Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TomG on February 21, 2025, 12:55:34 pm
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I am building a phono preamp based on circuits in the RCA Receiving Tube Manuals and I'm having an issue with the power supply. The B+ voltage is way higher than I expected. I'm using a 325-0-325V transformer, a GZ34 rectifier tube and a CLC filter. I expected around 350V but am getting a solid 500V on B+. I am very new to this. So it's probably something dumb I'm overlooking, but it has me perplexed. My goal is to get 275V using a dropping resistor. Not sure if that's doable or not, but I'm using a transformer that I had on hand. I built a similar power supply for the power amp using a 400-0-400V transformer and a 5U4 rectifier tube and got 460V on B+, so why this one is at 500V is beyond me.
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I am building a phono preamp based on circuits in the RCA Receiving Tube Manuals and I'm having an issue with the power supply. The B+ voltage is way higher than I expected. I'm using a 325-0-325V transformer, a GZ34 rectifier tube and a CLC filter.
B+ is so high because a preamp only puts a couple mA current drain on the PS. Put a resistor (call it R3) between L1 and B+. Make it a big resistor, maybe a 100K depending on the value of R2. Now R3 and R2 form a voltage divider. Adjust R3 and/or R2 until you have the desired voltage. Also put an additional 22µF cap across R2.
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I am building a phono preamp based on circuits in the RCA Receiving Tube Manuals and I'm having an issue with the power supply. The B+ voltage is way higher than I expected. I'm using a 325-0-325V transformer, a GZ34 rectifier tube and a CLC filter.
B+ is so high because a preamp only puts a couple mA current drain on the PS. Put a resistor (call it R3) between L1 and B+. Make it a big resistor, maybe a 100K depending on the value of R2. Now R3 and R2 form a voltage divider. Adjust R3 and/or R2 until you have the desired voltage. Also put an additional 22µF cap across R2.
Ok, that makes sense. I did put a 6k resistor at R1 and it dropped a few volts, so I'll try a bigger one on the other side. What is the purpose of the second 20uf capacitor? I assume that once it's hooked to the preamp, the values will need to be readjusted?
Thanks
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What is the value of R2?
I overlooked R1. You can use the circuit as you have drawn and just increase the value of R1 to achieve the desired voltage. This is what I'd do. BTW, that 200mA choke is way overkill for this application. A 50mA choke would be much smaller and work just as well.
Adding the third resistor as suggested means you no longer have a filter cap directly on the B+ line. The second cap in my suggestion is to provide filtering directly on the B+ line. I would just ignore this suggestion and go with your original circuit.
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What is the value of R2?
I overlooked R1. You can use the circuit as you have drawn and just increase the value of R1 to achieve the desired voltage. This is what I'd do. BTW, that 200mA choke is way overkill for this application. A 50mA choke would be much smaller and work just as well.
Adding the third resistor as suggested means you no longer have a filter cap directly on the B+ line. The second cap in my suggestion is to provide filtering directly on the B+ line. I would just ignore this suggestion and go with your original circuit.
R2 is just a bleeder resistor. I haven't figured out the value yet. I'll pop a 100k in at R1 and see what happens. Guess I didn't think about the low current when I tried the 6k resistor.
Thanks for the help.
Tom
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I am building a phono preamp ... I'm using a 325-0-325V transformer ... I expected around 350V ... My goal is to get 275V ...
SEL49 already walked you through your present situation, so you're set.
Let's look at how you might have approached this differently: Begin with the End in mind.
There are 2 common limits noted when AC Voltage is rectified into DC Voltage: "Peak Voltage" of 1.414x RMS Volts and "Average Voltage" of 0.9x RMS Volts.
You ran into the Peak Volts situation: 500v / 1.414 = 353v ---> about what one gets with 125vac applied to a PT wound for 115vac Primary, 325v Secondary.
If "275v" is the goal, shoot for that first: 275v / 1.414 = 194v AC needed. Assume the rectifier will drop nominal voltage (especially given it's a GZ34) ---> 220vac needed.
Check: if we have a 120v:240v PT and actually only have 120vac at the outlet, that rectifies to 240v x 1.4414 = ~340vdc (and little voltage-drop at the rectifier due to low current).
If you under-load the transformer, the voltage output will rise. If your outlet voltage is high, your voltage output will rise. Small current-draw from a preamp will probably drop less across the rectifier than you expect, and your output voltage will rise. And since no one ever seemed to complain that their build's voltage "is too low" it's better to "aim low" where possible when planning.
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B+ is so high because a preamp only puts a couple mA current drain on the PS. Put a resistor (call it R3) between L1 and B+. Make it a big resistor, maybe a 100K depending on the value of R2. Now R3 and R2 form a voltage divider. Adjust R3 and/or R2 until you have the desired voltage. Also put an additional 22µF cap across R2.
Success! A 39k resistor at R1 dropped B+ to 300V. Once the preamp gets added I'll adjust it to the required 275V.
Thanks again for the help.
Tom