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Author Topic: vintage high voltage mica caps  (Read 2487 times)
WA6NEQ
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« on: June 04, 2011, 12:47:01 AM »

I recently came into posession of about a dozen old high voltage mica caps. They are rectangular shaped, typically about 1.5 inches tall, about 2 inches wide, and about 3/4 inch deep. They have threaded studs on top. The body appears to be a brown colored phenolic or bakelite material. The body also has mounting holes on the base as if it was intended to be mounted to a chassis. The capacitance values range from 50pF to 0.001uF. Voltages range from 1.5KV to 5KV. 

In addition to the voltage and capacitance rating, they also have ratings for current (from 1.2A up to 7A) and frequency (from 1000 KHz to 3000 KHz).

Any idea what the intended application was for these? Are these suitable for use in an amplifier where you normally see a doorknob cap to couple RF from the plate to the pi-network? (I would think that the plate would be a high voltage, low current point in the circuit). Why the freq rating?

73, Bill
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KA5N
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« Reply #1 on: June 04, 2011, 02:22:36 AM »

Likely military surplus, WWII vintage.  Whether they are still usable depends on how and where they have been stored.  The spec for current is probably for RF current (you don't have DC flow in a capacitor!) and the plate circuit is HV DC and high RF current.
Keep them as a curiosity and not as substitutes for modern components.
Allen
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K2DC
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« Reply #2 on: June 04, 2011, 09:28:57 AM »

They might not be that old.  I believe mica caps like that were available at least into the 60's.  And as long as the encasement material is in good shape they should be just fine.  Mica is a mineral that was shaved or milled into fine layers to be used as the dielectric material in caps.  The stuff is already millions of years old, so it doesn't matter when they were made as long as the outside is sound.

73,

Don, K2DC

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WA6NEQ
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« Reply #3 on: June 04, 2011, 12:15:52 PM »

Thanks for the input. The case is actually two pieces. The main body and a plate on the bottom of the same material. On one of the samples the bottom plate has come apart from the body showing that the body is filled with a black material that looks like tar. Being potted in tar, my guess is that they are hermetically sealed. I'd attach a phot if I could figure out how.

Bill
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W8JI
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« Reply #4 on: June 05, 2011, 04:22:51 AM »

It all depends on what you mean by "door knob caps". Some door knob caps are totally unsuited for high current RF appliciations, and some are OK.

As a general rule all mica caps are pretty good for RF systems.  Based on your description the caps you have are fully suited for padding loading capacitors or lower voltage modest current applications where the signal or any harmonics that must be attenuated are at lower frequencies.

I wouldn't hesitate to use them padding the loading capacitor in an amateur power level amp. I wouldn't use them anywhere else though, except perhaps in a lower voltage antenna matching network.
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G3UUR
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« Reply #5 on: June 15, 2011, 02:53:42 AM »

Bill,

In answer to your question about why current ratings are necessary; for RF applications both the effect of ohmic (I2R) loss and dielectric loss need to be considered in capacitors. The balance of dielectric loss to ohmic loss changes with frequency and to keep the total loss within acceptable limits the capacitor current rating needs to be reduced as the frequency is lowered because the RF voltage across it goes up for a given current and increases the dielectric loss contribution.

I don’t agree that the plate coupling capacitor is a low current application. You shouldn’t assume that the capacitor from the plate to the tank circuit or the HV decoupling capacitor in an RF power amplifier is operating at a low RF current. The coupling capacitor current is always going to be higher than you calculate from the tube optimum load value and power output. It will be many times this value on 10m and 160m, for different reasons, and even on 40m where it’s likely to be lowest, the RMS value will still be about 50% more than a simple calculation would suggest.

From your description of these capacitors, they sound like the old WWII ones that were used in tuning units like the TU series for the BC- 375 and BC-191, but could be of more recent manufacture as Don suggested. In any event, I’d do an ESR test on them to see if they’re any good before using them in any RF power applications.

Dave.   
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W8JI
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« Reply #6 on: June 15, 2011, 07:58:55 AM »


I don’t agree that the plate coupling capacitor is a low current application.

I couldn't find anything where anyone said that, unless I missed it.

Quote
You shouldn’t assume that the capacitor from the plate to the tank circuit or the HV decoupling capacitor in an RF power amplifier is operating at a low RF current. The coupling capacitor current is always going to be higher than you calculate from the tube optimum load value and power output. It will be many times this value on 10m and 160m, for different reasons, and even on 40m where it’s likely to be lowest, the RMS value will still be about 50% more than a simple calculation would suggest.

True enough. The tube is not a sine wave load on the tank, and so the high peak currents cause heating to be much higher than expected. Plus reactance of the choke is somewhat lower on 160, and the shunt reactance of the tube is really low on 15 and ten meters.

Those mica caps though, as a general rule, make really good padding caps for the loading. Based on the description he gave, they would probably be pretty good at amateur power levels. Based on my experience with them, I would never trust one for a blocking cap or padding the plate side of the tank.

73 Tom
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G3UUR
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« Reply #7 on: June 15, 2011, 08:33:53 AM »

Hi Tom,

Either you missed it, or I misinterpreted it! Take a look at the 2nd and 3rd lines of the very first message posted by Bill.

I don't disagree with your advice on where to use these capacitors, but I would check them out first just to see what condition they're in now.

73,

Dave.
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G3UUR
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« Reply #8 on: June 15, 2011, 10:21:49 AM »

Sorry, I should have said take a look at the 2nd and 3rd lines of the last paragraph of Bill's first message on this thread.

Dave.
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W8JI
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« Reply #9 on: June 15, 2011, 02:22:50 PM »

Sorry, I should have said take a look at the 2nd and 3rd lines of the last paragraph of Bill's first message on this thread.

Dave.

Oh, I see now. I'm lazy and look for requotes in replies. :-)

Heck, a DM19 or CM19 sized silver mica will work for a loading capacitor padding cap up to several hundred watts. Those big micas are better than you think, except with some age on them I don't trust the voltage. :-) I always prized them when tearing apart WWII surplus stuff.
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G3UUR
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« Reply #10 on: June 16, 2011, 07:38:53 AM »

Quote
Those big micas are better than you think, except with some age on them I don't trust the voltage.

Unless I'm mistaken, I don't think I've voiced an opinion on how bad or good they might be, only that I think it would be wise to check them first before using any of them. Even silvered mica capacitors are not perfect. They have defects in the structure of the mica and cleaving the sheets introduces some damage in places, which the manufacturers make every effort to avoid but they can't be 100% sure of weeding out all the damaged areas. The silvering might have gone down into the defective or damaged regions of the mica sheet and compromised the voltage rating. Internal corrosion can also occur in old components, even if they are sealed, because the potting compounds and casings are not 100% pure and provide the necessary elements for corrosion to take place. So, the integrity of their electrical connections may not be that great now, either, and the ESR may have risen.

I'm new to eHam, so haven't quite gotten used to the way things are done yet. In fact I'm not sure whether I've used the 'insert quote' facility properly, or not. I'll find out when I post this.

Dave.
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