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Old 19th March 2006, 11:29 PM   #21
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Can anyone comment on these:

http://cgi.ebay.com/ws/eBayISAPI.dll...Pr4_PcY_BIN_IT

Supposedly they are Sprague brand motor run caps. I bought a few 50uf and 7.5uf as they met the specs.
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Old 21st March 2006, 01:01 AM   #22
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I just checked one of the 100uf ASC's out of the case of 25 that I got from Midori last week. Shaved one of the terminals with an exacto knife to see what was under the plating. The terminals are brass.

On the DC voltage rating they may be OK for higher than the 525 that Midori specs them for (your on your own if you try this...). Basis for this statement is based on the voltage rating for the last batch of 40uf 440 volt GE caps (97F5116F) I got from Allied. The GE's are rated for 440Vac and 1000Vdc, a ratio of 2.27x, quite a bit higher than the 1.414x usually used. Could be that DC operation is much less stressfull than AC operation.

Using the same ratio on the ASC's that GE uses the 370 volt caps convert to 840 volts DC.

I've also got some GE 8uf (23F1039G2) caps that are rated for 660Vac or 1500 Vdc which calculates out to the same 2.27x ratio.

I'm going to stick a pair in my modified Stromburg-Carlson PP 6550 amps that are doing subwoofer duties at this time. They run a B+ of 635V. We'll see if any smoke comes out...

Gary
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Old 21st March 2006, 02:06 AM   #23
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In the original intended application (motor run) the cap is used to create a phase shift for one of the windings in an AC motor. Some serious AC current flows through the capacitor. There is also a voltage across the capacitor. Since we now have a capacitance and an inductance (the motor winding) in series across the AC line, we have the desired phase shift. This means that the voltage and the current on the capacitor are not in phase with each other. It also means that 1.414 does not necessarilly apply.

Back to tube land. In a tube amplifier the first cap in the power supply has to deal with all of the ripple current and usually sees the highest voltage so it gets the most stress. The last cap in the power supply in a SE amp sees all of the signal current for both channels. These currents are in phase and add up for bass signals. The last cap in the power supply for a P-P amp provides the peak current demands for both channels. The parafeed cap sees the signal current for one channel.

The point here is that the lower the current demands are on a capacitor, the higher the voltage that you can apply to it. Just where the limit is is hard to say. Be warned that things may get ugly when the limit is found. Motor run caps are made to handle high currents, and thus have a low ESR (that is why we like them). If the power supply can source a lot of current (big transformer, SS rectifier) it is possible for the cap to explode.
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Old 21st March 2006, 02:24 AM   #24
rdf is offline rdf  Canada
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Quote:
Originally posted by tubelab.com
If the power supply can source a lot of current (big transformer, SS rectifier) it is possible for the cap to explode.
Hmmmm, something about your sig gives me the impression you didn't write this 'in theory'.
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Old 21st March 2006, 01:52 PM   #25
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I personally saw a 10 KV oil cap short, blow one of the terminals off and spray steaming hot oil all over the inside of a 3KW Gates shortwave transmitter. There was no other fault condition. Just a 20 year old cap that did not like being awakened form a long sleep. I was across the room (at least 20 feet away) and it still scared the **** out of me.

I have seen my share of electrolytics explode, but I have not seen a motor run cap fail catastrophically........Yet. The one in my air conditoner just died a silent death.
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Old 21st March 2006, 04:06 PM   #26
rdf is offline rdf  Canada
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Never had the dubious pleasure of seeing one go but once cleaned up the aftermath. A case of lightning strike meets Harris MW-50, a nasty transmitter with a 100 kwatt, 20 kV DC supply.
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Old 21st March 2006, 07:41 PM   #27
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Everybody's been discussing these wonderful PS and bypass caps, what do you guys use for coupling caps?
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Old 21st March 2006, 09:14 PM   #28
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Quote:
Originally posted by leadbelly
Everybody's been discussing these wonderful PS and bypass caps, what do you guys use for coupling caps?
I like AuriCaps. I replaced a 1.0uF electrolytic coupling capacitor in a solid state amp I built with a 1.0uF AuriCap, and that made a big difference in how it sounded. Much clearer and more detailed. Although that isn't always a good thing in that every defect in a bad mp3 now comes through loud and clear. Even some of the early CDs I have sound a bit "off".

Even though AuriCaps are a bit more expensive than your run-of-the-mill capacitors, they aren't outrageously so.
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Old 21st March 2006, 10:46 PM   #29
Sherman is offline Sherman  United States
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Quote:
Originally posted by leadbelly
Everybody's been discussing these wonderful PS and bypass caps, what do you guys use for coupling caps?

I'll second the Auricap. I put them in my amps and really like them. I replaced the Solen coupling caps in another amp I built with Auricaps and found an improvement in doing so.

Nice quality and not too overpriced.
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Old 21st March 2006, 10:51 PM   #30
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Thanks for the tip. Any fans of transformer coupling in the audience? How does iron compare to boutique caps for coupling?
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