Tube top caps

Fred Olsen fwolsen at EXECPC.COM
Fri Jun 1 23:58:30 EDT 2001

I don't think I implied that the cement made the electrical connection.
Of course one has to solder the wire to the cap, or to the base pin.
The point though is exactly the same as was made in the kit manuals, to
"make a secure mechanical connection before soldering".  The cement is
intended to provide the mechanical connection, the solder the

Since that's come up, perhaps we should touch on the proper way to do
it.  When dealing with heated glass you never want to tie it down too
tight.  That's why the correct cements have a particular hardness and
coefficient of expansion, to match the glass's needs.  Never just fill a
tube cap with cement and plop it on.  The trick is to lay a bead of
cement around the inside diameter of the (cleaned-out) cap, such that
the plate lead is free from the cement.  The lead should be formed with
some slight strain relief, a small S-bend or a portion of a helix, so
that the tube and cap assembly can expand and contract without stressing
the fret seal of the plate lead.  First have the cap and lead all
prepared, cleaned, tinned, ready to go.  Cement the cap and let it cure,
then solder the lead quickly and surely and get out.

Depending upon just the solder joint for mechanical strength will lead
to eventual failure of the plate lead seal, and a shot tube.  Prying the
connector off just that one more time will do it.  If the cement weren't
needed, the manufacturers wouldn't have used it.


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