Got 24" out of VTTC

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Original poster: "Ross Overstreet by way of Terry Fritz <twftesla-at-uswest-dot-net>" <ross-o-at-mindspring-dot-com>

Hi Guys,
I just managed 2 feet of really hot arcs out of my 3-1000z VTTC.  I decided
to live a little and push the tube into unknown territory.  The 2 foot arcs
really started happening about 5300VAC and 850ma on the plate.  I used to
back the power down every time the center of the plate started to get cherry
red.  I ran it tonight until the entire thing glowed yellow!  The arc to
power ratio is still pitiful, but finally I feel a bit of success in this
project!

I haven't updated my VTTC page with tonight's results yet.  I got some
QuickTime movies but didn't manage any pics.  I'll probably get around to
documenting it over the next few days.

The main thing that I changed was MORE VOLTAGE on the tube.  I was
previously running at around 2400VAC, 800ma(plate) and only getting 15-18"
arcs and they didn't have many branches.  Now I get 24" inch strikes and the
arc is a lot more "bushy".

Here are some observations:

1.  Grid leak R is now at 750 ohms.  This seems to clamp I(grid) at around
250ma.  It's interesting to watch I(grid) increase until arcs start to break
out.  Then it increases a tiny bit more and then starts to back down to some
steady state value for arc lengths from 2" to 24".  The arcs seem to clamp
it at some value.  I noticed that I(grid) climbs dramatically as R is
decreased and I(grid) decreases as R is increased.  It's quite possible that
I may be able to get the same length arcs with more R(grid) and less
I(grid).  I'll have to test this out another night.

2.  The resonant freq of the coil seems to change significantly between when
it's producing 10" sparse arcs and when it's making 24" bushy arcs.  I
noticed this while playing around with vacuum variable cap in parallel with
my tank C.  My primary has a few too many turns in it and I expect the
system to be in instrument tune with C(vac) at its minimum position.  Sure
enough, I can set it for 10" arcs and make them almost disappear by moving
C(vac) to its max position.  However, if I leave C(vac) at a middle position
and really pour the coal to it, the VTTC seems to perform better than when I
put it in instrument tune!  By the way, I can swing Primary F(res) about
50khz with C(vac). (I realize all this is subjective and nonscientific - I
felt more like goofing around tonight than being serious.  Better
measurements are in order)

I think the best route for these animals may to be to instrument tune the
primary with any variable C at its minimum position.  Then you can increase
primary C as necessary to drop the primary res freq to match the lower
resonant freq of the secondary.

3.  There is a clear maxima as you raise the grid coil above the primary
coil if you leave grid leak resistance fixed.  It's surprising how far apart
these guys want to be in my coil.  Matter of fact, I have to wind another
grid coil because I just had it sitting on top of the  primary coil's PCV
former with nothing between it and the secondary but 2" of air.  This wasn't
sufficient at 5+KVAC and a nasty flash over got it.  In order to play for a
few more minutes, I stuck a zip lock bag between the two and kept running.
The zip lock worked, but it started to get warm?!?  Weird, huh?  I'm
guessing corona was going to it from the partially burned grid coil.

Enough rambling.  I guess the moral is to take reasonable measures to ensure
you are in tune and then just pour the power to these things until big fire
comes out the top! (or components melt down - either way makes a good show).

BTW, Wild Bill (of TCBFW Fame) and I are both getting 24" arcs out of our
VTTCs now.  He is running 800ma -at- 6KV and I'm running 850ma -at- 5.3KV
(completely different triodes).  Anyone in the two foot range on raw AC
doing better than this?

Cheers,
Ross-O