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In Reply to: RE: How do I estimate cathode bypass capacitor voltage? posted by 91derlust on March 18, 2017 at 18:34:45
i would say 300vdc rating cap is more than safe for your amplification
Thanks for the advice... it opens up a few (expensive!) options - not sure I want that :)
Still, I'd like to know how to work it out.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
With simple Ohm's Law, 250V/4750 Ohms yields .0526 mA; this was the only parameter you did not mention in your OP.
With the cathode "sitting" at 200V DC to ground, a bypass cap needs to be at least that value. If you were bypassing a la Ultrapath, since the cathode is at 200V and the plate at 250V, believe it or not there are vintage schematics which would specify a 50V bypass cap for the Ultrapath. Of course, there is no "safety" in cutting the voltage rating. If your driver stage can drive the opt into grid current, driving more than 152 VAC, then the "needed" cathode bypass cap voltage would have to be increased.
Thinking about the very vintage schemes, the amps would use paper in oil (ie: bath tub or stand-up types, as well as chunky tubulars) for the cathode bypass duty. Most very vintage PIO are underrated for their acceptable voltages.
Personally, I agree we should use safety measures, for longevity and minimal failures. Since you are using Direct Coupling, if the 2A3 tube shorts, then there could be very high voltage on that cathode/plate. Thus, for safety sake, even though it's going to be expensive, you may want to consider a voltage rating higher than your opt's total HV rail.
If you use vintage USA PIO, rather than the boutique plastic or teflon caps for your opt cathode bypass duty, 200V to 400V caps should be fine. BTW, if you were to later choose Ultrapath, the same 200-400V rating would be my choice...
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