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Old 5th January 2006, 07:07 AM   #1
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Default Computer Grade Capacitors

I recently came into posession of some 450 wvdc 3600 uf caps and was wondering if they would be suitable for use in the power supply of a tube amp, or should I get some normal (not soup can) sized electrolytic caps.
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Old 5th January 2006, 09:03 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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That is LARGE! Even a tenth of that capacitance would be a lot...

Depends what you are going to build. If its a PPP 750V KT88 monster, you could probably put a couple in series for 1800uF.

However for most practical purposes they are too big?
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Old 5th January 2006, 09:17 AM   #3
EC8010 is offline EC8010  United Kingdom
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More to the point, they are extremely dangerous. Fully charged, they each store 364.5 Joules of energy. and that's enough to lift someone weighing 80kg (176lb) by 464mm (18").
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Old 5th January 2006, 09:24 AM   #4
Giaime is offline Giaime  Italy
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They are LARGE. If you put them just after a SS rectifier (a tube would die) you'll have LARGE switching noise injected to the mains, disturbing all your equipment.

But anyway save them, useful for a cap-coupled OTL amp.
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Old 5th January 2006, 11:06 AM   #5
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Use them L-C and save diodes
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Old 5th January 2006, 11:37 AM   #6
SY is offline SY  United States
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Check the date code on those before using! Lots of surplus CG types are around that are 20 years old or more, not a good thing.
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Old 5th January 2006, 12:22 PM   #7
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Quote:

Check the date code on those before using! Lots of surplus CG types are around that are 20 years old or more, not a good thing.

Is it possible to reform these older caps or is this just myth? I have never done it so I cannot comment on the percentage of success or failure.
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Old 5th January 2006, 12:50 PM   #8
SY is offline SY  United States
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Any large cap should be reformed before using. Problem with these older ones is that even after reforming, there can be quite a bit of leakage current (that means heat!). And the capacitance may have greatly deteriorated over the years from loss of the electrolyte; reforming will not help this.
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Old 5th January 2006, 12:55 PM   #9
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SY,

It is better, safer to use or replace with new caps? I have read articles about reforming caps in old radios but have often wondered if the best approach might be to hide a newer cap under the old one.
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Old 5th January 2006, 01:12 PM   #10
EC8010 is offline EC8010  United Kingdom
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It's kindest to gently form any large capacitor regardless of age before serious use. All you need is a variable power supply, a DVM with a mA range, and a 1k resistor (to limit current). Connect them all in series with the capacitor and gently increase the current from zero whilst ensuring that it doesn't rise above 5mA. With a large capacitor, it can take some time to reach the capacitor's full rated voltage. Leave it at full voltage, and the leakage current will gently fall. Most capacitor data sheets give an expected leakage current (Rubycon quote the smaller of 0.02CV or 3mA).

But if all the electrolyte has evaporated, then it won't make any difference what you do...
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