Bill: You do not seem to be getting an answer to the specific question
you asked.
1. The insulating material in the coil form should be capable of
withstanding high temperatures. The same applies to the end plates.
Ceramic or Mycalex are used in good rotary inductors. If you are not
familiar with it, Mycalex is a hard grey material that looks like
marble.
2. The coils are made out of either round cross section wire or
rectangular cross section material. The rectangular stuff is used in the
better low loss inductors.
3. The contact design really separates the low quality units from the
better ones. If round wire is used in the coil, the moving contact is
usually a round grooved wheel of brass which rides on the inductor wire
and has a hole in its center which slides along a round rod. The really
good rotaries have a curved shoe which rides along the top edge of the
rectangular inductor. It may also have flat spring arms which ride on
the sides of the conductor. This moving contact assembly slides along a
large rectangular bar and usually has contact springs or contact buttons
riding along the bottom and sides of the bar. Rotaries of this type were
found in WW2 surplus xmtrs and are highly prized but still appear in
swap meets now and then. I think E.F.Johnson built some or all of these.
4. There is an intermediate quality of inductor which uses the edge
wound inductor and has a circular contact wheel which is made of 2
washers which pinch the conductor between the 2 wheels. I believe these
inductors are being manufactured now and are sold by Surplus Sales of
Nebraska. A similar or identical rotary is used in some Henry amps and,
I believe, in some of the top line antenna tuners. I believe the
insulating material is some type of plastic or fiberglass material. I
hope this helps you. 73, Dan, N5AR
Bill Smith wrote:
>
> What qualities should someone look for in a Rotary
> Inductor intended to be used in an amplifier Tank
> Circuit?
> 73,
> Bill
>
> =====
> Bill Smith KO4NR
>
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