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Old 8th March 2012, 12:44 AM   #1
bichela is offline bichela  United States
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Default Capacitor WVDC. is higher really better?

I am going to be doing three different amp projects. one is a recap of an older tube amp in a Sony TC-200 (first version), recap a working mono Philips 4 track unit and the third is my 6BM8 Push Pull amp (Fire and Ice).

I need to buy caps for all three. but I only want to deal with one set. the caps I was told it is fine to go with higher than the rated voltages. in the TC-200 it shows that for the power supply filter caps are 3 100uf 180WVDC whereas in my tube amp (fire and ice) caps are needed at 300VDC ( 25uf, 25uf and 40uf))

Here is my question. is it wise to use higher WVDC caps in a circuit? One guy says no as the electrolytic won't be stable. Another says "Not a problem". I want to order the needed caps for the projects in larger quantities. Oh yeah, in my philips tube reel amp they are 35uf, 35uf and 50uf at 350WVDC. therefore I was looking to getting 400-600WVDC instead of having lots of voltage versions.
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Old 8th March 2012, 01:10 AM   #2
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It's both wise and recommended. Be conservative (about voltage and temperature ratings).

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Chris
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Old 8th March 2012, 02:59 AM   #3
kevinkr is offline kevinkr  United States
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Modern electrolytics are much more stable than those of yore and operating a high voltage cap on a significantly lower voltage should not cause an appreciable reduction in capacitance.

As long as proper voltage derating guidelines are followed the real cap killer is temperature. 105C types will generally live a lot longer at any given operating temperature than a device rated for 85C. (And a 10C reduction in cap temperature doubles life - so keep as cool as possible.)
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Old 8th March 2012, 04:49 AM   #4
bichela is offline bichela  United States
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Quote:
Originally Posted by kevinkr View Post
Modern electrolytics are much more stable than those of yore and operating a high voltage cap on a significantly lower voltage should not cause an appreciable reduction in capacitance.

As long as proper voltage derating guidelines are followed the real cap killer is temperature. 105C types will generally live a lot longer at any given operating temperature than a device rated for 85C. (And a 10C reduction in cap temperature doubles life - so keep as cool as possible.)
And what are these guidelines exactly? As for "keeping cool" the Phillips is a plastic heat box I don't think I can do much about that. Sony has a good air flow as would my amp design. Yes, I am speccing 105c parts
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Old 8th March 2012, 05:13 AM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by bichela View Post
And what are these guidelines exactly? As for "keeping cool" the Phillips is a plastic heat box I don't think I can do much about that. Sony has a good air flow as would my amp design. Yes, I am speccing 105c parts
The manufacturer will generally provide derating guidelines in the data sheet or an app note - if not I would not exceed 80% for an aluminum electrolytic.
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Old 8th March 2012, 05:21 AM   #6
bichela is offline bichela  United States
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Okay so a 400wvdc cap at 160 would be okay as using the 70%. Or 280 means 120vdc would be the "floor"
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Old 8th March 2012, 07:18 AM   #7
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Cornell Dubilier say 50% voltage derating is common in space and military applications because it extends capacitor life. So obviously no harm in using a higher rated cap at lower voltages.
The only time to worry is if the ripple current is very high and a higher voltage cap is replaced where a same size but lower voltage cap has been used. The higher voltage rated cap will have lower capacitance and probably lower ripple current rating. This isn't applicable in your situation.
I read some paper by a chemist that using an electrolytic at lower voltages for a long time, essentially created a new rating for that cap. The cap would need to be reformed if it were to be reused at higher voltages.
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Old 8th March 2012, 07:52 AM   #8
M Gregg is offline M Gregg  United Kingdom
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Just a thought,

If your caps are close to working voltage and you have a heater supply fail or your fuses are after the caps on the HT rail. Then the increase in the B+ with no load could exceed the WV of the caps creating a capacitor blow out. I don't think you should ever have less than B+ off load working voltage.

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Old 8th March 2012, 08:30 AM   #9
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Quote:
Originally Posted by M Gregg View Post

I don't think you should ever have less than B+ off load working voltage.

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M. Gregg
+1

Especially if you cannot entirely rule out that you will create off-load conditions in case of a failure (and in most cases you cannot rule this out).
Easier to buy a higher voltage cap (or use them in series) than to remove all that gooey junk in your precious amp after a blow-up.

Martin
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Old 8th March 2012, 09:07 AM   #10
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In the first post he says the highest value cap to be replaced is now 350WVDC. He plans to get 400WVDC caps. So isn't he covering his *** pretty good?
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