Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on October 30, 2018, 05:44:59 pm
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I want to use a 50uf and a 22uf electrolytic capacitor in series for a screen supply node. Both are 500v but there is likely going to be near 600v+ on them.
Can I use these two capacitors in this manner if I strap some bleeder resistors in parallel with them.
Also all I have is 100K and 390K 3 watt resistors.
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I found this which I think helps re voltage per cap, but I'm too tired to sort through it just now: http://www.capacitorguide.com/applications/capacitors-in-series/
I don't know how to use bleeder resistors with unequal caps. Without bleeders, the voltage per cap will be derived by the capacitance voltage-divider formula. This is always true, but it's easy to divide voltage by 2, when 2 caps of equal capacitance are used.
The combined capacitance is 15.2777777778uF
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I want to use a 50uf and a 22uf electrolytic capacitor in series for a screen supply node. Both are 500v but there is likely going to be near 600v+ on them.
Can I use these two capacitors in this manner if I strap some bleeder resistors in parallel with them.
Also all I have is 100K and 390K 3 watt resistors.
You can put those caps in series but the value resistors you have will not work. The voltage across series caps of different capacitance will be different. You must use unlike resistors to force the voltage division correctly.
For example, if you put a 100K across the 50µF you will need to put a 227K across the 22µF. (220K would be close enough) The idea is to keep the RC time constant equal, ie, R1*C1 = R2*C2.
It's much easier to use equal value caps. Then you can use equal value resistors.
Look at this schematic for a good example of this...
https://el34world.com/charts/Schematics/files/Fender/Fender_65_deluxe_reverb_reissue_sm.pdf
220*100 = 47*470 (well, very close)
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Thank you very much for the replies.. I goofed up ordering capacitors that I thought I could use in series so I'm stuck with different values.. for now. I will get a single 50uf cap shortly to balance them.
I did find some 220K resistors and I have the 100K resistors so I can proceed to fire this Ampeg B15S up .. the info is much appreciated Sluckey.. I need to review the time constant formulas.
Curiously the B15S originally has two 40uf caps on the screen supply.. one is rated 450v and the other is in a can and is 500v rated. They did not use any voltage leveling resistors (bleeders).
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It's all about the leakage current. Using a common formula, a 50uF @ 500V cap. has 1.5mA leakage and a 22uF @ 500V cap. has 0.8mA. 1.5mA with 500V looks like a resistor of 333K and 0.8mA with 500V looks like 625K. So the 50uF cap. in series with the 22uF cap. looks like a 333K resistor in series with a 625K resistor.
The balancing resistors are selected such that the parallel combination with the 333K is the same resistance as the parallel combination with the 625K so that the voltage at the middle is half the voltage at the top.
A 100K balancing resistor across the 50uF cap. results in a total resistance of 77K. After a couple of strokes on the calculator, the 22uF cap. needs an 88K balancing resistor for a total resistance of 77K.
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Thanks 2Deaf .. leakage current sounds like what is happening for sure.