voltage rating of coupling capacitor

Could there be any issues with replacing an aluminum electrolytic capacitor in a coupling position with the same value but a higher voltage rating. I know it "works" as I have done this many times without incident. I was wondering if there are any issues WRT the biasing and forming of the capacitor.

Within reason. A 350V cap working at 250V can be replaced with 450V, no problem. Maybe better reliability, though modern electros are incredibly reliable even at their rated voltage.

There are stories about 450V caps used to bypass 2V cathodes, and failing in months. According to recent literature, that should not happen; maybe it is only true of old production. Still probably a bad idea to go overboard.

This assumes it is just a cap. An electrolytic can act as a Zener and clamp overvoltage. It will start to cook and eventually explode; but apparently this effect has been relied on to protect other parts from short-term overvoltage. It seems like a very dumb idea, especially today when real Zeners and tranzorbs are cheap.

there is a theory that higher voltage caps will change the charachteristics of the audio more than a lower voltage of the same value due to the thickness of the metal foil.. but I have yet to hear this or see the proof myself..

I guess I'm wondering if caps are designed to form at their stated voltage rating. In other words will a cap rated at 350v form properly if it only sees 35v.

Modern 'lytics are much less picky about forming voltage, and their shelf life unpowered is much longer than the old days. I don't think 10:1 is a big problem, but why not (for a 35V circuit) just get a 50V or thereabouts part? 350V 'lytics are big suckers!

[quote author="bcarso"]but why not (for a 35V circuit) just get a 50V or thereabouts part? 350V 'lytics are big suckers![/quote]

I do try to use the nearest rating. This was more of a theoretical question. I've found it hard to find some values is low voltage ratings. So I end up using a 35v part as a replacement for a 6v. I've never encountered a problem but I wanted to see if there could be one.

I haven't tried to use >100V lytics in low voltage apps, but I've used plenty of 50V parts in equipment with ~zero volts and they have lasted indefinitely.

Metalized film caps often require high voltage in order to 'self heal'. They get little pinholes eroded in their films that can be repaired with high voltage across them (got this info from Bob Pease's Troubleshooting Analog Electronics). I'm not sure what the long term implication is for a coupling cap made of say metalized polyester though.

I've alse heard (but never verified) that if you use a high voltage rated electrolytic in a low voltage app long enough that it's dielectric will no longer work for high voltages. Not sure I beleive that one, or we'd really have to worry about storing caps on the shelf without polarizing voltage.

Cheers,

Kris

As a first guess i would stay in a logarithmic kind of order of magnitude...
(shoot me, i was a tech translator and physics student once upon the time...lost almost everything :grin: ).
My personal fist rule would be 'not more than 10 times the original voltage rating' in our audio related domain...
(OK, not very scientific...)

Kind regards

Martin

[quote author="DrFrankencopter"]Metalized film caps often require high voltage in order to 'self heal'. They get little pinholes eroded in their films that can be repaired with high voltage across them (got this info from Bob Pease's Troubleshooting Analog Electronics).[/quote]
As I understand it, the holes get there because of overvoltage spikes - which is why the mains filter caps are self-healing. I don't think it's an issue for normal audio use.

Best regards,

Mikkel C. Simonsen

yep, mcs, i second that, this self healing effect is not important for our uses. read an article about that recently.