Vintage Tubular Capacitors: Paper or Polymer?

This page contains an extensive list of vintage non-electrolytic tubular capacitors, specifying the dielectric material found within each type. With few exceptions, this dielectric is either paper, plastic film, or a layer of each, which I will call hybrid, though it is also often called Difilm, which was Sprague's trademark. My main intention is to save myself and other technicians effort during "re-capping"—the process of replacing all paper and/or electrolytic capacitors in a vintage electronic device—by indicating which types are plastic film or hybrid, and should therefore be excluded from re-capping unless proven faulty. Having a list is valuable because capacitors with different dielectrics can look identical from the outside; for instance, Sangamo 33 and 33M are indistinguishable aside from the "M", yet the former is paper (and often troublesome), while the latter is plastic film (and always good, in my experience). As well, my list may be useful for finding types with rare high-quality dielectrics such as Teflon and polystyrene, and knowing which paper dielectrics are impregnated with PCB oils.

Paper capacitors are notorious troublemakers, because as the paper absorbs moisture and decomposes, electrical leakage increases, and so does capacitance. Lifespan varies significantly with paper type, its impregnation (wax, oil, polymer, etc.), seal type (wax, plastic, rubber, glass, etc.), and storage conditions... but in any case, most paper capacitors have significantly degraded by now, due to their inherent instability coupled with age—they have not been in common use since the 1960s. The worst ones tend to be wax-coated cardboard-bodied types, since those are the least well-sealed and chemically stable, besides being the oldest. Plastic- and ceramic-bodied types generally hold up better, but some are notoriously bad, such as "bumblebees" with oil-filling spouts. The only types that consistently hold up well regardless of age are those with metal bodies and glass hermetic seals; these usually test like new, so I tend to leave them in place. Without knowing the reputation of a given type, re-capping should only be considered after measuring one or more suspect parts and considering factors such as package design, impregnation, and so on. Consider, too, that there are some circuits where even somewhat degraded paper capacitors can be left in place without worry for a long time. For these reasons and more, I do not encourage replacing all paper capacitors as a rule.

As for plastic film capacitors, there is usually no reason to replace them preventatively, because all plastic dielectrics age extremely well in comparison to paper. Even the earliest types from the '50s almost always test like new, which suggests that they have a vastly longer average life than paper types. 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 have never seen occur. Note that on rare occasion I have found hybrid capacitors that show minor leakage, perhaps due to imperfect overlapping of the layers, leaving an area of plain paper.

There is plenty of misinformation about this topic online. For instance, 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 plastic-bodied capacitors in vintage tube electronics are plastic film, and that the plastic decomposes much like paper. This is also completely false; most of them are paper, with the transition to plastic film happening primarily in the '60s alongside transistorization, and those that are polymer do not develop leakage the way paper does.

My list 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. Note that I have only included the most misleading cardboard-bodied types, since these are nearly all paper.

Soviet-made tubular caps are included because they are common and inexpensive 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 discontinued in the West. They are often found in Soviet tube and solid-state electronics.

"Good" dielectrics are bolded, and one very rare and practically unknown dielectric is italicized. PCB oil names are colored red; I do not condemn these capacitors, especially since the hermetic types hold up very well, but some people consider it important.

Note that except where transistions 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.

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 a 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 (such as KEMET/RIFA's so-called "safety" types). Yet, in my experience, it will always show no leakage on a polymer capacitor, even those from the 1950s or '60s. Hybrid capacitors almost always show no leakage, but some are occasionally slightly leaky, perhaps due to imperfectly overlapping of the two layers.

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 pungent vapors or PCB oils. 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. Polymer 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 harder to identify the material because one cannot as easily see its color or opacity, but the same tests can be applied: stretching and tearing will reveal whether it is paper or film.

Below shows a metal-can paper capacitor opened 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 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.

If you'd like to contribute to this list, I certainly welcome it, though I require proof: either provide a scanned datasheet (or other official reference), or pictures showing the capacitor's label and dissected insides. Here are a few types that I am especially curious about:

• Astron Metalite in yellow molded plastic with green or black text; these look much like "Blue Point" caps, but are probably metallized paper.
• Cornell Dubilier type ST "Budroc", a ceramic-bodied type. Probably paper.
• Good-All types 623 and CMI-1. My suspicion is that both are plastic film.

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