Richard Knoppow wrote:

I don't think I have ever found a bad OD; nor have I ever
found
a bad BB that wasn't of the oil filled variety.

-Chuck

Well, I don't know. The BB's I've gotten from Hammarlund
receivers and some other equipment date from the mid 1950's
and are definitely not oil-filled nor do they have the
filler tubes. I've taken them apart: the dielectric is paper
impregnated with plastic. It looks a little like the very
fine paper used in Chinese cooking, thin and translucent.
These all have stripes on them and no printing. Some of
these have a thick end from the way the end leads are
fastened to the foil. I really think there must have been
two versions of the BB series. Perhaps the plastic type was
a replacement for the oil-filled kind. They would have been
easier to make and free from leakage.
All the BBs I've check on a capacitance bridge, even
those which appear new, have somewhat low capacitance and
somewhat high dissipation factor. Of course, both could be
accounted for if they were actually oil filled and the oil
is gone to the extent of leaving no trace whatever. It would
be interesting to know what the molded cases were actually
made of. The material does not quite look like Bakelite but
there were many variations of resins sold as Bakelite and
under other names like Durez. Old catalogue date could
answer some of these questions.

Hi Richard,

My experience with BB's is primarily with Tektronix and HP
equipment from the very late 1940's to the early 1960's.
They both used the banded BB's, and they were all oil filled.

After the oil filled BB's failed in droves, Sprague changed
the BB's to a plastic dielectric, which is, I believe, what
you have seen. There might have also been a difference depending
on the WV rating of the capacitor. The oil caps were 600V rated
as I recall.

I have never seen an oil filled BB, that had failed, that wasn't
all fuzzy and goopy on the outside. The mineral oil definitely
didn't just evaporate.

-Chuck