re: 70kv system

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

Original poster: "Peter Lawrence by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>

Johnathan,
          I too built one of those (almost 20 years ago). The spark-coil
powered TCs are always going to be disappointing. I got maybe 1" sparks
from the top of my TC that way, but the spark-coil that powered it could
do 1" sparks so what was the point of the TC in that case?

I soon obtained a (very small) 6kv-20ma Neon Sign Transformer, and got
4~5" sparks right away. Cool.

It is impossible that small changes like #22 verses #24 wire size are going
to make any difference.  (I've been experimenting with difference wire
sizes and number of turns, everything from #24 all the way down to #36, and
its hard to see any difference with less than 4 wire guage sizes appart).
Same thing with winding diameter. Same thing with primary wire guage and
diameter. I've used everything from 3/16" tubing to #14 (0.064") solid wire,
its all the same on these small sized TCs. You get close to some reference
design, and then adjust your primary tap point to tune the coil, it always
works.

Here are some of my observations on small systems:

get a NST power supply (MOTs have too much current for a static spark gap),

build an MMC cap (contact Terry Fritz for the caps, I use 2 or 3 strings of
12 caps each for my 6kv-20ma, 7.5kv-30ma, and 9kv-30ma NSTs; 12 long strings
is overkill for these voltages but hey I've never blown a cap this way!),

the best static gap I've used has tungsten-carbide balls for the electrodes
(brass and stainless steel corrode and the corrosion tends to diminish TC
performance, requiring frequent cleaning),

get a torroid for the top (a ball top will not suppress the corona on the
last few windings of the secondary),

if you are going to use more than 7.5kv input then put a 2" ball about 4"
above the center of the torroid as a break out, this way the sparks go UP,
rather than OUT; the difference is the outward sparks will tend to hit
the primary or the NST or anything else nearby, maybe even you.

built a safety gap, or you will blow your (possibly expensive) NST. This
warning comes from experience!!!

put 1-kohm-25-watt resistors between the safety gap and the primary capacitor,
that way the cap won't put out an explosive discharge across the safety gap
when it fires. (dont worry about power loss in the resistor, the NST has an
internal dc resistance of 20-50 kohms, so you are only adding 2-5%).

For years I thought I was doing something wrong not getting the 8~10" sparks
that the plans said were possible. Not so, the only way to get that long
of a spark on a 12" high coil is with 7.5kv input, and 9kv will get 12~14".


-Peter Lawrence.