Capacitor WVDC. is higher really better?

[Chris Hornbeck]

Line voltages in the USA are very often 5 to 8 percent higher now than the nominal 115 volts of yore. Just another reason to be conservative.

Thanks,
Chris

 

[tomchr]

One minor issue is the difference in ESR for higher voltage rated caps. The higher the voltage, the greater the ESR as a general rule.

Personally, I would measure the voltage with all the tubes removed, and high line applied via a variac. Then I would pick the cap with closest higher rating and a surge rating of at least 50V above that voltage. I would also check in the mfg data sheet to compare ESR and ripple current ratings of it and the next higher voltage rated cap.

+1

The tolerances on the line voltage in most western countries are typically pretty tight. +/- 5 % or so. I tend to be a bit more conservative and use +/-10 %.

So for US gear, I'd measure B+ without load at 120 + 10 % mains in voltage (132 V). I'd pick a cap with a voltage rating higher than that, leaving at least a 10 % margin. This will ensure that the cap can handle start-up under worst-case conditions. Under normal conditions, the cap is probably going to run somewhere around 70~75 % of the rated voltage.

There's no harm in picking a capacitor that's rated for a much higher voltage than what you need. But it'll be physically larger, have higher ESR and ESL, be much more expensive, and harder to find.

~Tom