Back in the mists of time I built a K2RIW PA for 432MHz. That used a pair of
4CX250BMs in parallel with a reverse airflow system, and would seriously
endstop a 1kW Bird slug in class-C without overdissipating the tubes
(much!). The chimneys for this amplifier were made with Teflon sheet formed
into a cylinder, and the edges were stuck together with cyanoacrylate
superglue. They lasted the lifetime of the amplifier in my ownership - about
15years.
At the time I built the amp. I was working in the Materials Science
department of a venerable University, and I sought advice from a number of
people who should have known. They pointed me towards the superglue route,
suggesting that I should fairly agressively clean the mating surfaces before
gluing.
The capacitive 'flappers' which (in my version of the amplifier) tuned both
the input and output networks were driven by polypropylene string made by
separating the strands of some conventionally laid 6mm dia. polypropylene
rope, and twisting three or four strands together. Polyprop. is a good
dielectric material, and I never suffered the problems experienced by other
people with nylon fishing line burning up in the intense RF fields present
in the output network.
I tend to regard Tufnol as a 'legacy' material. Good quality glass-epoxy
composites are stronger, and have better loss characteristics, although they
are perhaps a bit less pleasant to machine. Also, epoxy-glass doesn't
absorb moisture significantly, which is one of the reasons why it used to be
a popular high-end boatbuilding material before carbon-fibre, and kevlar
based materials became popular. Polyester-glass composites are another
matter, however.....
73
Chris
GW4DGU
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