Re: [Amps] Teflon Chimneys

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Chris Bartram wrote:
> 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.
I once used a cyanoacrylate adhesive formulated specially for teflon, 
and this came with a tiny bottle of 'primer' to pre-clean the surfaces. 
That was simply heptane - a light hydrocarbon typically found in 
petrol/gasoline. It worked well, but I'm not sure that the special 
adhesive and heptane worked much better than ordinary superglue and 
petrol would have.

The usual trick of roughening both surfaces *before* you clean them will 
always help, of course.

John B's option of RTV will also stick teflon fairly well. I use that 
routinely at the top-cover ends of teflon chimneys, to seal around the 
gap and make sure that all the hot air exits the cabinet. With a few 
holes punched around the edge, the chimneys hold very firmly in place.


> 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.
Part of the trick is not to tie the line directly to the flapper, 
because close to the metal surface is where the field gradients are the 
worst.

The Arcos amps (W2GN) used a 'mushroom' shaped piece of teflon, pushed 
through the flapper from the plate line side. This serves two purposes. 
The head of the mushroom is a stopper to prevent the flapper from 
touching the hot plate line. The bottom end of the stalk is maybe 1/4in 
away from the underside of the flapper, and is cross-drilled to tie on 
the pull cord. At that distance, any kind of cord seems to work - 
fishing line, button thread...

The insanity prize went to a certain GJ, whose answer to melting drive 
cord in the K2RIW was a rigid brass rod going downward through a hole in 
the base of the anode compartment... AKA feedback link...


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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