Measuring old PIO Capacitors accurately

I am looking for a means to measure old PIO capacitors accurately that may have some leakage, where a Fluke or other typical DMM cannot read them accurately. Not looking for any expensive bench test unit, but more towards a small simple accurate means to see what the value actually is on a few old PIO caps. My Fluke is fine and accurate for film and others but is reading PIO caps differently every time I try to measure them and from what I have found out, reads them way too high. Just want a simple inexpensive means to measure PIO caps with some accuracy without buying some expensive bench test meter.

Welcome to the forum.

Since oiled paper dialectic capacitors were considered obsolete well over 50 years ago I doubt there is any inexpensive modern test gear that can test for their unique problems.

Also be aware many capacitors that used oil in the dielectric contain polychlorinated biphenyls (PCBs).

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Hello tmicsa1,

Are these little capacitors? Here the term little implies a 400 to 600 VDC rating. Such capacitors would typically be used for coupling between vacuum tube amplifier stages.

Are these larger capacitors? Here large implies a 600 to 3000 VDC rating. These larger capacitors would be used in the power supply of the vacuum tube amplifier.

Is this something else? For example, large capacitors used for power line applications.

With regards to leakage, the capacitor is usually analyzed at full rated voltage. As you suggested, they may measure as acceptable with the Fluke but may change at the higher voltages.

I’m not aware of any modern low-cost solutions. Also, there is a balancing act between safe, simple, and inexpensive. For example, we can describe a simple and inexpensive circuit, but it will not be safe.

The best I can do is recommend you watch online auctions or HAMfests and purchase:

  • Spraugue TO-6 or TO-4
  • Sencor “Z-meter”

Either of these instruments will get you to 600 VDC. However, be careful. These units were manufactured in a different era. Safety standards have changed: 600 VDC at the end of an open wire is considered unsafe today.

There are also a few videos on YouTube describing some DIY solutions. Again, there is that balance between safe, simple, and inexpensive.

Sorry, I couldn’t give you a better answer.

Best Wishes,


P.S. Out of curiosity, what is your application for these capacitors?

P.P.S. For reference, from my collection, here is a picture of two “little” paper in oil capacitors:

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Thank You.
These would be consisdered “little” as in your pictures of the sprague and pyramids. Consequently they would be Luxe reproduction caps as copies of older “bumble bee” caps made by Sprague in the value of .022 MF. (uf).
Problem is that the maker insists that they are a matched pair measuring that listed value but my FLUKE meter shows them reading upwards of .08 for one and over .1 for the other. So I am looking for a simple means to verify their value through a more reliable means than my FLuke, which through its process of estimation seems to be quite off the mark.

400V rating

I’d ask the manufacturer about what gear they recommend for measuring their performance.

With boutique ultra-high cost reproduction components, the manufacturer should have a lot of technical info they are willing to provide.


Hello tmicsa1,

Do have access to test equipment? Specifically, do you have a function generator, oscilloscope, and power supply? If you do, there are a few experiments you can perform that will give you a reasonable comfort level with regards to capacitor matching:

  • 555 timers: Construct a 555 based timer circuit using the capacitor as the timing capacitor (C1) as shown in the schematic below. The frequency of the oscillator is a function of capacitance. You will be able to perform A / B comparisons to determine the quality of the capacitor matching.
  • Rise time: Construct a series using a series consisting of a resistor (10 kΩ) and the capacitor. Drive the circuit with a function generator and observe the results with an oscilloscope. A digital oscilloscope would be particularly beneficial as you could store the waveform allowing easy A / B comparison.
  • Resonance: Construct a parallel circuit using an inductor and the capacitor. Drive the parallel circuit with a function generator via a 1 kΩ resistor. Adjust the equipment so that you can find the resonance. If you knew the value of the inductor you could calculate the value of the capacitor. However, as I understand A/B comparison would be sufficient.

On a related note, I will assume you are using these capacitors in a push-pull vacuum tube amplifier. The precise value of the capacitor may not be that important relative to the other variables in the circuit e.g., the phase splitter stage or the differences in power amplifier tubes. Occasionally we find AC balance potentiometers in these old circuits. This allows the load resistance in one arm of the phase splitter to be changed. This is not a common feature. It’s also rather difficult to adjust as you typically need access to a distortion analyzer. There is also a question of matching at a particular frequency vs a flat response across all frequencies. Sometimes it’s best to let Harold Black’s negative feedback take over and just enjoy the music.

Best Wishes,


P.S. This does nothing to measure the leakage you enquired about earlier.

Picture of DigiKey's online 555 timer calculator.