[AMPS] more poor mans chimneys and glassware

[Ian White, G3SEK]

Skip S Isaham wrote:

>I realize a lot of this sounds silly, but I brought the topic of Glass
>Melting Temps up with the professional Glass Blower where I work.  This
>was a short time ago when I approached him about repairing vacuum
>variables and chimney repairs. 
>
>To sum up his information, he told me not to get too excited about the
>different glass melting temps and that mixed glass and quartz glass was
>better, but not by a great deal. I'll have to ask him more about the
>pyrex types of glass in the next day or so. 
>

FWIW, I've been using Pyrex glass chimneys with 4CX250Bs for well over
20 years. They were cut from stock tubing that was a close slip fit over
the anodes, the ends ground square, and simply glued on to the base with
silicone rubber. Works fine.

>He's a
>retired professional glass blower hired back when we couldn't find anyone
>else that had the skills.
>
>He has a very nice working shop... 

Takes me back to the time when all the experimental rigs in our physical
chemistry lab were built from glass, so everybody was taught the basics
of glassblowing. For the difficult stuff, there was sufficient work for
a full-time scientific glassblower. If you've never seen someone at work
with a glassblowing lathe, book tickets now - it's absolutely
fascinating to watch.

(In a glassblowing lathe, the headstock and the tailstock are both the
same, and both rotate at exactly the same speed. The rotation allows the
glass to be heated uniformly, all the way around, by a fixed burner or a
hand-held torch. The whole tailstock moves along the bed while still
rotating, so that the ends of two separate pieces of glass can be heated
and then butted up and fused together. The two large hollow chucks have
soft padded jaws, and allow the user to blow into the workpiece through
a rubber tube while the whole thing is rotating.)

>I'm still looking for a dead vacuum
>variable to practice on...

Which brings me to the point: unfortunately there are very few things a
laboratory glassblower could do to repair a vacuum variable, because the
manufacture required very special facilities. 

If the copper parts are distorted, no amount of glassblowing will fix
that. If there is a crack into a glass-metal seal, it's almost
impossible to repair by hand. If the repair involved softening the whole
glass envelope, then you wouldn't be able to maintain the precise
centring between the two metal parts because a glassblowing lathe is not
a precision tool. That's before you even begin to think about the
effects of heating on the ultra-clean copper surfaces inside - the
manufacturers almost certainly used vacuum ovens for all the heating
processes. Probably the only place where a naked flame would be used
during manufacture would be for sealing off the neck when the finished
item had been pumped out - and that is very much a one-way process. 

Sorry to be discouraging... why not ask him to make you some Pyrex
wineglasses instead, then sit back and watch the show!

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

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