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Old 8th September 2005, 10:12 PM   #1
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Default are "super pup" caps any good?

A local guy has a small box of CDE 'super pups'. They are 2uf__ 200vdc__ #MPY 2w2. Are they worth grabbing for $0.25 us each or so? What are they made of?? They have a paper outer cylinder but what's inside.??

Thanks in advance..
Marc
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Old 8th September 2005, 11:11 PM   #2
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Probably leaky paper caps. They tend to absorb moisture.

You can check them with a DC supply more than 80V and a neon light. Connect the neon in series with the capacitor and connect up the power. The neon will blink once as the capacitor charges to somewhere around +V, minus 10 to 60V (depending on the neon light and amount of current pulsed). If it blinks at all within thirty seconds, the cap is leaky and should be discarded. (At 1uF and around +120VDC, this represents 30Mohm. 100V across such a leakage followed by a 220k will put 0.7V on the output, a generous portion of an audio signal.)

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Old 9th September 2005, 12:11 AM   #3
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FWIW, if you can measure the peak voltages in such a circuit, the exact formula for determining average leakage resistance is:

R = t / [C * ln (Vs-Voff / Vs-Von)]

Where R is the effective leakage resistance, C is the capacitance in farads, t is time between blinks in seconds, Vs is supply voltage, Von is the turn-on (highest peak) voltage of the neon and Voff is the minimum (lowest peak) voltage of the neon, when it turns off.

The natural log comes from taking the inverse of the RC exponential curve.

Typical values I would expect:
Vs = 200V, Von = 80V, Voff = 40V, C = 0.1uF (0.0000001 F), t = 60s, R = 2 x 10^9 ohms (2 gigaohms).

At that rate, some careful watching of a VOM should give accurate enough voltages for high precision with this formula.

Oh, and if you aren't using those caps for much DC, or very slow AC (low frequency oscillator for example), the leakage won't matter much. They'd probably be great for speakers.

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Old 10th September 2005, 04:02 PM   #4
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Thanks ..

Any opinion on thier use as a cap in the signal path... as in a preamp etc. I didn't have in mind DC use.
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Old 12th September 2005, 08:12 PM   #5
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Well there are only three things a preamp would want with a cap of that size, coupling, bypass or tuning a bass filter. Only the bass filter probably has no issues with leakage. The others depend.

If you mean like, how they sound, yeesh...

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Old 13th September 2005, 01:14 AM   #6
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If you mean like, how they sound, yeesh...
well then..I guess I won't buy any more! Thanks for yer input!
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