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Re: Still mystified by odd TC behavior, what gives?

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

Tesla list wrote:
> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> Hi Jonathan,
> If you can add another ~0.025 cap in series and it suddenly works, it does
> sound like there may be a problem with the first cap.  It may be arcing
> over internally or something but it would also be smoking and burning up.
> However, if both caps are good, the effective capacitance will be divided
> in half to around 0.012 which may simply be a far better point for you
> cap/NST combinations to work at.  If the gap fires well with a given cap
> but there is no output, that sure sounds like the primary circuit and the
> secondary circuit are not tuned together.  I would try getting more turns
> on the primary first.
> The "sweet spot" should not be all that hard to find.  I wonder if your
> primary has too few turns and you are simply not able to tune down low
> enough in frequency.  You may want to figure out the primary and secondary
> frequencies with one of the programs or equations to see where you are at.
> There was not enough info here for me to do it.
> Cheers,
>         Terry
> At 07:03 AM 5/19/2001 -0700, you wrote:
> >Howdy All,
> >
> >I have been struggling for over a week to figure out what the heck is
> >going on here. If anyone can help me, I'd be soooo happy. Here is the
> >situation:
> >
> >Basic coil specs:
> >
> >Small secondary, 450 wraps of 22 AWG on a 3.5" form.
> >Primary: Either a 12 turns 12 AWG auto wire on a 6.5" form, or 15 wraps
> >10 AWG auto wire on a 8" form.
> >PSU: Either a 12kv/120ma or 15kv/60ma.
> >Vacuum single gap
> >RF filter ALA Terry
> >
> >I recently got a Resonant Research Inc. .025 (actually tests to .023)
> >tank cap. When I try to use it, with either transformer or even with
> >both ganged up in parallel, I get no output from the secondary,
> >regardless of how I tune the primary or set the SG. I can draw a wimpy
> >spark to a ground rod, but that's it. The gap fires fine over a range of
> >settings. As soon as I add another cap in series to the .023 and lower
> >the capacitance, it fires up. Yet the gap isn't acting as I have seen in
> >the past with too large a tank cap, which has always resulted in erratic
> >SG performance.
> >
> >The only thing I can think of is that the "sweet spot" is really small,
> >and as I only have taps on each turn on the large primary I haven't hit
> >it right. Is it possible for the tuning to be so sensitive that one gets
> >no output unless really close? The gap sounds mean and hot, but is it
> >possible that the .025 cap is damaged? It seems unlikely considering the
> >fury of the gap.
> >
> >If you can help, you're my Hero!
> >
> >TIA,
> >
> >Jonathan Peakall (The other OTHER Jonathan)
> >

	There isn't enough information there to really understand how your coil
SHOULD behave.  Things which would help:

1. Length of secondary,

2. Terminal capacitance,

3. Length of the two primaries

4. Capacitance of "the other cap".

Making a rough estimate here I find that the resonant frequency of your
secondary with no added top loading is of the order of 1000 kHz.  The
inductance of your smaller primary is about 30 uH, that of the larger is
about 40 uH. The resonant frequency of the smaller primary with 0.023
ufd would be about 195 kHz, that of the larger would be around 165 kHz. 
As a rough guess, the thing should tune with around 6 turns on the
larger primary and 5 turns on the smaller.  It seems to me that it is
quite possible that you never got the primary tuned to the right
frequency and that your new capacitor will be OK.  You didn't mention
what happens when you hook it to the transformer and gap without the
primary coil in series.