Re: VTTC Tubes

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Loudner, Godfrey by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>
> 
> Hi Winston
> 
> I often wondered if a crude but working transmitting tube could be
> homebrewed and place inside a bell jar. It seems like one would need
> constant access to the insides to make repairs. A quickly removable bell jar
> would be ideal. On the other hand, tube construction seems impossible for
> the homebrew artist. The specialized equipment is probably too expensive.
> Stringing and attaching tungsten filaments!---I just don't know.
> 
> Godfrey Loudner

	You can't just "throw the parts together" and expect the thing to
work.  The geometry of the grid and plate are important, and the
filament must be sized right to give enough emission without requiring
way too much filament current.  For a HV transmitting tube you'll need a
vacuum at least as good as 10^-4 mm, which requires outgassing the
electrodes by heating them to white heat while on the pump.  Taking all
those things together, I fear that any attempts along this line will
result in disappointment.  The use of "demountable" and continuously
pumped tubes used to be common in very high power BC transmitters, and
in some of the early British "Chain Home" radar transmitters.  Advances
in vacuum techniques and tube materials led to the development of
sealed-off tubes which worked at least as well and were much easier to
use.

	I have made and operated small diodes and triodes here, using a setup
about like you describe.  Was able to get them to "work", but very
poorly and had trouble with the tungsten filament being so hot that it
melted the copper grids and plates which I used.  Never could keep the
pressure low enough to allow operation without ionization ("blue glow")
for voltages much above 100.  Can obviously be done with more care than
I put into it, but more for the sake of just doing it than the hope to
make power tubes.  A friend of mine, now deceased, did make a number of
small tubes in his lab.  He had a large induction heater (4 833A's in
parallel) for outgassing, abd access to tungsten filament wire in many
sizes, plus molybdenum for the plates and grids.  I never tried any of
his tubes, but sure they would have worked. I still have a little soft
X-Ray tube he made; it works well "on the pump".

	For more information on vacuum-related stuff take a look at these
sites:

 http://www.tiac-dot-net/users/shansen/belljar/
 
 http://www.thesphere-dot-com/SAS/SciAm/1996/Vacuum/Vacuum.html

 http://nyquist.ee.ualberta.ca/~schmaus/vac.html#top

 http://www.lesker-dot-com/

And get a copy of John Strong's "Procedures in Experimental Physics"
from Lindsay publications.  Because the book is about 60 years old it's
full of really good construction stuff which should be of interest to
almost any experimenter because in those days a lot of physicists built
their own equipment.

Have fun,

Ed