Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on January 25, 2022, 06:22:28 am
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800 build. The PI supply voltage from the filter cap is 400 v. I would like to reduce this say 40-50 volts. I tried putting a 10K resistor in series from the cap to the PI supply but it still reads 400V. This filter cap isn't feeding with any other section of the amp, it only feeds the PI B+ lug . 1.) why didn't the voltage reduce? and 2.) how can i get this voltage down?
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Hey Joe ! If the tubes are unplugged then the voltages will be higher then expected.
Show us the schematic for the amp please, it is easier to understand what is going on.
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I would like to reduce this say 40-50 volts. I tried putting a 10K resistor in series from the cap to the PI supply
WRONG!!! Don't do that.
To lower the voltage at B+3, increase the size of the 6.8K dropping resistor between B+2 and B+3. Probably gonna need more than 10K. Experiment with different resistors until you are happy. Be aware that dropping the voltage to B+3 will also drop the voltage at B+4.
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ok i think i know what the issue is. This B+ wire coming from Cap B+3 isnt connected at this juncture. I couldn't tell from the diagram if it was and i didnt connect it. Man o man. Shoulda read the schematic
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This B+ wire coming from Cap B+3 isnt connected at this juncture.
Really? If that's true you would have zero volts at the PI plate resistors.
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The wire comes from the cap can to the plate resistor junction for the PI. The reading there is 400V with the amp running. i guess this multi cap can passes the voltage through to the B+3 tap running there. Amp sounds fine this way but i will get that connected to the dropping resistor to lower the voltage.
i'm 90% sure it's not connected at the dropping resistor
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Show us the schematic for the amp please, it is easier to understand what is going on.
What Uki said . . .
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Show us the schematic for the amp please, it is easier to understand what is going on.
What Uki said . . .
Enjoy :laugh:
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Equally, you could increase the value of the 10k tail resistor to around 39k (or so), raising the voltage at the cathodes by 40-50V (give or take). Functionally the same, but will also improve the balance of the PI by acting more like a CCS.
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I would like to reduce this say 40-50 volts. I tried putting a 10K resistor in series from the cap to the PI supply
WRONG!!! Don't do that.
To lower the voltage at B+3, increase the size of the 6.8K dropping resistor between B+2 and B+3. Probably gonna need more than 10K. Experiment with different resistors until you are happy. Be aware that dropping the voltage to B+3 will also drop the voltage at B+4.
Well you were right. (once again!). I swapped the 6.8k with a 20k and the voltages are happy. 320v PI supply, perfect for this amp! :worthy1:
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I am curious as to what your B2 volatges were before and after the swap.
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I am curious as to what your B2 volatges were before and after the swap.
400v to the phase inverter supply. acceptable but early 800's were around 320v there ( also lower voltages to the preamp section)