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Polarity of "mylar" capacitor?

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Posted on July 31, 2020 at 09:42:55
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 I recently purchased the 3nF/4kV capacitor shown in this eBay ad. It is said to be mylar and to have been made in Japan by Kyocera. Since it is supposed to be mylar and therefore nonpolarized, I wonder about the red band at one end of the capacitor body. I even tested one sample in both directions, using my Sencore capacitor tester, and it did work both ways, so I guess I shouldn't worry. If it's nonpolar, why the red line? Thanks.

 

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RE: Polarity of "mylar" capacitor?, posted on July 31, 2020 at 09:56:52
coffee-phil
Audiophile

Posts: 1444
Location: Shingle Springs CA
Joined: January 7, 2010
 Back in the day with paper caps the band indicated the outside foil. I'm pretty sure that is still so with capacitors of similar physical construction.

Phil

 

I think you're right., posted on July 31, 2020 at 10:16:06
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 It just occurred to me, after posting. It's actually very easy to see which lead connects to the outer foil on these capacitors, and the red line is on the end that seems to go to the inner foil. But at least there is some rationale. Sorry for wasting space here. I just wanted to be absolutely sure the caps were nonpolar, even though said to be "mylar", and even though the caps pass my Sencore testing in both directions, up to 600V (the upper limit of the leakage test via the Sencore). In use, they'll be subjected to +/-3000V.

 

RE: I think you're right., posted on July 31, 2020 at 13:20:35
coffee-phil
Audiophile

Posts: 1444
Location: Shingle Springs CA
Joined: January 7, 2010
 It is not a waste of space. I would guess that that is not common knowledge.
With 3000 volts you really want the low side near ground.

Phil

 

RE: I think you're right., posted on July 31, 2020 at 16:35:25
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 agreed... not a waste of space at all!

Not sure what you mean by the low side, but the generally accepted orientation is to have the outer foil at the node with the lowest impedance to ground. This means to the plate of the driving tube as a coupling cap and ground as a bypass cap. "guitar folk" simply hook a cap to the input of an amp and the orientation that shows the least increase in noise when the outside of the cap is touched has the + input connected to the outer foil. An oscilloscope will do the same thing. The idea being that with the outside foil of the cap being at a lower impedance to ground it shield the rest of the cap from stray noise pickup.

dave

 

RE: I think you're right., posted on July 31, 2020 at 20:09:45
Chip647
Audiophile

Posts: 2645
Location: The South
Joined: December 24, 2012
 So if the coupling cap had one end attached to a 50K plate load resistor and the other end hooked to the grid with a 100K grid resistor to ground, which end has the lowest impedance to "ground"?

 

RE: I think you're right., posted on July 31, 2020 at 23:08:19
Tre'
Industry Professional

Posts: 17292
Location: So. Cal.
Joined: February 9, 2002
 The 50k plate resistor is connected to the decoupling cap (last cap in the power supply filter) at one end and that's AC ground. That 50k is in parallel with the plate resistance of the tube. So the plate of the tube is something considerably less than 50k above AC ground.

The 100k grid resistor is connected to the grid of the following tube and, in a circuit that doesn't draw grid current, that grid has no real connection to ground except for the 100k grid resistor.

So the plate is a lot closer to AC ground than the grid.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

I agree!, posted on August 1, 2020 at 13:13:04
Chip647
Audiophile

Posts: 2645
Location: The South
Joined: December 24, 2012
 Cap orientation is really dependent on the circuit. Or if cap orientation is really something to be concerned about at all.

 

Thanks, guys!, posted on August 6, 2020 at 08:24:59
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 What I have actually done is to construct a dummy load for a Beveridge direct-drive amplifier that mimics a Beveridge speaker panel. I measured the speaker, and it is purely capacitative, with 3nF between each stator and the screen, and therefore 1.5nF across from one stator to the other. I connected two 3nF/4kV caps in series with the junction connected to the screen and each free end connected to one stator or the other. (The caps are installed in a well insulated box, and the flying leads are 20kV insulated wires with HV banana plug terminations.) The amplifier puts 3200VDC on one stator and -3200VDC on the other. I am guessing the outer foils of the capacitors on both sides should be connected to the screen, which is held at zero VDC and very low impedance, per Beveridge's unusual concept.

 

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