over voltage on coupling caps.. how much margin?

[diddymix]

Hello people, I was hoping someone could help with regard to their experiences and knowledge on having a coupling cap that has a voltage that exceeds its rating go across it,... and if theres ever any slight margin for the limit on it not failing.. Basically I have a 400V coupling cap I want to use on my amp.. but Ive noticed that in its position when measuring the voltage on the plate of the V1 tube that my amp gets a surge of voltage on start up for about 2-3 seconds and it does exceed this and go up to around 420V but only for that short burst before falling back down to the 200V where it settles and stays on the plate.. Would this still be a no no for using it and not be safe... could I expect it to blow up if I tried?

Thanks

[sluckey]

I use caps rated for 630V and don't worry.

[diddymix]

I use caps rated for 630V and don't worry.

Ayshh I know!! Its annoying the CDE film caps I'm using are only available at Mouser UK and theyre shipping is like £15... I just dont wanna pull that much on a cap... I could just use something else and be happy tho lol

[pompeiisneaks]

Really it's a crap shoot. They do support going over the max rating on SOME caps, that's just the known rating they should never fail at. On average, some would fail the moment it exceeds the ratings, others will last quite some time, (but all of them should meet anything under the maximum to some expected failure rating). But it does guarantee a shortened life of the cap even if it only hits it for a super brief moment. It's always safer/better to overdo the cap voltage rating, to ensure safety and longevity.

~Phil

[roberto]

I agree with Phil.

Another point that I noticed not many consider is the PI coupling caps, where on the other side you have negative bias, that you have to sum on the supply voltage of the stage, to stay on the safe side.

[M Fowler]

Buy your caps from Mike at https://modulusamplification.com/

[Murrayatuptown]

Varies with manufacturer and particular construction, but if they are metallized film instead of film/foil, they are in a better position to 'self-heal' if pinhole arc-overs actually occur. Metallized polypropylene is the best type of polymer film dielectric regarding non-carbonizing 'healing'.

I am studying one particular capacitor type's data sheets and catalogs obsessively for another reason, and for that particular product family, they do have specs on what over voltage ratio and how often (among other parameters) they should handle vs. 'nominal' rating. For that particular part family the brief (DC) overvoltage ratio is significant, something like 150% of 'rated'.

I think I read you are talking about CDE which is a long-standing reputable brand, so they probably have conservative safety margins in their designs.

What keeps me awake at night is thinking about AC ratings of DC capacitors...having worked in the fluorescent ballast inverter industry...At high frequencies and continuous high voltages, you actually have to worry about power dissipation in film capacitors, even polypropylene.

I probably don't need to worry about the brief amount of time there is high voltage AC on coupling caps if not producing continuous sine waves with a guitar (who knows, might be a popular new genre ), but I DO wonder if that has anything to do with bloated vintage paper caps.

Internal construction can vary by mfr. and their particular parts, especially at higher voltages (like 1600-2000 VDC parts), and also X and Y AC line capacitors.

BTW, metallized polypropylene capacitor 'end-of-life' is considered 3% loss of capacitance due to self-healed arc-overs. That's not end of life to me...I wouldn't even suspect a problem if they were 10% tolerance parts!

[roberto]

Study a datasheet is not the same thing of having tested constantly that the specific caps can resist over years at that stress.
I would never suggest to use something out of specs just because a datasheet says that for "brief periods" it can resist.
Also because "gutta cavat lapidem" and a big number of brief periods makes a big period of time.

I would buy from another seller with lower prices, or go for another brand.

[pdf64]

I’m not normally an advocate of the standby start up ritual but as your amp, as typical with Marshalls, has an otherwise unprotected direct coupled cathode follower, to avoid damaged to that tube it would be beneficial to power up in standby, then switch to operational mode after ~45seconds.
That will also remove over voltage stress from your 400V caps at power up.
A trickle charge standby bypass would mitigate the risk of cathode poisoning, eg if you inadvertently left it in standby whilst on vacation
http://www.valvewizard.co.uk/dccf.html
http://www.valvewizard.co.uk/standby.html

[diddymix]

Thanks guys what awesome information. Yeh Im just gonna get some slightly different 600v parts, whilst it might not blow up the first few times I switch on I get a feeling it might not that long down the road. BEtter to be safe! Thanks for all the input guys