Wax Paper Capacitor Test

[salvarsan]

I've repaired a small few old guitar amps from the 50's and wondered why wax paper capacitors have such an effect on the overall sound.
tested a wax paper capacitor marked as .05 uF/400V (= 50 nF) and found it was 25% high (at best), very lossy at low frequencies, yet much better above 1kHz.

As an audio coupling capacitor, you would use it for its pathological behavior, not its fidelity. Since the DF decreases so much with frequency, it might do okay as a voltage spike snubber.

In contrast, a polystyrene capacitor is a nearly pure capacitance at all audio frequencies, its 0.5% DF measureable only at 100kHz.

F (Hz)	Cs (nF)	Cp (nF)	DFs %	DFp %	ESR (ohms)
100	92.95	85.54	30.0	30.2	5150.00
120	89.24	83.01	27.9	28.2	4160.00
1k	69.74	69.19	9.7	9.8	0.22
10k	65.25	65.21	3.1	3.1	7.70
100k	63.13	63.09	2.0	2.0	0.53

• F = test frequency
• Cs = serial capacitance
• Cp = parallel capacitance
• DFs = serial dissipation factor
• DFp = parallel dissipation factor
• ESR = equivalent series resistance
• Rp = parallel resistance
• tester: UNI-T UT612 LCR meter
• 07/22/2013

[Enzo]

Of course those are 50-60 year old caps. We have no new wax caps to compare.

[salvarsan]

Consider also that the guitar tone filter rarely has any C components that pass signal, rather they are used to filter high frequencies out. Caps that might sound lousy as pass elements could be the bees knees for a tone filter.

IMO, tweaking a guitar tone cap is an expensive exercise in rearranging minor results.

In these old amps, the bass roll-off in the coupling caps gives it a brighter sound ... until it's in early distortion mode, in which case it has some appeal.

Many of the 70-100 yr old patents on capacitors (AKA "condensers") use paper without impregnation (no wax, oil, tricky organic compounds) yet seal the device in wax. If you've potted pickups, you know that the wax doesn't penetrate deeply. Simply, what we call "wax paper capacitors" are as often as not "waxed, paper capacitors" -- they were paper dielectric devices sealed in wax.

High quality paper caps in the .001 to .1 uF @400-630V are still made and inexpensively available. Look for names like Evox, Rifa, Wima, Kemet.

[R.G.]

I have a book I bought from used book seller titled something like "Power Capacitor Technology". I bought it thinking it was a textbook on electrolytics. It wasn't.

It was a history and treatise on power caps - the real ones, used for mains power purposes. It contained the history all the way back to telegraph days, and dug into why there are always two layers, why they were impregnated in oil or wax, and why/how they became supplanted by paper/plastic film combinations and finally plastic film only.

I'll go see if I can find it in the boxes of old textbooks.

As I remember, the paper was stuck in there not for any electrical purpose at all, just as a spacer to keep the metal foil plates from touching. It was soaked in oil or wax for the enhanced insulation properties that the oil/wax did give. And paper was subject to conductive impurities on a % chance per square foot, so they played statistics and used two layers. The chances of two conductive impurities coinciding were infinitesimally small.

Plastic film, usually polypropylene, got into the act because it let manufacturers completely eliminate one layer of paper, and make smaller caps. Paper stayed because of problems with ensuring that the polypro film was durable enough under the field stress at high voltages. Later the polypro film improved and it was used all by its lonesome.

There was a huge amount more info. I'll go see if I can find it.

It's funny. The musical industry seems to use parts almost entirely for their side effects, not what they were intended for.

[trobbins]

In these old amps, the bass roll-off in the coupling caps gives it a brighter sound ... until it's in early distortion mode, in which case it has some appeal.

Can the tester do different signal voltage levels, and what signal level was used for those results?

I would think that even 5k ESR would incur a pretty negligible effect in most circuits of the time.

[salvarsan]

I'm reading antique patents on paper condensers.

One paper defect rate was stated as 6-10 conductive microscopic metal defects per square inch, what you might expect from something successively pressed out between steel drums. The double paper layer was a good bet that the defects wouldn't line up on top of each other.

If the caps were for benchtop or tabletop devices (< 1000VDC working), some builders used a hard finish paper single layer and did away with the wax until the potting. Once vacuum deposition of a conductor was possible, some capacitors were wound under 2 or 3 hundred volts AC to burn out the conductive bits.

Re: testing

The cap tester does series and parallel mode calculations both. The parallel resistance values got lost in the copy-paste, but they suggest that, for audio <15kHz, a wax paper cap might be adequately simulated with the right parallel RC element tacked on.

F (Hz)	Rp (ohms)
100	61400
120	56600
1k	23400
10k	7700
100k	1250

It's funny. The musical industry seems to use parts almost entirely for their side effects, not what they were intended for.

Boy, does it ever! Compression from power supply sag, 2nd harmonic sweetening from dV/dT effects in carbon comp resistors, polyester capacitor microphonics, overdriving output tubes with a high gain phase inverter, running the output transformer near saturation mode -- makes me shake my head.

I think the music industry has more fun than the AudioPhile industry, though.


PS -- Abe Books has a Power Capacitors by R.E.Marbury, 1949, so old there's no ISBN. With luck, someone local can scan it. Thanks for the suggestion.

PPS -- "Capacitor" paper is still manufactured, and to exacting standards. A German firm, Spezialpapierfabrik Oberschmitten, lists their different paper types in their catalog PDF, noting that two are suitable for metallization on both sides.

[Enzo]

It's funny. The musical industry seems to use parts almost entirely for their side effects, not what they were intended for.

Today, well, I don't know about the industry so much, but the basement tweakers, most certainly.

Remember in the days they actually had wax capacitors, no one was remotely thinking about overdrive, early breakup, etc. Wax/paper caps were used because they were cheap available caps. Mojo as a concept was decades later. Factories like Fender or Ampeg or any other volume maker bought caps that met their needs in terms of cost, reliability, and availability in production quantities. They were not concerned with excess graininess in the upper midrange.


It would be interesting to know in those paper caps, what, over all the years, might have absorbed into the cap paper, moisture primarily I'd imagine, and what effect that would have.

A lot of my early years learning were spent in TV set chassis, and cheap wax caps were king. The high voltage in TVs attracted dust and dirt in the air, and a certain amount of carbon soot was released. So after an afternoon putzing in these things, you would up with your hands coated with this wax/dust/soot black stuff that was darn difficult to wash off.

But I did find long ago that when you are overhauling a chassis full of waxed caps, and the wax is foggy so you can barely read the values printed on the part, a heat gun (which all shops should have anyway) melts the wax off the surface nicely leaving a nice clear view.

[Steve Conner]

Great results, Salvarsan! I always suspected those waxed paper capacitors would be bad, but I never would have expected them to be as bad as that. My theory is that they are full of moisture. Water has a high dielectric constant and a hefty dissipation factor at low frequencies.

I guess they might have been better when new, but anyone using them now will be using old ones, so this is the relevant test, unless some audiophile company started making them again.

The electric music industry itself is just a side effect of the electronics industry, so it's hardly surprising that we take components intended for another job and use them for their musical side effects. I believe this is possible without sacrificing reliability or manufacturability, but some people would differ.

The UT612 looks like a fine geek toy.

[salvarsan]

I always suspected those waxed paper capacitors would be bad, but I never would have expected them to be as bad as that. My theory is that they are full of moisture. Water has a high dielectric constant and a hefty dissipation factor at low frequencies.

Most astute! I should be ashamed that I didn't see that given as much chemistry as I know. Note to self: a hobby should not be a blinkered obsession.

I guess they might have been better when new, but anyone using them now will be using old ones, so this is the relevant test, unless some audiophile company started making them again.

There are AudioPhlake grade polished turds available, but not one has wax. Mostly they are paper, polyester and secret sauce oil, with some decoration by copper/silver/gold in electrodes or leads ... or packaging.

As a side project, I'm trying to cruft up a simple RC network to approximate a wax cap. Any suggestions?

The UT612 looks like a fine geek toy.

It's a little more difficult to use than the Extech, which I still prefer as the primary tool.

There's a UT612 review and tear-down over at EEVblog.

The tester noted the test conditions as:
... voltage
without DUT - - - - - - - - - 0.613V RMS
with 47.0 Ohm resistor - - 0.159V RMS
with 0.24 Ohm ESR cap - 0.014V RMS
The meter impedance is 120 Ohms and the test current is 4.5mA~5mA RMS.


They missed a coupla points on the USB side.

1)the USB jack and plug are a non-standard mini-B found on some cameras. If the tear-down photos are accurate, a micro-USB jack could be trivially installed on the jack PCB.

2) the serial-to-USB interface chip is not broadly supported, particularly under Linux. I am debating rebuilding the kernel to include it, but WTF.

[Cygnus X1]

I have a very few paper caps sealed with wax, NOS.
Never been installed.
Got them in a box of radio kit parts that was not assembled.

But I do not have a. true capacitance reader.

Just multimeters that roughly measure capacitance,
a Tek 466 scope, and multi function signal generator.

[salvarsan]

...I do not have a. true capacitance reader.

Just multimeters that roughly measure capacitance,
a Tek 466 scope, and multi function signal generator.

It's a good way.

I just A-B tested a polystyrene cap against the wax paper one (both .05 uF) with a CGR101 scope+sig-gen.

This particular wax paper cap was 6 db down at 100 Hz compared to the PS cap. That's 1/4 of the energy passed if it's used as the input stage coupling cap to the 1st grid. While you may not hear much difference when the amp is played clean, the sound is distinctly less farty in distortion modes. Bottom E2, A2 guitar strings are tuned to 82Hz and 110Hz, respectively

Below is a syscomp CGR frequency sweep of the two caps from 10 to 999 Hz, measured across a 1.63k resistor.
RED=wax paper cap, GRAY = polystyrene cap.

[Cygnus X1]

Thank you salversan.
I'll try to get this set up when I can.
Also, I have some "dry" caps I have saved from pulls.
They looked too well preserved (and cool) to toss out.

[Rhodesplyr]

Plastic film, usually polypropylene, got into the act because it let manufacturers completely eliminate one layer of paper, and make smaller caps. Paper stayed because of problems with ensuring that the polypro film was durable enough under the field stress at high voltages. Later the polypro film improved and it was used all by its lonesome.

R.G., Aren't you leaving out Mylar/polyester film? The reading I've done suggests that a lot of R&D went into developing a polyester film that was thin enough, sufficiently free of defects, and flexible enough to wind into capacitors without breaking. Polypropylene starts to show up in the 1970s, partly because of digital circuits that could be affected by Mylar's slight dielectric absorption.

I did a lot of research on paper capacitors in connection with their use on Hammond Tone Generator filters, where replacing them is controversial, depending on which sound you like. What I found was that the specs on paper capacitors during the 1950s were generally pretty close to that of Mylar, but I have tested some, special-ordered from the John E. Fast Company by Hammond, that show lower DF than typical Mylar caps, even at 50 years, probably due to impregnation with PCB oil. These were sealed in brown, hard plastic tubes, and I've yet to see one of these fail. I recently measured one in my impedance bridge, and it measured 0.04705uF with a nominal 0.047uF value.

Speaking more generally of paper capacitors, the concept I found in 1950s literature was that they were considered a composite dielectric; in other words, the dielectric combined the characteristics of Kraft paper and whatever oil or wax formulation was used. That untreated paper was used is a new idea to me. I do know that getting the paper dry enough for use in winding capacitors was a challenge.

The wax-sealed capacitors I've tested from 1950s Hammond tone generators are often 30-80% over their nominal value, and have a dissipation factor of 10x what spec sheets show they should have had when new. These are brands like E.U.C., John E. Fast, and CDE's Grey Tigers.

I only have a bridge that tests at 1kHz and an ESR meter that tests at 100kHz, so I can't do continuous curve measurements at different frequencies.

[R.G.]

R.G., Aren't you leaving out Mylar/polyester film?

Not intentionally. I still have to go through the boxes of textbooks and find the actual book, but my memory is that I was surprised that this text said polypropylene, which I also thought was a later item.

The reading I've done suggests that a lot of R&D went into developing a polyester film that was thin enough, sufficiently free of defects, and flexible enough to wind into capacitors without breaking. Polypropylene starts to show up in the 1970s, partly because of digital circuits that could be affected by Mylar's slight dielectric absorption.

I've read similar. It is possible that the work on Mylar was done for economic reasons, or for reasons that polypro had consistency or cost problems. I don't know. I wasn't there. When I find the book, I'll correct myself if I remembered wrong. It is possible that the book itself was wrong, too. Actually, a lot of R&D went into any plastic film with enough syllables trying to find thin enough, consistent enough, stable enough, yada, yada.

Speaking more generally of paper capacitors, the concept I found in 1950s literature was that they were considered a composite dielectric; in other words, the dielectric combined the characteristics of Kraft paper and whatever oil or wax formulation was used.

Strictly speaking, anything at all that goes in the gap of a cap forms a composite dielectric, to the extent that it nominally fills the gap. Paper is itself a composite material, being composed of a felted matrix of cellulose fibers which have been dissociated, cleaned and treated with chemicals to some extent, felted, compressed, mechanically treated, and then "sized" (or not). The interstices are filled with whatever the surrounding fluid or gas is - air and water for most papers, then chemical treatments, filler particles, sizing, coating, whatever. The more the paper is compressed in to a monolithic block of pure cellulose, the more it would be consistent, but it probably never gets all the way there. So actual papers in practice are combinations.

Here's one interesting capsule look at it: http://users.df.uba.ar/sgil/physics_..._const_2k4.pdf and a quote from the first page:

However, this experiment consistently results in a nonlinear relation between capacitance and inverse dielectric thickness and yields dielectric constants that are much too low pre-dominantly because of air trapped between the layers of the dielectric.

Many instructors choose paper as the dielectric material. Paper has the advantage of being thin~small plate separations result in large capacitances with a reasonably uniform thickness. Most introductory texts give a value of roughly 3 for the dielectric constant k of paper. However, the values given for k in various textbooks range from 1.7 to 4.0, and none of the textbooks provide a reference to an original source.

There are multiple factors that can cause a variance in the dielectric constant of paper. Paper can be wood-based, rice-based, or rag-based, for example. Different inks and bleaching processes are used in its production as well as various surface finishes. Wood-based paper -presently the most common paper -undergoes a drying process and chemical process with aging. Residual acids left on the surface from the manufacturing process cause the paper to yellow and become drier over time. In addition, paper is hygroscopic, and the dryness of paper is very important for the value of the dielectric constant. Water has a dielectric constant of k=83 and wood has a dielectric constant that can be as low as k=2. In short, we cannot rely on a specific dielectric constant for paper.

Soaking oil or heated wax into paper helps with the voltage and dryness, and adds its own dielectric constant into the mix.

That untreated paper was used is a new idea to me. I do know that getting the paper dry enough for use in winding capacitors was a challenge.

Yep. Paper is hygroscopic, and will suck in water when it can. Water's high dielectric constant of 83 means a little goes a long way in changing capacitance, as well as it's effect on leakage and conductivity.

The wax-sealed capacitors I've tested from 1950s Hammond tone generators are often 30-80% over their nominal value, and have a dissipation factor of 10x what spec sheets show they should have had when new. These are brands like E.U.C., John E. Fast, and CDE's Grey Tigers.

Again, just speculating, water absorption has a possibility to do both of those.

[salvarsan]

more random notes on wax paper caps --

The wax+paper and wax+oil dielectrics were later used because they provided more capacitance per volume, obviously. What is curious is that either paper or wax(paraffin) or oil (assorted phthalates, monoglyeride esters, and PCBs) all have a dielectric constant ranging 2-3; in combination with paper, the net dielectric constant was approximately additive, i.e. impregnated paper dielectric constants seem to range 4-6.

Purchasing and testing a box of ~100 pulls revealed the variability in wax paper cap quality.

Tested at 1kHz, about 1/2 of them have a DF <1%, 1/3 range DF ~1-2%, and the final 1/6 range DF ~ 3-25%.

The low dissipation factor ones are usually within 5% of their marked values while some of the others are as much as 50% high.

Wax-dipped capacitors dominated the defect pile while caps with plastic/epoxy/polymer cases age well.

So it's not that all wax paper caps are good or bad, but
that there are many good and bad instances
that you won't find without testing.

A 1951 Dubilier Capacitor catalog shows several grades of wax paper capacitor around page 18.