Re: VTTC Tubes (homemade!)

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

Original poster: "Marry Krutsch by way of Terry Fritz <twftesla-at-qwest-dot-net>" <u236-at-earthlink-dot-net>

Hi Jim.

	Comments scattered about.

Tesla list wrote:
> 
> 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.)

	The reason I chose those is because A) I have them, and B) I have a
5VAC -at- 400 amp XFMR.  Also, construction of a filament using thin wire
would be more complicated than one made from rods (support structure). 
The rods could even stand up on their own.  But, I will keep my eyes
open for tungsten wire.


> >
> > 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.

	Actaully, all the lithium I have is from batteries.  A single AA
battery has quite a bit in it, in the form of a long piece of thick foil
(convenient size).  I have the extraction procedure down pretty well,
and it is really very simple.  I don't know about the type of chemistry
the batteries use.  I use Energizer batteries.

> 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 plugs would be brazed in place to make the final seal.  I don't
think that a good joint would leak.  (I could be wrong, though.)
 
> 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".

	As you say, this is far from practical, most likely.  However, the
"cool project" aspect may be too much for me to resist :-)).

> 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.
> >
> >
> >