[ARC5] Caps & Crystals

[Michael Tauson]

Last week (Week before?) I said I’d post the transmitting micas I need
as well as the variable caps needed.  Here’s the list (so far) …

Fixed micas: 45, 50, 100, 200, 400 & 500uuf, and .001, .002, .004 and
.005uf.  Variable caps: 100 & 140uuf (both full sized and mini like
are in the BC-610 tuning units), 50uuf and dual 100uuf (total 50uuf).
In both cases, the working voltages won’t exceed 1000vdc.

There is one exception to the working voltage requirements, that being
the high power (813?) rig which will be operating around 1500vdc which
means 2500 or 3000 volt micas and corresponding variable caps, the
exact values dependent on the design used.

When I wound my own paper caps, the working voltage wasn’t all that
important since the receivers never used a b+ higher than 150vdc and
more were closer to 90 volts.  The caps were made of aluminum foil and
Kraft paper from paper bags – pretty common stuff.  A few of the
antique designs are asking for “.01uf, 400vdc tubular transmitting
paper caps” for use as heater bypasses.  Paper caps being what they
are, it looks like an opportunity to make my own again.

Since I had rolled caps using Kraft paper and foil, I did some web
research and discovered that Kraft paper has a dielectric strength of
1000-1250vdc/.001” (or 1600vdc/.001” for oiled paper) and a dielectric
constant of 4.4.  Cool.  Then I measured a bag from an exceptionally
high quality source – BurgerKing.  Dependent on where I measured, it
was between .003” and .0035” showing to the attention to quality
control used in the paper’s manufacture.  The old paper bags were
heavier – maybe .004-.005” – so had a higher breakdown voltage.  I
think wrapping paper was about the same thickness but I can’t
remember.

The caps then used bare paper but after they were assembled (with the
actual capacitance in part calculated then finished using the tried
and true trim ‘til it’s right method) I dipped them in wax paraffin,
or candle wax if that was all I had, to seal them.  Now I’m curious if
shellacking or waxing the paper would do any good in any way other
than maybe helping hold to the capacitors together after they’re made.
 I’m also interested in how waxed paper or the white coated wrapping
paper sometimes used in fish or meat markets would work as a
substitute.  This should be fun.

Now, for the crystals part …

FT-243 crystals are common but they and FT-241s were WW II products.
What I need is holders available to civilians during the 30s.  Among
these are the famous Bliley round holders, an earlier version of the
FT-171 (which can be approximated by modifying FT-171 holders),
possibly DC-34 & DC-35 holders and a few others.  I could very easily
be wrong but it seems that the crystals with 3/4” and other wide pin
spacing were likely prewar designs.

While it would be nice to acquire some of these holders, a good second
best is dimensional drawings for some of the older holders so I can
alter the designs for use with some blanks intended for FT-243
holders.  At the moment, I have photos of the Bliley BC-3 holder so
it’s covered but information on others would be helpful.

Best regards,

Michael, WH7HG
-- 
http://www.nationalmssociety.org/chapters/NTH/index.aspx
http://wh7hg.blogspot.com/
http://kludges-other-blog.blogspot.com
Hiki Nô!