The Quick and Dirty VTTC

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

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Folks!


Well my word!  I've had more good old fashioned FUN this
evening than in a long time.

Thank you Marc Metlicka and Ulrich Haumann!

http://members.aol-dot-com/uhaumann2/plasma/eplasma.htm

This thing works!  Leastways as a VTTC.  I haven't done
anything about modulating it yet, but it looked so cute on
these webpages that I simply _had_ to try it.  Well, I
wasn't going to try the 813s just thrown together anyhow on
a piece of wood (and certainly not first time with a 2kV dc
psu directly connected to the TC base - see the circuit
diagram on the above page) as Bad Things can happen with the
larger rf tubes if they don't like what you've done to them.
However, it was worth a ferreting session in my plastic bag
of old bottles, and lo and behold - no colour TV deflection
valves (Ulrich's version uses a pocket battleship of a
bottle, the EL519) but an old, war-weary Mullard EL37 which
I rescued from a long-defunct constant voltage transformer
some years ago.  This thing is a direct drop-in equivalent
of the KT66, with 25W plate dissipation.  Anyhow.

Tesla coil 37 turns of 2,2mm wire rescued from an
autotransformer, a bit over one inch diameter and around
95mm long on a junkbox ceramic former.  The quickest TC I
ever wound.  Filed a point on the top end wire.  Basically I
used the circuit diagram on the webpage above.  Minor
changes - g2 connected to plate supply via 4,3k resistor
with no provision as yet for modulation; g2 bypass 0,5uF.
Grid leak 15k 1W.  Plate choke 164uH, homebrew.  Plate
supply thanks to a scrap transformer I rescued from a ham
rally a couple of years back  450-0-450 -at- 120mA, plus type
83 mercury rectifier with homebrew filament tranny, 10H
choke input filter, 4uF smoothing cap with 30k bleeder.
Nominal 420V output.

So I give the 83 filament a couple of minutes and hit the HT
switch.  Nothing.  The 83 lights up blue :-)  The EL37
lights up blue 8-[]  Hard blue rather than soft blue :-)  I
congratulate myself on rescuing a good tube.  I poke around
a bit.  With due caution I bring a neon bulb near the TC.
It lights around 2 inches away.  Ha!  We have oscillation!
I touch the tip of a well-insulated screwdriver to the point
of the TC.  Instant plasma flame :-)  It is also blue, all
of 10mm long (pardon me but that's not an accurate
measurement, the ruler isn't going to survive the
temperature, never mind the volts, and the TC base is
connected directly to the valve anode and hence the HT+,
which does not suffer fools or take prisoners, nor encourage
physical intimacy ;-) and *silent*!  The transformer
laminations are making more noise than the arc!  The tip is
greenish, indicating that the copper wire is vaporising
slightly.

The nominal 420V 120mA supply has dropped to 325V on load -
and this is a choke input psu! Vg2 is 270V.  The 1W 15k grid
resistor is *warm* (I switched it all off before feeling
this ;-).  No metering for Ia or Ig1 or 2 yet, and only
guesswork for the frequency, it's probably in the ballpark
of 20-30Mc/s (I like accuracy ;-)

The Tesla arc is non-self starting and is easily blown out
like a candle flame.  Ideal for that electric birthday cake.
Once "lit", it is extremely hot.  A piece of glass tubing
was cautiously introduced into the arc (no idea if it was
sodalime glass or borosilicate) and as expected the arc
turns yellow.  What was *not* expected was that the point of
contact between arc and glass is white hot and emits bright
white light.  Also, since the glass becomes conducting when
hot, it can be moved to one side of the arc, dragging a
yellow arc with it, whilst a blue arc continues straight up
simultaneously.  Two for the price of one!

It must have taken all of a couple or three hours in between
meals etc casually putting this thing together.  Built like
a rat's nest, breadboard-fashion on a bit of particle board,
and nothing nailed down either.  Quick and dirty.  My "first
light" with my first VTTC:-)  I've seen clever ham designs
for "no-tune" transverters and the like.  Well, now there's
a no-tune VTTC.  This I really like a lot.

I haven't had this much fun in ages!  It must have been
good - I'm sat here at 00:45am typing this ;-)

Dunckx