Can metalized film caps be used where paper caps were used in radios?

Yep, that is one of the common replacements for paper/wax capacitors.

Curtis Eickerman

"Paper" capacitors have layers of foil with paper separating them.
Metalized caps have a layer of plastic that is coated with metal. So they both contain layers of metal separated with an insulator. The plastic lasts better than the paper.
You must be new here, replacing paper caps with plastic ones is one of the main elements of restoration.

As a follow up I would note that I stock a range of values for restoring a variety of vintage electronics. I buy only 630 volt plastic caps - one (voltage) size fits all.

Generally speaking you can use metalized film capacitors to replace paper ones, but not everywhere. Some RF and IF circuits are particularly touchy about secondary characteristics like self resonant frequency, dissipation, "Q," and equivalent series resistance or inductance. The circuits may not function as they did originally when the caps are replaced or you may find that the circuits become unstable. In some cases modifications have to be made, like increasing lead lengths or putting small resistors in series with the caps. Also, metalized film caps offer relatively little effective shielding regardless of which way they are installed. Paper caps often had their outside foils labeled so they could be used to shield the inner foils from electrostatic noise and signal coupling. If you replace paper capacitors with metalized film ones you lose that shielding and may get hum or feedback as a result. One other place to look out for is where a capacitor has substantial AC or pulse currents passing through it. Metalized film caps have relatively high ESRs and low current capacities and may not last too long in such applications.

Since paper capacitors are not readily available in convenient sizes for antique radio repair, you pretty much have to use plastic film caps. Polyester (Mylar) comes the closest to matching the original paper characteristics so it should be used in preference to polypropylene in most cases. Polypropylene is superior in a number of ways, and it is fine in equipment designed to use it, but it gets you further away from the characteristics of the original paper capacitors you are trying to match. You can find wound metal foil/plastic film capacitors which have more shielding than metalized film. They are pricey but worth it in critical situations where you need more shielding in the capacitor or where the circuit involves AC currents or high pulse energies.




I'd love to see proof of that claim. All evidence is, plastic film capacitors age and deteriorate faster than paper ones. The difference is in how they degrade. Paper capacitors tend to get leaky when they are EOL; plastic films develop high dissipation or loss. That's not necessarily a bad thing; many circuits will continue to work safely but perhaps at reduced performance levels when a cap develops high dissipation. A capacitor with high leakage can damage other components. Paper capacitors withstand high energy pulses or spikes better than plastic, which is why paper capacitors are still available for EMI filtering applications. The pulse-withstanding capabilities for plastic film caps are often spelled out on the spec sheets because it is an area of concern; it was not for most paper caps.

Paper cup compared to a plastic cup.

I'd like to see your source for data showing the lifetime of plastic capacitors.

At any rate, plastic dielectric caps are available in a greater range of values these days than paper one.

Additional shielding is easily added when needed. The trick is to know when it is needed.

I have seen videos recommending identifying the lead connected to the outer foil layer on plastic capacitors so that that lead can be connected to the ground terminal for better shielding. Is this worthwhile in your opinion?

Thanks,
Jerry

I have never attempted to observe the outside foil layer when replacing radio capacitors and have never noted any adverse effects from it.
YMMV.

It depends on the capacitor. If it is a wound plastic film and metal foil capacitor, you might pick up enough shielding to make it worthwhile. But if it is a metalized film type, the metalized film doesn't provide enough shielding to bother with regardless of which way you connect it. It looks impressive on the u-tube videos but if you look closely there's only a 10:1 voltage difference which is 20 dB. Decent shielding should produce 70, 80, 90 dB or more (gets really hard to shield better than that).

Neither did I, but in some critical audio applications with some type of capacitors there might be a (slight) difference.

The pic below is taken from an old unbuilt Lafayette preamp kit and the (paper) capacitor outer foil is clearly identified by a stripe and the right orientation marked "O.F" on the PCB parts legend printing, the assembly manual also instruct to respect the capacitors orientation. And since there was no audiophool-nonsense yet among audio design engineers in the late 50ies, I guess there must be a technically valid (and measurable) reason for this.

BTW, I checked the original Pyramid "IMP" capacitor (from the kit) and it showed no sign of leakage @ 500VDC, the value was within the 20% rated tolerance (actually closer to +/- 10%). I have no idea about the technology used in these IMP capacitors but according to some "guitar gurus" experts they are a mix of paper-mylar called "DiFilm" and (of course) these "black beauties" have a "magic" sound quality... (info to be taken with a grain of salt of course !)

Higher frequencies and all the components laid side by side tightly together, shielding becomes a bit of an issue.
Not so much in an AM radio.

A simple way to polarize an unmarked cap - touch either lead to a tube grid in a working radio. The lead with the loudest hum is connected to the outer foil. Never tried it, but it seems workable.

Once again, old paper capacitors were big. Inside a piece of foil rolled up with paper. It's easy to see how the outer side might pick up signals, so best to make that the ground side. (Or for coupling, the pkate side)

It's not because old equipment was finicky, but the way the capacitors were made became an issue.

Newer capacitors are smaller, and made differently, so the same problem no longer there.

The only time in fifty years that I've really noticed marked capacitors was in old tube equipment.

The outside foil (on capacitors so marked) is not a problem. All wound tubular capacitors have an inside foil and an outside foil, it comes with the territory.

In some applications this can be used to advantage. If the outside foil is grounded or connected to a place in a circuit that has a low impedance to ground, it can serve to keep external electrostatic fields from being picked up by the capacitor, or it can prevent the capacitor from radiating electrostatic fields elsewhere. This is not VHF or UHF phenomena, because tubular capacitors have too much inductance to work much above a few MHz. This is shielding for audio, IF, and medium frequencies.

If only the outside foil itself is grounded, the shielding is about 90% effective because the ends are still exposed, which is why many paper and plastic film caps with foils do not have outside foil markings. Better quality capacitors from the 1930s and 1940s with the outside foil marked had that foil extended over the ends (except for a tiny hole where the lead to the inner foil passed through), giving them nearly 100% shielding.

Now it turns out that the effectiveness of any electrostatic shield depends on its thickness. The lower the frequency, the deeper it penetrates the shield. If you disassemble some metallized plastic film capacitors you'll discover the metallized layers on some of them are so thin light can pass through them. So if they cannot block light they may cause some attenuation but they will not block lower frequencies. And with a metallized film dielectric there is no chance of shielding the ends of a capacitor; the end terminations are glued in with a conductive adhesive and sealant. As said before, very little worth bothering with there.