Cap question

[500VDC]

I need to replace a couple of capacitors out of an old ss amp that are 250 mfd at 15 volts.

The closest I can find is 15 volts at either 220 mfd or 270 mfd.

Looking for some education here - if these two are my only options, which one do I choose and why? Or are there other options?

Is there a general rule when these situations come up or is it case by case? Hope the questions aren't too generic and thanks in advance for any constructive help.

[Witek]

Find yourself some 250uF (MFD = uF (millifarad)) caps rated at something higher than 15volts, if you change the rating the tone changes, but putting a bigger cap in (one that can handle more voltage) and it won't do anything to change the amp.

[mark norwine]

250uF (MFD = uF (millifarad))

Not to nitpick, but actually MFD = uF = microfarad [a millionth of a farad].....not "milli" [thousandth of...]

[Witek]

Not to nitpick, but actually MFD = uF = microfarad [a millionth of a farad].....not "milli" [thousandth of...]

Ah my bad, at least I got corrected, us mechanical engineers don't have too great an electronics knowledge. :messedup

[BJF]

I need to replace a couple of capacitors out of an old ss amp that are 250 mfd at 15 volts.

The closest I can find is 15 volts at either 220 mfd or 270 mfd.

Looking for some education here - if these two are my only options, which one do I choose and why? Or are there other options?

Is there a general rule when these situations come up or is it case by case? Hope the questions aren't too generic and thanks in advance for any constructive help.

Hi,

The exact value won't matter much quite likely and in reality a 220uF cap could easily be 250uF...

Might want to try higher voltage rating

Have fun
BJ

Affiliations
www.mpamp.com
www.bjfelectronics.com

[John Phillips]

Chances are the old caps have drifted up in value anyway, so if you want to keep things as close as possible the same, use the 270s. (Rising capacitance doesn't make old caps 'better' BTW - because internal resistance and leakage also increase as they get old, so they become less effective.) Also, as BJ said, electrolytic cap tolerances are usually pretty wide (typically 20%, and often more upward than downward, eg -10%/+20%), which means there's not a lot of practical difference between a 220 and a 250 or a 250 and a 270 anyway.

It's rare that increasing a filter cap value to the next higher value is anything other than an improvement anyway - it reduces hum. The only real exception would be if they're driven by a component (eg a tube rectifier) that can't handle the lower charging resistance of a larger cap. Going to much larger values isn't always a good thing in amp power stages even with a solid-state rectifier, because it changes the dynamics too (makes the amp stiffer-sounding), and might not be desirable even if it's 'technically' better. From those values I assume they're in the preamp supply though, so again no particular reason not to use larger ones if they're available.

Raising voltage rating has no penalty whatever, although it isn't necessary to go too high.