Re: Model T First Light

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi David,

Great to hear the smaller caps are working!

Regarding helix and spiral:
When you run the spiral, check the coupling. When you run the helix, check
the coupling. If you
don't increase the inner diameter of the helix, K is going to climb way up.
You need to adjust
the diameter to get the coupling back down. Once you adjust coupling,
you'll much fewer turns,
but also, you'll see just how wide that helix needs to be (you may not like
it).

The number of turns from spiral to helix is large in your case because the
new cap is now
"really small" and your making up for this in more inductance. The number
of turns between
helix and spiral (not changing coupling via diameter) will is dependent on
frequency and
primary cap size. Once you adjust coupling, you'll have much much fewer
turns (actually, you'll
probalby end up less than a spiral because the helix will need to be quite
wide - try it,
you'll see what I mean).

My preference is a spiral. A spiral will induce the secondary just as well
as a helix as far as
coupling is concerned. Might as well build a simple spiral and keep the
primary as far away
from the spark as possible.

Take care,
Bart

BTW, updated JavaTC to version 9.1 to repair an issue with primary to
secondary height which
John Corture made me aware of (it's good for 4 inch adjustments above and
below the secondary,
but falls off with values beyond this). May need to reload the browser.

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 it is
> a 45 degree conical primarily because I couldn't decide between a spiral
> and a helix (it seemed a good compromise at the time and it does look
cool :-).
>
> 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
>
> =========================================
> Dave Kyle
> Austin, TX USA
> Email: dave-at-kyleusa-dot-com