Re: VTTC Tubes (homemade!)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


>
> Hi Again.  Here is the "plan".
>
> My idea was to use several thoriated tungsten 1/16" rods as the
> filament

You'd need a pretty high current supply to get those suckers red hot so
they'll emit.  I suggest getting some tungsten wire, or scavenging a
filament from something else (i.e. lightbulb).  Tungsten wire is fairly
available mail-order from places doing special effects supplies (they use it
to "invisibly" suspend things.. 1 mil wire is invisible at normal distances
and will hold up several pounds) under the trade name "suspendo".
Straightening out a coiled lamp filament is probably out of the question
(brittleness, etc.)

, window screen, or fine stainless steel mesh as the grid, and a
> 2-3" ID stainless steel tube as the plate.  Glass isn't needed, as the
> plate tube would also serve as the envelope.  I realize that having the
> plate out in the open isn't the safest thing to do, but there really
> isn't any reason to touch a running tube, is there?  The support
> structure would be made from wire-wound resistor cores (ceramics), or
> maybe some sort of porcelain plate.  The electrical input would be via
> spark-plugs brazed into a plug, which would then be brazed into the
> bottom of the plate/envelope tube.  Another plug would fit into the top
> of the envelope (closing the tube).  The top plate would have a threaded
> hole in it, which a plug would screw into.
>
> Evacuating the tube would be crude, but possible (no vacuum pump).  The
> getter, which in this case would be lithium (extracted from lithium
> batteries), is cut into strips, and dropped through the hole in the top
> of the tube into a pan mounted to the top of the filament/grid module.
> Now the fun part!  Oxygen is jetted in through the hole, displacing the
> air in the tube.  The plug is then screwed down tight, sealed (brazed),
> and the whole tube is placed in a hot oven for a few hours.  During this
> time, the lithium ignites, and consumes the O2 in the tube.  This system
> gets rid of the leftover noble gas problem.

I'd suggest using a adsorption pump instead of all the oxygen and lithium
stuff.  Extracting lithium metal from a battery is going to be expensive and
pretty darn tricky.  Lithium is really, really reactive, and if your battery
is a Lithium Thionyl Chloride chemistry, there are other bad news aspects to
it.  The Amateur Scientist (and "the Bell Jar") described how to make a
adsorption pump, which can easily pull down a small enclosure in a "one
shot" scenario very cheaply.  You'll need a small amount of liquid nitrogen,
but that is readily available.  You might need to do a double pumpdown to
get the pressure low enough, depending on how much "junk" there is in your
original tube.

Threaded plugs in any form are going to cause troubles with virtual leaks.
Real tubes that are made "one off"  (I'm familiar with Traveling Wave Tubes,
in particular) use a piece of metal tubing for the vacuum port that is
crimped to seal it off (they use a thing that looks like a bolt cutter with
custom jaws.. Felco is one mfr).  I suggest that you figure out a way to
heat your tube (externally or internally) while pumping, to help with the
outgassing.  A couple hundred degrees C would work nicely.

The getter is usually something like barium, and a bit of research will tell
you how they do it..  External/internal heating  is a good way.  Neon sign
places will have stuff for gettering.



>
> Some of the evacuation procedure is questionable, but I will test it.
> I have some experience with lithium, and believe that this MIGHT work.
>
> The design is hard to follow, but it is a bit clearer in my mind.  It
> sure would be nice if we could find an alternative to expensive tubes.
>
> Thanks for putting up with my stupid ideas. :-/
> Winston
>
> PS- I'll try a diode first, to test the idea.



It might be cheaper, parts cost, than purchasing a tube, but you're going to
spend a LOT of time, which is worth something. Your tube might wind up
having the performance of a $10 vacuum tube, except built with $50 in parts
and $1000 in time.  On the other hand, it is a "cool project".

Don't expect long life and reliability.  That's what a tube mfrs expertise
gets you (and repeatability, etc.)

You should get, if you don't already have it, a copy of John Strong's book:
"Procedures in Experimental Physics".  It is at just this level... Also, get
a copy of the Kurt J. Lesker catalog (big vacuum supplier) because of the
useful information in it.
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