Re: [Amps] insulation

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

W0UN -- John Brosnahan wrote:
> > The opposite example is that I once used Delrin for the PA tuning 
> > shaft in a little 6522 2m transmitter. The capacitor rotor was 
> > *supposed* to be at RF ground, and that TX couldn't have been putting 
> > out more than 25W, tops - but the Delrin melted in seconds. Wrong 
> > place, I guess.
>
> Hi, Ian--
>
> Are you SURE that the material was Delrin?  Sure sounds like NYLON
> to me.  I have been using Delrin as the form for the plate RF choke on
> my 3CPX5000A7 amps at 49 MHz.   These run 50-100 KW pulse and
> 2 or 3 KW average and often transmit for WEEKS at a time--in some
> cases for YEARS.  I have NEVER had a failure of a Delrin RF choke
> or ANY hint of any heating.  I have been building these amps--at
> least 100 in the field--for the past 20 years and not one single problem
> with Delrin as the RF choke form.
>
> So I am quite surprised with your results--to the point of believing
> that it was probably NOT Delrin.

It was a commercial molded shaft extension, and my memory is that the 
catalog had said "Delrin". After more than 30 years it's hard to be 
"SURE" of anything - except that it was definitely not nylon.

Sooo... if Delrin is OK for a plate choke, it *can* be good for a lot 
more RF applications too.

In the end, it all depends on the field gradients you allow to develop 
through the material. These are hard to forecast because they depend on 
little things like where you choose to put the fixing screws or other 
metal parts.

If you get it as right as John obviously has, a wide range of materials 
will probably be OK. But if you get it as wrong as I did, you can 
probably set fire to just about anything. There may be only one way to 
find out... welcome to the world of QRO.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek
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