Re: [TCML] Tuning idea

[Ed Phillips <evp@xxxxxxxxxxx>]

any idea on how a normal
air-vane trimmer (ie. one from a tube radio) would hold up
if it were placed under oil... (ive got one 60 year old one
that has a plate spacing of about .5 mm, wonder if it will
work )... also its value would be insreased by the high
dielectric constant of the oil, especially if refrigerator
(silicon) oil were used..."

	Not worth the effort as the breakdown voltage will be low [haven't figured it for  0.5 mm and of course depends on what type of oil].  I think I mentioned in an earlier note that I have such a capacitor comprised of a whole gang of 4-section MIL surplus capacitors in a big plexiglass tank.  Spacing is quite a bit more than yours and it breaks down around 3000 volts.  The oil in mine carbonizes and the breakdown results in contamination which I can usually clean out by turning the whole assembly on its side [normally plates are parallel to the earth] and rotating the thing until the stuff is moved out of the air gap and into the main oil bath.  Since the case isn't entirely oil tight this also results in some oil on the bench......................  I didn't make this thing.  Someone gave it to me and I decided to try it on a TC.  Not worth the effort for sue.  Oil-filled HV capacitors are certainly possible and were used in "the early days of wireless" but they had large spacings and were usually run in open tanks to make cleaning easy.

	I do have a beautifully-built MIL surplus variable which is oil insulated.  The case has metal ends and a heavy wall molded plastic center and is sealed with an expansion bellows at one end and, so far as I can tell, no air in it at all.  Picked it up surplus and don't know what the voltage rating was but the tuning range is from about 65 uufd to 980 uufd.  I haven't figured out how the seal to the external shaft is designed.

Ed



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