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Re: Oil cap questions

9/3/1999 4:38 PM
Re: Oil cap questions
I would say there is no difference, except that the oil caps have an actual capacitance much closer to their nameplate rating than electrolytics, especially electrolytics produced back in the tube rectifier era. The rectifier ratings for peak charging current take into consideration that an electrolytic may have an actual capacity of 80% higher (to 20% lower)than nameplate. So the rectifier can safely handle actual peak currents associated with initial capacitance almost twice the rating given in the tube data sheets. Stating it another way, you could probably get away with using a higher value oil-paper (or poly film)cap than the max stated on the tube sheet. Actually, oil cans work better as a filter than an equivalent value electrolytic, so you wouldn't need to try using a value even as high as the max for an effective filter. But then the higher efficiency is due to a lower impedance path, so maybe the max stated value is actually max for an oil can or dry poly cap, too.  
I wish I knew more about the actual tube design process. All the old-timers that developed the tube types we currently use and try to understand are either dead or don't realize that there are still people who want to know what's in their heads.
8/31/1999 1:00 PM

I haven't been keeping up with my forum monitoring.  
The peak voltage of a sine wave is 1.414 times the RMS value, which is what AC voltages are quoted in. A 480Vdc cap will withstand 480*1.414 = 678Vdc.  
No reforming needed - ever! If they're NON-PCB types, they will say on the can. If they're not NON-PCB, don't buy them. These caps are actually metalized polypropylene film capacitors in an oil of some kind. They work to higher temperatures than electros and last forever in amps.  
They are, as you've found, big, heavy and ugly.
8/31/1999 2:02 PM

"The peak voltage of a sine wave is 1.414 times the RMS value, which is what AC voltages are quoted in. A 480Vdc cap will withstand 480*1.414 = 678Vdc."
Did you mean to say 480VAC not 480VDC?  
I ran across a pile of cheap 370VAC poly caps and wondered if they would work fine in the power supply at 500VDC.  
8/31/1999 2:49 PM
yes they will work fine up to 370*1.41 or about 520VDC. They'll even work higher than that, but up to that point you KNOW you're playing it safe.  
8/31/1999 4:21 PM

Of all the things I miss, it's an output channel better than typing. Yes, 480Vac, not 480Vdc.  
Thanks, Bruce.
9/1/1999 3:06 AM
Pat F
Dang!! I was there yesterday on an unscheduled visit in the middle of the day after my company car tossed a tred on the freeway and I had to get new tires at a place around the corner from my electronics salvage place. I looked at them again, but forgot to see if they are NON-PCB. I'll check again next trip. Oh yeah, what about polarity? I don't recall seeing any polarity markings on them.
9/1/1999 12:57 PM

You won't find polarity markings. In addition to being tougher, better able to withstand heat and ripple current and having no built-in wear out mechanisms, they are non-polar. Hook'em up either way.
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