Re: Tuning, toploads and tank caps

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

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

Hi Jonathon,

On 25 Jul 01, at 11:30, Tesla list wrote:

> Original poster: "jpeakall by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jpeakall-at-mcn-dot-org>
> 
> Howdy all,
> 
> Before I ask my questions, let's make sure I have this right:
> 
> More caps in tank circuit = lower tuning on primary
> Less caps in circuit = higher tuning on primary

That obviously depends on whether primary inductance stays the same 
or not.

> Larger topload = lower tuning on primary
> Smaller topload = higher tuning on primary

If the secondary coil stays the same.

> Increasing gap = lowering frequency
> Reducing gap = raising frequency

No. Increasing the gap width = increasing the primary (shot) energy, 
and vice versa. 

> And here are basic coil specs:
> 
> Secondary: 14" of 22ga wire, close wound, on a 3.25 form
> Primary : Have several, one I am trying now is helical, 10 wraps 10 ga on a
> 9.5" form
> PSU: 15/120 Franceformer, have tried a 12/120 and 2*15/60
> Tank caps: see below
> SG: Single vacuum gap ala Lau
> Topload: 3" duct / 9" pie pan torrid or a copper toilet float
> 
> I normally run this TC with about 7.4nf SW caps. This tunes at around four
> turns. I have two caps that connected in series give 14.3nf. I have tried
> and tried, but I can't get the coil to break out with these caps. When using
> the toilet float, best output is on one turn, judging by drawing arcs to a
> ground rod. With the torrid, best tuning is at 3 turns. At most, with the
> torrid, I can draw about 8" sparks, but no break out. I have raised and
> lowered both the secondary and topload, and am not undercoupled, even with
> the large primary (I have to keep the secondary raised to avoid racing
> sparks). SG action is hot and heavy, and has been with all the PSUs I have
> tried, so I think the caps and trannys are ok.
> 
> My questions:
> 
> Why the heck won't it break out?

Firstly, your cap claulation is in error. Putting two in series 
halves the capacitance and for the same gap setting, cuts the primary 
energy in half. Leaving losses out of it that means your output 
voltage will have dropped by SQRT(2).

> Is it possible to have a tank cap too big for a coil, regardless of PSU
> current? Inductance?

In a word, no, but for a given secondary, increasing primary 
capacitance demands a reduction in primary inductance and you will 
see primary losses climbing dramatically as inductance drops and the 
point will be reached where you lose more power in the gap than the 
secondary.

> Shouldn't closing the gap allow for higher tuning on the primary?

No.

> Is it possible that the primary  "sweet spot" is so small that I am missing
> it? I only have taps every turn.

Your biggest problem is the operating frequency of your coil which 
demands a low primary inductance be used. You aren't left with too 
many tapping options by the time only three turns is in the circuit. 
Ironically, you should be tapping more turns in for your series cap 
combination and that will reduce you primary losses and might well 
offset the drop in output voltage caused by the energy reduction.

> Would using a smaller topload help?

No.

> Isn't a 15/120 more than adequate for the 14.3nf caps?

In my opinion, your caps are far too small to get the best from that 
transformer.

Malcolm
 
> Any ideas or advice would be most welcome. This is like a loose tooth when
> you're a kid, I just can't leave it alone, and it's bugging the !#%^&* out
> of me!!
 
> Thanks all,
> 
> Jonathan Peakall
> 
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