Vintage Tubular Capacitors: Paper or Polymer?

Non-polarized capacitors (aka condensers) with more-or-less cylindrical bodies have been used for the entire history of commercial electronics, and many varieties are still produced. They are simple components, each consisting of two metal electrodes rolled into a cylinder with a dielectric material between them. With very few exceptions, this dielectric is either paper, plastic film, or both wound together as in hybrid capacitors (often called Difilm, which was Sprague's trademark). This capacitor roll is then encased in a tubular package, typically made of either hard plastic, ceramic, cardboard sealed with wax, or metal sealed with rubber or glass.

When restoring or rebuilding vintage electronics, especially vacuum-tube equipment of the 1960s and earlier, "re-capping" is often done. This is the act of replacing every electrolytic and paper capacitor in something, for the sake of improving reliability, and sometimes as part of necessary repairs. As for electrolytics, suffice it to say that their main problems are evaporation of the liquid electrolyte and "deforming" of the oxide dielectric; these defects basically result in failing open or shorted repectively, with the former being most common. In some applications, an electrolytic failing short can damage expensive and/or difficult-to-replace parts, such as power transformers and rectifier tubes. Luckily, electrolytic capacitors are easy to identify because of their distinctive packages, large size, high capacitance, and polarity.

Paper capacitors are also among the most troublesome parts. The paper tends to decompose and absorb moisture, causing increased electrical leakage; eventually, the capacitor may fail short. Depending on application, this failure mode can destroy transformers and tubes, or in rare cases create a shock or fire hazard. Now, keep in mind that reliability depends somewhat on the type of paper, its impregnation (wax, oil, polymer, etc.), and the tightness of the package's seal (with hermetically-sealed parts faring best). The worst types tend to be wax-paper capacitors with wax-coated cardboard bodies, since those are the least well-sealed, and the wax is not very stable, besides that such caps are mostly over 60 years old. Some plastic-bodied types are also notoriously bad, such as "Bumblebees" with oil-filling spouts. Without knowing the reputation of a particular type, recapping should be considered after measuring one or more suspect parts and considering factors such as package design (e.g. hermetic or non-hermetic), type of impregnation, etc.

Conversely, there is no reason to replace plastic film capacitors preventatively, because all polymer dielectrics age extremely well in comparison to paper—only in the rare event of failure should they be replaced. I also tend to leave Hybrid capacitors in place, since if the paper degrades, the permissible working voltage may decrease (and capacitance may drift slightly), but at least the polymer prevents leakage from developing prior to a breach of the dielectric, which I've never seen happen so far. In fact, my main purpose here is to prevent wasted effort replacing plastic caps, not to encourage replacing paper caps, which can sometimes be left in place without worry.

So, replace some paper caps, leave plastic caps. No problem, right? Well, there is a problem: they often look identical. There is no way to reliably distinguish between them without either breaking them open, or already knowing what's inside the given type. There is a lot of misinformation about this topic online. I have read in multiple places that "Bumblebees" without spouts are mylar or hybrid. They aren't. Every one that I've dissected was just paper. I've even read that most molded-plastic capacitors in vintage tube electronics are plastic film, and that the polymer breaks down just like paper. This is also completely false; most of them are paper, with the transition to plastic film being made primarily in the early '60s, and those that are polymer do not develop leakage the way paper does. In fact, even the earliest polymer types usually test as good as new, with virtually no leakage, and capacitance well within tolerance.

Tired of misinformation, and more often a plain lack of information, I decided to make a list of vintage tubular capacitors and their dielectrics. It contains only info that I have personally verified or taken from official documents. By "personally verified", I mean that I have broken open at least one capacitor and looked inside. Also note that I have only included the most potentially misleading cardboard-bodied types, since these are nearly all paper. Many were made before the commercial application of polyester—the first common polymer dielectric—dafter all. The list would be a real mess if every cardboard type I'd ever seen were included!

Also included are many Soviet-made tubular caps, which are common on eBay and similar sites. Notoriously unreliable paper types (such as БМ-2, БМТ-2, and МБМ) were produced in great quantities well into the 1980s, long after such capacitors were abandoned by the West. They often appear in Soviet-made tube and solid-state electronics.

"Good" dielectrics are bolded, and one very rare and practically unknown dielectric is italicized. So, without further ado, here is the list:

Note that except where transistions (from paper to plastic or hybrid) are known, my list generalizes; I assume that all of a particular type of capacitor use the same dielectric, based on only one or a few examples.

Again, I should state plainly that I am not encouraging replacing all paper capacitors; in many cases, especially with relatively late molded types, they can be left in place without problems. Rather, my main intention is to save myself and other technicians valuable time, effort, and resources by pointing out which types are plastic film and hybrid, and should therefore be excluded from recapping unless proven faulty. As well, where I've specified dielectric materials (based on official documents), my list may be useful for finding certain rare dielectrics, as well as knowing which paper dielectrics are impregnated with PCB oils.

If you have capacitors not listed that you are curious about, here are some suggestions. If you don't wish to destroy the capacitor, a reliable test can be done using Mr. Carlson's Lab low-voltage leakage tester. The design is only available on his Patreon page, and you have to build it, but it performs magnificently. The extremely sensitive "forecasting / mica" setting will almost always detect leakage on a paper capacitor, even some brand new ones. Yet, it will always show no leakage on a polymer capacitor, even those from the 1950s and 60s. Hybrid capacitors also nearly always show no leakage.

While the Carlson tester gives you a good idea of whether an individual capacitor is worth worrying about, breaking it open is the only way to be certain of what's inside. I find that squeezing between large pliers is sufficient to break away the shell of most molded plastic types, and metal cans can be pierced by flush cutters. Wear eye protection and disposable gloves, and work in a well-ventilated area, since you may encounter nasty vapors, possibly those containing PCBs. Once the shell is breached, the rolled layers can be cut with a blade and picked apart using fine tweezers.

From picking at the layers, you can easily discern the material. Paper is always translucent to opaque, and white to brown in color. It tears easily, giving a ragged fibrous edge. This is compared to polymer, which is transparent to translucent, and will sometimes stretch before tearing. When it tears, it does so in a clean line, or with some scalloped irregularity (if stretched to breaking), devoid of fibres. Most often, the dielectric will be easy to separate from the aluminum foil electrodes. Sometimes, the dielectric is "metallized", meaning it is coated in aluminum, comprising the electrode instead of a separate foil layer. It can be slightly harder to identify the material as a result of being unable to distinguish its color and opacity, but the same tests can be applied: stretching and tearing will reveal whether it is paper or film. Also consider that metallized polymer types are more common than metallized paper.

Below shows a metal-can paper capacitor cut open with flush cutters starting at the rubber seal. Note that the paper is brown, translucent, fibrous, and easily torn. This capacitor showed no leakage even on the "mica" setting of a Mr. Carlson tester, suggesting it might be plastic. No, it was just an especially good paper-in-oil. The paper was still noticeably wet; the rubber seals really did their job on this capacitor.

Below shows the inside of a 1950s polymer capacitor: a Good-All 600UE. This tested perfectly in both capacitance and leakage. Breaking it open, the film is very obviously plastic, since it is translucent blue, and stretches slightly before tearing cleanly and without fibres.

Lastly, here is a dielectric sample from a classic yellow-bodied "Blue Point" Astron capacitor, showing that it is hybrid. You can see the transparent plastic base layer, on top of which is a partially torn translucent paper layer. The plastic stretches and has no fibres, while the paper tears easily, giving a fibrous edge. Note that Astron Blue Point capacitors are one of several series that transitioned from plain paper to hybrid at some unknown point!

If you'd like to contribute to this list, I certainly welcome it, though there is a certain standard of proof necessary: either provide a scanned datasheet (or other "official" reference), or pictures of the capacitor both in unbroken and broken states. Here are a few types that I am especially curious about:

• Astron Metalite in yellow molded plastic (and green or black text); these look nearly identical to "Blue Point" caps, but are almost certainly metallized paper.
• Cornell Dubilier type ST "Budroc", a ceramic-bodied type. Probably paper, but don't know for sure yet.
• Good-All types 623 and CMI-1. My suspicion is that both are plastic film, but I am not sure.

Links

• Allied Radio & Electronics Catalogues (archived) — Many capacitor listings from 1929 to 1981.

• Electronic Component Catalogs & Databooks - Hallman Labs — Various catalogues sorted by brand.

• Free Electronic Component Datasheet Downloads 33audio.com

• Large selection of assorted catalogues on technicalaudio.com.