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Old 26th October 2005, 12:37 AM   #1
Tubesin is offline Tubesin  Macao
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Question PIO caps question

A question of PIO capacitors on coupling. I found that the caps coupling from driver tube at 320vdc to the 300B in cathode bias, the grid shows a positive voltage on 3 volts! (it shoud be zero), but the sound seems OK, does this will destroy or impair the full function of the power tube or the sound? More, even between voltage tube to driver tube, the cap a 0.lmf/630v coupling from 150v also shows up 1.2v on the grid of the driver tube also in cathode bias. It concerns me on using the PIO caps because the polysterene type doesn't shown like that!!! Please help....
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Old 26th October 2005, 01:36 AM   #2
anatech is offline anatech  Canada
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Hi Tubesin,
If the other caps show no positive voltage, then the current ones are leaky. Get them out of there before you ruin your tubes.

-Chris
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Old 26th October 2005, 03:06 PM   #3
Tubesin is offline Tubesin  Macao
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Thank you Chris, that means it is dangerous to let it run like that.
It is strange, these oil caps of good reputation brand and happen this fault, I lost money trying to upgrade. So, anyone please take care on buying paper in oil caps on this concern. But, when ordering through the mail, I have no way to check its condition. Before soldering to the circuit, I used the 10K setting on the ohmmeter to check it n found only very slow discharge to zero, but after soldering it shows up leaky voltage!!!!
Do someone have this nasty experience?
I try to return to seller, but, they say I passed the warranty time!!
Anyway, loss big bucks...pity
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Old 26th October 2005, 06:01 PM   #4
anatech is offline anatech  Canada
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Plastic foil caps perform well. Older technologies should be avoided.

-Chris
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Old 27th October 2005, 12:51 AM   #5
Tubesin is offline Tubesin  Macao
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Yes its old technology, but, in their ads they claim using modern tecnique and materials like copper foil in impregnated oil dielectric and copper tubing and silver leads out!!!!!! And these are from a reputable manufacturer in the audio world!!!!!!
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Old 27th October 2005, 01:16 AM   #6
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Hi Tubesin,
I hate to say this, but sometimes the higher the price, it's more whatever will make it sell. The "sound" of older materials sometimes suggest a colouration. Or just nonsense.

-Chris
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Old 27th October 2005, 01:42 AM   #7
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Yes its old technology, but, in their ads they claim using modern tecnique and materials like copper foil in impregnated oil dielectric and copper tubing and silver leads out!!!!!! And these are from a reputable manufacturer in the audio world!!!!!
I really hate to tell you this, Tubesin, but you wuz had. Paper-in-oil is just so 19th century! Even by the early 1890's, PIO capacitors had largely been abandoned in favor of the much higher Q mica capacitors for just about everything, except for apps requiring very large capacitors (motor start/run caps.). PIO's have always had potential leakage problems. Not so critical for a motor start cap, but disasterous where you're counting on these things to protect your precious VTs. Sure, the 300B's might not mind that slight positive bias now, but rest assured, this situation will not improve with age.

When various capacitors were actually put to the test, PIO really did not test out all that well. Perhaps better than ceramics (e.g. Sprague "Orange Drops") but not by much. You'd do considerably better with metal film polypropylenes, such as AuriCaps. (And, no, I'm not connected with them in any way, shape, or form, other than as a satisfied customer.)
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Old 27th October 2005, 02:35 AM   #8
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This is the only test I could find comparing PIO to other caps. Rudimentary for sure:

http://members.aol.com/sbench102/caps.html

I'd be interested in seeing others.
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Old 27th October 2005, 02:58 AM   #9
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Interesting. Still, the polyprop's come off pretty good. However, I haven't seen any PIO's that didn't show some resistance when checking them out with a digital multimeter. That usually came to about 1.0 -- 3.0MEG. Just enough to make a nice voltage divider between a plate and next stage grid resistor. Use the PIOs for PS bypass caps, not interstage coupling caps.
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Old 27th October 2005, 01:12 PM   #10
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Yes, and when the PIO's start to run warm, yank them. They will eventually dry out in a few years anyway. Solen Fast caps might be the way to go, but any decent plastic foil will make you much happier.

Miles,
Sprague "Orange Drops" are metalized plastic are they not? Generally I avoid ceramics all together.

-Chris
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