First light - or rather disappointing performance

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

Original poster: "Mudford, Chris by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chris.mudford-at-agresearch.co.nz>

Hi all

A few specs and a few questions.

This is a small coil that I was given and was fairly poorly built.  I
have used it as a learning experience before I build a larger coil.  In
its original configuration it had a single spark gap mounted on plastic
which ended up melting, hence widening the gap and blowing the cap.  The
primary coil was a bundle insulated wire with 3 turns of bare wire
outside of this for tuning.  It has a 90 mm diameter secondary of 250 mm
winding height of about 500 turns with a small spun aluminium toroid
atop (200 mm main diameter and 50 mm minor).  It was fed with a 6 kV 30
mA NST through a wound wire as a choke.

 >From my readings this coil was inadequate so I decided to rebuild most
of it and give details as follow:

6 kV 30 mA NST.
Filter on HV side (safety gap between each leg and case, one 1000 pF 10
kVdc ceramic disk cap between each leg and case, 1150 ohm 55 W resistor
(5 smaller values in series) on each Leg).
Multiple static gap (4 gaps between 25mm diameter copper tube 40 mm
long) 3 tubes on one board 2 on another, moving boards apart adjust
total gap.  With fan.
Resonant MMC capacitor, 4 strings of six film-foil polypropylene
capacitors 1500V 0.022 uF (RS 114-468) for 9 kV 14.7 nF. Safety gap.  33
mOhm bleed resistors.
Primary coil of 15 turns of 4.2 mm copper refrigeration tube with 4.2 mm
between turns as flat archimedes spiral.  1st turn 37.5 mm from
secondary.
Secondary as per original, bottom turn on same plain as primary.
Toroid as per original, 50 mm above final secondary winding.

I wired it up with small diameter cable of short length (didn't think it
would matter at 180 W) and found tune point to be at turn 2.  This gave
me 250 mm to a grounded wire from a break out point and 4-5 200 mm
streamers into free space.  I was only using a small tin box (200 x 300
x 500 mm) as a counterpoise.  All arcs were of low intensity and needed
the lights out to see them and are far from the calculated 22" possible.

Now for the questions:

Setting safety gaps (across capacitor and on filter board).  I used 1
kV/mm to set each leg of filter board for 3 mm and across capacitor to 6
mm.  Should these be calculated for peak voltage eg 8.5 mm.

Setting main spark gap distance.  I ran it connected to the NST alone
and adjusted the gap so that it was smoothly firing, however, I could
only see arcs in the gaps at each end.  Note the tubes aren't exactly
parallel nor are the gaps exactly the same.

Secondary coil.  Is the aspect ratio of 2.8 too low?  Is 90 mm diameter
too great?  Are 500 turns not enough?

Is this type of spark gap over kill and too lossy?  Would I be better
with two large brass bolts?

Any comments, suggestions or questions are most welcome.  At the moment
I think I need to wind a new secondary at 5:1 aspect with 1000 turns.  I
can't change my secondary diameter unless a greater primary to secondary
seperation is not a concern.

This coil is probably over built for the power input but I plan to feed
it with 15 kV 120 mA with a resonant capacitor.  But for the moment I
want longer and brighter streamers from it.

Thanks for your time.

Chris. NZ.