Gap Firing Characteristics (was Re: Tesla Coil Blunderbusses)

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi John,

On 10 Apr 01, at 12:25, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <FutureT-at-aol-dot-com>
> 
> In a message dated 4/9/01 8:54:54 PM Eastern Daylight Time,
> tesla-at-pupman-dot-com writes:
> 
> > >I have seen exactly that effect. I started with a smoothly running
> >  >coil, then tuned the primary down to enhance the lower sideband
> >  >response. Operation went from smooth to erratic but the sparks
> >  >increased considerably in length (about 20% from memory).
> >  >
> >  >Regards,
> >  >malcolm
> 
> Malcolm, all,
> 
> Thanks to all who have responded.  It turns out I was aware of
> this effect, but wasn't thinking.  I wonder why the operation became
> erratic at the new tune point?  
> 
> John Freau

Good question. It's something I've not yet investigated (lying in the 
in-tray as usual). This isn't the first time I've noticed a marked 
difference in gap characteristics when altering something in the 
primary resonant circuit alone. (BTW - altering the tune in that 
fashion is akin to altering k in a way). One line of thought suggests 
that the altered transfer time affecting gap ignition time is having 
some impact on the charging circuit. Expt will eventually tell.

       Let me recount a tale about a coil which I've told before 
where mods to the resonant circuits similarly affected gap operation. 
The gap in this coil was a single static gap with 1/2" W-C electrode 
tips. The coil had a rather modest topload for its power supply 
capability and also suffered from a low primary inductance. After 
being inspired by a piece from Scott Myers on how he improved the 
output of his coil, I eventually got around to building a larger 
oblate topload for the coil. Secondary Fr went from about 155kHz to 
130kHz, a figure which serendipitously allowed me to add a single 
turn to the primary to maintain tune, boosting primary inductance by 
about 50%. (Aside - the primary ringdown showed an enormous reduction 
in losses). 
      Up until the mod was done, the gap work smoothly with no 
airblast. Following the mod and with the gap in no way altered, 
operation was totally erratic. Much messing around failed to get it 
running smoothly. At that point, I started running a jet of air 
through the gap and through much adjusting of the airflow and gap, 
got closer to good operation but still very erratic. It would fire in 
bursts of half a second to a couple of seconds and then just stop. 
Finally, I got rid of the 2mH of chokes between the transformer and 
gap (I had been intending to anyway for quite a while). Suddenly, the 
coil started to behave well and fine tuning of airflow and gap 
yielded some real results. Output of the coil had improved by a good 
20% with sparks showing the brilliance and loudness one associates 
with a decent energy store at the top. I posted a few pictures in odd 
places of the coil in action at last year's open day at the 
university.
      Due to the still rather low primary inductance, I have not been 
able to offset tune the coil to date much to my regret.

Regards,
Malcolm