A curious observation regarding 2 capacitors of same value

[pjd3]

Hello,

While doing this JCM800 2204 build I will often take the caps to be used into work to run them on the nice LCR bridge, just a curious thing mostly, to see how close to rated spec they are, a look at the dissipation and also Rp, which I assume is parallel resistance, please correct me if I'm wrong.

I"m primarily using Mallory 150's for coupling caps but, I did pull out an old big yellow Astron .02 600 volt cap from a very old receiver. I looked at it on the LCR bridge and it appeared to be something useable, was around .023uF, has a dissipation of around .02 and the Rp was a few hundred kilohms, all this at 1kHz/1 volt.

However, with all the Mallories and this old Yellow astron cap soldered onto the board, I decided for shits and giggle to throw the Fluke ohm meter across a Mallory 150 and then the old Yellow Astron. What I observed was that Fluke digits took less than a second to run up in resistance and go to infinity, which, is what I expect. You then reverse the polarity of the meter probes and it repeats the same thing. However, when doing this with the old Yellow Astron, it took around 3 seconds for the Fluke to quickly ascend through the digits and go to infinity. Reverse the meter probes, same thing, at least 3 seconds for the cap to charge till the point of the meter seeing an open circuit.

What would you suspect would cause the Yellow Astron to take 4 times as long to look like an open circuit to the Fluke meter if it measured approximately the same as the Mallory in terms of capacitance? These caps are all the same value, the .022 or .02 uF coupling caps. Does this timing difference tell you anything specifically about a particular characteristic of one cap vs the other? A couple of things come to mind but, I'm not yet able to rely on my theories as fact as of yet.

Thanks for taking time on this, I just want to make sure there isn't some bad characteristic that isn't obvious that would reak any havoc on perceived amp tone once this is done.

thank you!
Best,
Phil D

[thetragichero]

my guess is that since you mentioned they were soldered to the board the astron has other components connected to it that are effecting the readings?

[pjd3]

Hey, thank you for your input.

And sure, most often I would never attempt to measure a component in circuit but the ends of the cap were not yet connected to anything. I haven't got to attaching the wiring as of yet. Thanks for that reminder though, could bite someone in the ass if they weren't aware of that.

Sometimes I just toss an ohm meter across a cap just to make sure there isn't any conduction or shorts in the cap. I just look to see if the digits ramp on up to infinite ohms. And of course the time it takes is generally a function of the cap value. That's why I was taken by the fact that the old Astron took many times longer to reach infinity than the nice new Mallory 150's. The things that come to mind without really knowing would be: 1. a long dielectric absorption time. 2. A higher series resistance. 3. A DC leak making it take longer to charge. These are only speculation, just using imagination to think of what a cap is, and what things would cause that much more time. And sure, it could be the input impedance of the Fluke meter reacting with some impedance of the cap itself. I"m not entirely sure of all the characteristics that can change in a cap when it gets that old. But, people seem to be very content leaving these old Astron yellow caps in their vintage amps, and even pay good money to get NOS caps of the same type to install into brand new amp builds. I just thought it would be cool to have one old, moldy, NOS'y, grandpa of a cap in there. Ha. I may be only fooling myself. I had ordered 6 Sozo's .022uF 630V caps from Amplified Parts but they were out of stock at that point, well, just last week. I will probably play the amp for a while with the Mallorys and then try some Sozo's just see if I experience any difference.

So lets see if anyone else has some insight into this cap thing.

Thanks!
Best,
Phil D

[R.G.]

It would be useful to get out the manual for the LCR bridge and read up on what "Rp" means for that meter. If it's parallel resistance as you guess, that might amount to DC leakage resistance, and a couple of hundred Kohms may well mess things up if it's used as a series coupling cap to a grid. It's worth checking out. Or it could be that the LCR meter is not as good at measuring leakage resistance as the Fluke.

[pjd3]

Thank you RG,

I haven't yet found the manual to the LCR bridge but did take some to measure Rp on all the types of caps we have here in stock at work. (electrolytics, tantalems, films, multi-stack ceramics) and they all measured from 10's of megohms to many in the 100's of megohms. A little research on line has led me to believe that Rp is in fact reflecting parallel resistance in a cap or components.

Where my cool little ol' yellow Astron was a mere 200K to 300K ohms, I think I'll just have to give up the antique NOS mojo nostalgia for a known reliable cap! I've read some story's of the things that can happen using caps with leaky DC. Doesn't sound like any fun.

Thank you again RG, as I move through builds I like to use each one as an opportunity to become more familiar with the characteristics and vicissitudes of the various components. It does scare me a little that a bad component will get in there and I'll be scratching my head not knowing where to look for the trouble for long extended lengths of time. This happens to be for a 50W JCM800 2204 build.

Best,
Phil D.

[R.G.]

Another thought - talking about hundreds of megohms brings up a nagging thought from the back of my head. You might want to wash the old cap in soap and water, dry it, rinse it with isopropyl then dry again, and re-test. In the really high resistance ranges, surface contamination can be bigger than the internal resistances.
Maybe.

[pjd3]

RG, I am going to do just that. I made the decision to replace it with a good new Mallory 150 so, there's the opportunity to see if that makes any difference.

I'm entertaining the idea of setting up a circuit with some high DC voltage and a resistor, apply the voltage and see if I can measure any potential across the resistor. A leaky cap or Rp would involve some kind of current moving through the cap I believe. Should that be so, there ought to be a voltage across a resistor in series with the cap. I think. Why not.

Thanks for the idea on washing the cap. The units that I pulled these caps from smelled like a basement flood that happened in the 50's! So, it would be no surprise that layers of contaminants a grime are basting the surfaces of these components.

Best,
Phil D.