• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

PSU capacitors/electrolytics

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lndm

Member
Joined 2006
Hey jaymanaa, good luck with the searching! Sorry I can't help you with the case isolation issue. I sometimes ask questions which noone seems able to answer.

Anyway, one things for sure, any 370VAC cap will handle 520VDC. When I first posted to this thread, I looked up specs for some motor caps. It seems some can handle well over twice the AC rating (but for short bursts). I feel the way to be sure would be to read the specs for your actual brand.

Cheers
 
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jaymanaa

Member
Joined 2006
Thanks indm, you have been a great help throughout this first stage of my first amp. I hope that someday I will be able to help someone getting started in DIY tubes. I have a good source for motor runs through a supplier I use at work. I will see if they can supply some more specific data. If so I will post it here. Thanks Again, Jay:cool:
 
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jaymanaa

Member
Joined 2006
Well I did some experimenting with a 500v meggar. I tried Mallory, GE, Mars, and Eccol motor run caps rated at 370 and 440v. Measuring from either terminal to case @ 500v the ohms readings on all was infinity (no leakage to case). This would indicate to me that direct chassis mounting would not pose a problem. I think I will disect one just for curiosity's sake. None have any markings indicating neg. or pos. so I assume they are not polarity sensitive.:smash:
 
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AndrewT

R.I.P.
Joined 2004
Hi,
I think I have read that the case of electrolytics should be assumed to be near negative in event of failure. Not sure who's site I saw that on.
But here is a quote from United Chemical
"Insulation From The Aluminum Can

The aluminum can is not insulated from the cathode, and when the internal element needs to be electrically insulated from the can, capacitors specially designed for these insulation requirements should be used. Also, the dummy terminal is not insulated from the cathode and must not be connected electrically to the anode or cathode. "

Hope this helps.
 
kevinkr

kevinkr

Administrator
Joined 2004
kta-hifi.net
Paid Member
Definitely true for electrolytics, not relevant for motor runs which are non polar film caps designed for ac line operation. The case is well isolated from both capacitor terminals, however it is not a bad idea to ground the case for safety in the event of an internal insulation breakdown.
 
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