Re: First light - or rather disappointing performance

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

Original poster: "Peter Lawrence by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Peter.Lawrence-at-Sun.COM>


Chris,
       I've built LOTS of "small" TCs, all on 3 and 3.5 inch (76 and 90 mm)
coil forms, with many different sizes of wire and hence number of turns,
and started with a 6kv-30ma NST also.

I never got more than 6 to 8 inch sparks from a 6-30 NST,
10 to 12 inch from a 7.5-30 NST
and 14 to 16 inch from a 9kv-30ma NST.

Small TCs just don't perform according to the "formula" the way larger TCs do.

In my experiments, more turns (via finer wire) makes a small improvement,
but only an inch or so, nothing dramatic.

In my experiments I've tried aspect ratios of 3, 3.5, 4, 5, and they all
perform about the same.

(caveat, I have gotten longer sparks, but only briefly, after which my NST
burned up, because I openned up my spark-gap too wide. since then I've
been using a safety gap religiously).

(note, I have not tried a SRSG yet).

(last note, a "blown gap" does add noticably to my spark length over a non-
ventilated one).

this hobby is all about experimenting, enjoy,
Pete Lawrence.




 >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.
 >
 >