Re: Model T First Light

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "David Kyle by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dave-at-kyleusa-dot-com>
> 
> The new caps came in and with a few modifications I got them fitted to my
> battery powered Model T spark coil powered Tesla Coil. I put the two
> doorknobs in series for a value of 1000PF at 80KVDC (well at least the caps
> should never fry from over voltage!). I fired it up and with a few
> adjustments to the spark gap got consistent firing with about a 1/8 inch
> gap. Being totally out of tune I did not expect much but was pleasantly
> surprised when I could draw 1/2 inch sparks from the top load. No breakout
> but it did light a florescent tube a few feet away.
> 
> This seems entirely consistent with Ed's results from several days ago. It
> also suggests my original problem was in fact way too much capacitance for
> such a low powered coil. At .01uf I was effectively showing the spark coil
> a short. In fact 1000pf is probably the maximum this one can go and a wider
> spark gap could no doubt be had at 500pf.
> 
> Now to tuning and primary geometry:
> 
> I am going to have to rebuild the primary since I had never planned on such
> a small cap and have far too few turns to get it in tune. At present 

> Since I probably need all the coupling I can get, I am now leaning towards
> a helix. What say you all, spiral or a helix?
> 
> The only downside that I can see to a Helix is that it takes far more turns
> to tune with a helix. Using JavaTC I find I need 26 turns with a spiral and
> 58 turns with a helix. Does this seem correct and why?
> 
> Thanks again for all your help,
> 
> Dave

	Sounds like you're on your way.  I'd use the helix/solenoid as it's
much easier to built and at this power level should be totally
satisfactory.  Don't know diameter of your secondary (if you posted it I
got rid of it when I deleted the original message) but in my case it is
about 1-3/4" and the "new" primary diameter was 3".  I made the primary
2" long so I could have it pretty close wound with #17 wire off a spool
which was sitting within easy reach.  Problem is that I do tend to get
some discharge from the top of the primary to the secondary.  It the
diameter were 3-1/2" or 4" there would be more safety factor and getting
enough coupling should be no problem.  Used the 3" diameter stuff as it
was the first cardboard tube I encountered when I went out to my shop 
to start the project.  If and when I start over and make a better
primary think I will use something like #22 wire instead, to reduce the
length (height).  The primary impedance will be high enough that I don't
think there would be any loss of performance due to the use of the
smaller wire.

	On the subject of small and low power coils I wonder if anyone has ever
given any thought to the optimum secondary size, if there is one? 
"Conventional wisdom" sort of suggests a relatively small secondary, but
then conventional wisdom often isn't!  If my memory is correct Richard
Hull once described a little coil he built, but can't remember the
details; it wasn't this low power of course.

Ed