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How do I estimate cathode bypass capacitor voltage?

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Posted on March 18, 2017 at 18:34:45
91derlust
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Hi all,

I'd like to learn how to calculate and select a safe voltage rating for the output stage of my next amp build. I have read a few posts, but I still don't seem to have it straight in my head.

The input stage is directly coupled to the 2A3 output stage. Output stage grid sits nominally at 152VDC, the cathode at 200VDC and the plate at 450VDC (bias -48V, plate to cathode 250V). Cathode R is 4,750 Ohm, bypassed to ground by the cathode bypass capacitor (not "Ultrapath").

How do I go about working out the required voltage rating? I assume it is not as simple as the VDC across the capacitor.

Please refrain from posts about the best "sounding" cap or capacitance, unless it is directly relevant to my question - I am interested in how to choose a cap with a safe voltage.

Cheers.
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

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RE: How do I estimate cathode bypass capacitor voltage?, posted on March 19, 2017 at 01:08:17
lovetube
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Location: Melbourne Australia
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i would say 300vdc rating cap is more than safe for your amplification
LT

 

RE: How do I estimate cathode bypass capacitor voltage?, posted on March 19, 2017 at 02:04:43
91derlust
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Posts: 531
Joined: December 25, 2014
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December 0, 0000
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.

Cheers,
91.


"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: How do I estimate cathode bypass capacitor voltage?, posted on March 19, 2017 at 08:26:36
Triode_Kingdom
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Posts: 6079
Location: Texas
Joined: September 24, 2006
The only issue is the DC voltage. If there's sufficient capacitance to bypass the cathode resistor, very little AC voltage will be present across the cap. I tend to be conservative about capacitor voltage ratings. 2:1 is my preference, which in this case would mean a cap with a voltage rating of 400V or greater.



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Buy Chinese. Bury freedom.

 

You know the cathode bypass capacitor minimum voltage..., posted on March 19, 2017 at 11:24:02
Interstage Tranny
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Posts: 2988
Location: Eastern
Joined: October 4, 2006
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...

 

Thanks LT, TK, and IT! - comprehensive and exactly what I needed. nt., posted on March 19, 2017 at 13:05:08
91derlust
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Posts: 531
Joined: December 25, 2014
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.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

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