Jeff Dutky wrote on 4/21/2022 8:42 PM:
Were the tubular "dogbone" capacitors unique to Tek, or were they a commonly available third-party component in the 60s and 70s? Does anybody have any information on how these were made? Was there a "correct" jargon for them?
Hello, Jeff--
IIRC,? tubular ceramic capacitors were (or are available as NOS) for
many years-- at least into? the late 1930s. The process begins with formulating
the ceramic dielectric mixture that gets extruded into tubular bodies which
undergo firing.
Next, a silver solution gets applied to the inside and outside of the tubular
ceramic body.? The capacitor undergoes another heating cycle, which
converts the silver solution to internal and external metallized electrodes,
leaving clearance to solder wire leads to the exposed silver on the body's
ends.? A varnish, paint or resin coating protects the outside electrode and
provides a surface for marking capacitance. Breakdown voltage is dependent on
the ceramic tube's wall thickness, and capacitance and temperature
?coefficient are determined by the ceramic mix.
In jargon, they're indeed called "dogbone" (or "dawgbone") capacitors for their shape.
I'm looking through my batch of tubular caps and finding a 1000 mmf
N750F "Ceramicon" dogbone made by Erie that's? 2 inches long by 1/4 inch diameter.
A DE-5000 LCR meter reads 1016 pF at 100? KHz; I'll? guess that it's a 500 volt part.
On the other extreme, there's a dogbone cap that measures 1/4 inch long
by 1/8 inch diameter which reads 12.6 pF at 100? KHz; it's marked
with colored dots which denote 13 pF; based on size, I'll guess it's rated
for 100 volt maximum.
I don't recall ever replacing a dogbone cap in any equipment although
?I've seen them used in European-manufactured tube radios of the 1950s.
Failure modes might include silver migration due to a combination of
a flawed external coating, high ambient humidity and applied DC voltage.
73--
Brad AA1IP
-- Jeff Dutky
--
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