[Boatanchors] Ceramics help? Yes, it's boatanchor-related.

[Carl]

The 8791 looks very similar to the 7213 & 7214 RCA of the past which 
were used in a number of VHF/UHF military cavities that easily tune to 
222 & 432.

Carl
KM1H


----- Original Message ----- 
From: "mikea" <mikea at mikea.ath.cx>
To: <boatanchors at mailman.qth.net>
Sent: Wednesday, May 06, 2009 11:06 AM
Subject: [Boatanchors] Ceramics help? Yes, it's boatanchor-related.


> Amateur radio leads into all manner of interesting side-topics,
> including in this case needing to do some fairly high-precision 
> ceramics
> construction. I'm also a Really Cheap Bastard who's unwilling to pay 
> as
> much as the manufacturer wants for three small ceramic widgets if I 
> can
> make them myself.
>
> I refer the interested reader to
> those parts of <http://mikea.ath.cx/tubepix/> that refer to the 
> Jettron
> socket, and in particular to the image of the insulating ceramic 
> spacer
> to be found at <http://mikea.ath.cx/tubepix/8791-socket-washer.jpg>.
> (caution: 1,548,661 byte image)
>
> Those spacers are geting pretty crumbly,and fell apart on me as I was
> inspecting things. They are each 1/2" OD and 1/16" thick at their
> widest, with a 5/16" long 5/16" diameter shoulder, drilled to pass a 
> #6
> screw. The manufacturer wants $167.50 for three of them. Yes, $56.00
> each, pretty near, for each of those little yellowish pieces of molded
> porcelain. The manufacturer's quote came as rather a surprise, though 
> I
> should have expected it: the socket costs something like $500 new.
>
> The spacers are nothing special, just unglazed fired porcelain. I
> have access to a porcelain kiln, and I have a drill press and bits
> suitable for making molds. Now I need a low-shrinkage porcelain with
> good insulating properties and reasonable mechanical properties. The
> potential across that 1/16" shoulder will be about 1000 Volts, and it
> will be under no more than 50 pounds compressive force from the nut 
> and
> washers. That's not a lot of force, and the flat washer spreads it out
> evenly.
>
> I'm thinking about machining the mold cavities from split plaster
> blocks, using flat-bottomed Forstner (or Pforstner) bits, though wood
> might well suffice. I see a fair (or perhaps even unfair) amount of
> experimentation coming up.
>
> I'm also thinking about getting a chunk of PTFE rod and finding a 
> small
> lathe. That may turn out to be less work and less expense overall.
>
> Comments? Suggestions? Advice? Pointers?
>
> -- 
> Mike Andrews, W5EGO
> mikea at mikea.ath.cx
> Tired old sysadmin
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