value to shoehorning a 4K7 somewhere on the board in series with the 220K (or does somebody make a 225K?) and putting in a 1K trimmer as a variable resistor to GRD?
You could tie the 4k7 and 220k together vertically in the 220k footprint. Uses vertical space instead of taking up horizontal space.
 
I know space it at a premium, but the trimmer is UBER sensitive up in the desired range. If you use 4K7 as a fixed value and "bias" it by varying resistance to GRD would there be any value to shoehorning a 4K7 somewhere on the board in series with the 220K (or does somebody make a 225K?) and putting in a 1K trimmer as a variable resistor to GRD?
You could use a "Trimmit" board with a 250k trimpot, or even a 500k.
 
Tie the 4K7 & 220K vertically and install a 1K trimmer. Just clip the pin on the "220K side" and instead jumper that eyelet back to Pin 2? For that matter, would the slight add'l resistance created on the "220K side" by installing per plan just shift the "balancing act" slightly, but still accomplish the same end without all the jerryrigging? Is 5,260 vs 5,700 total ohms across that point a deal breaker?

BTW MichaelW. . . I looked at the Trimmit and it appears to be a means of replacing a typical fixed value resistor "bore" with the Trimmit that then allows the insertion of a trimmer to create a variable resistor in its place. In this application the PCB is bored for a trim pot so that's what I gotta deal with.
 
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Tie the 4K7 & 220K vertically and install a 1K trimmer. Just clip the pin on the "220K side" and instead jumper that eyelet back to Pin 2? For that matter, would the slight add'l resistance created on the "220K side" by installing per plan just shift the "balancing act" slightly, but still accomplish the same end without all the jerryrigging? Is 5,260 vs 5,700 total ohms across that point a deal breaker?
Nope, doesn't matter. I've used 10k trimpots on these when I ran out of 5k, still works fine, just a little harder to dial in. The ratio is more important than the actual value.

BTW MichaelW. . . I looked at the Trimmit and it appears to be a means of replacing a typical fixed value resistor "bore" with the Trimmit that then allows the insertion of a trimmer to create a variable resistor in its place. In this application the PCB is bored for a trim pot so that's what I gotta deal with.
I think he meant you could replace the 220k with a Trimmit and dial it in to 225k so you don't have to worry about tying two resistors in to one footprint.
 
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