"Difilm" paper/mylar capacitors

[Rhodesplyr]

I just finished recapping a Leslie 122 amp, and, sometimes out of curiosity, I test the capacitors I remove in the process. This amp was built in 1972 with Sprague 160P capacitors, the ones that I think are known as Black Beauties, and I read online that these were made with a hybrid paper/mylar dielectric known as "Difilm."

Though they weren't leaking--yet--they were 20-30% over nominal value and had dissipation factor readings about eight times that of known-good mylar or paper capacitors. In contrast, some 1968 CDE "greenies" I removed from another recent rebuild tested like new.

That leaves me wondering--what was the reason for developing hybrid paper/mylar capacitor dielectrics in the first place?

[J M Fahey]

Absolutely out of my hat: maybe "funky newfangled technologies" weren't easily accepted.
I know for a fact that some steamships in the mid and late 1800's still had sails, besides the steam engines.
Even today, you can still buy a mylar+pressphan (treated paper) insulating material for transformers.
There must be a valid reason for that, because that's not a Mojo_driven market.
Believe it or not, in the XXIst Century some amp modelers *still* had actual glass_and_metal tubes in them, although in their discharge it can be said that quite a few were unconnected, or used the filaments as expensive pilot lights.

[Steve Conner]

I believe the paper is used as a wick for oil. If you just roll up a load of Mylar, the oil won't necessarily be able to get between the layers, so there may be air pockets left where corona can form. But adding paper to the roll will bring the oil right in there by capillary action. The oil-soaked paper is also a reasonably good dielectric in its own right. I believe this is how large industrial PFC capacitors are made: a mixture of metallized polypropylene and paper, all impregnated in oil.

Nowadays, both Mylar and paper can be metallized (coated with a thin film of metal on one side) to serve as electrodes. But maybe when Difilm capacitors were made, they hadn't got the hang of metallizing Mylar reliably yet. So that's an alternative explanation, maybe they wanted to use the new-fangled Mylar for its dielectric properties, but needed the paper for the electrodes.

I think high capacitance and high dissipation factor is caused by moisture. Water has a dielectric constant of 80 but is very lossy.

[J M Fahey]

Yes. As I said, it's in use still today, my insulation supplier carries it.
I use straight Mylar but I understand in certain industrial and power line distribution transformers which *will* be submerged in oil anyway, maybe not for insulation but for refrigeration (your friendly utility, pole-mounted transformer in the corner ), or just to follow outdated municipal or whatever specs, they use the dual-film, pressphan/mylar foil.
Pressphan is treated paper, disrespectfully called "fishpaper".
On its own it's already an insulator, but it absorbs oil beautifully, without air pockets.
For an English link:
Inventory Items
One example is: "PM10/2X1000 Presspahn/Mylar, 0.25/0.05 X 1000mm"
One funny detail is that pressphan thickness goes from .1 to .5 mm, yet Mylar is always .05 mm .... because it still outclasses paper in oil. Oh well.
PS: you gave me an idea, i'll get my trusty scissors, some old wire and beeswax and start manufacturing "wide stage imaging" DiFilm capacitors for the Mojo loving people.
I might buy a new BMW after all

[guitician]

I think the Sprauge 160P is a foil&film type. I put some old Pyramid IMP caps(DiFilm) in my amp the other day for heck of it, replacing some Mallory 150's, and they do add an enjoyable quality to the tone, but mojo.... I don't know about that.

It looks like some early Sprauge 160P used paper, then they switched to mylar? I'm not sure, but I thought they were film/foil, without paper, and then more reliable because of it.

[Zipslack]

Paper dielectric

Absolutely correct, Mr. Conner. I worked as an engineering technician for a company (Potter) that made EMI filters, surge supressors, and caps. 90% of the caps we used were designed and wound in-house. The paper works as a dielectric, but vacuum-impregnating with a oil solution (actually more like syrup/varnish that is used to coat transformer cores) drastically increases dielectric constant and breakdown voltage while decreasing corona and eliminating any water. The caps would also be stored in a "warm-room" for a day or more prior to vacuum-impregnating to help eliminate moisture from the paper. Since the oil is pretty thick, it can take MANY years for it to dry out enough to affect the condition of the capacitor, especially if it is embedded in epoxy or otherwise sealed. We never used any metallized paper...just metallized mylar/polyester/polypropylene with paper added, or plain foil with paper. BTW, these caps would be production tested with 1200-1400 VDC for breakdown. I've dealt with caps that worked to military spec for 20-30 years (generator rooms on Navy ships, etc) using these methods.

[km6xz]

The only common caps to deserve replacement on sight are the Black Beauties for several reasons but not much used them later than about 1960 so maybe these are not the Sprague caps so well hated by techs.
They often leaked since they were injected with oil AFTER assembly through a small brass filler tube on one end. These were sealed with solder. Does it have little balls of solder on the leads right at the end of molded body? Those were paper and foil with oil caps, I don't think they used Mylar.
Unless I can measure a problem I do not change caps wholesale as done by modders and tweakers do without realizing it does not make any difference in reliability or performance. Much of M.I. hobbyist mythology is following by a couple decades the smoke and mirrors of esoteric audio and new age spiritualism, and none can stand up to any rigorous tests. But it does make people money.
Does anyone actually play their instruments any more, or just tweak them?

[Jazz P Bass]

Does anyone actually play their instruments any more, or just tweak them?

Or learn how to play them.
Rock on.
I look forward to seeing your posts.
Kind of like halving a Prof standing over your shoulder.