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Re: Tuning, toploads and tank caps
Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>
> Howdy all,
For the second time, hi :)
> 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
If by higher tuning you mean higher resonance, then yes. If by higher tuning
you mean less turns, then no it is the opposite. As a rule the smaller the
cap the higher the resonance, the less turns on the primary the smaller the
resonance. If you want to use a smaller primary cap, then you need more
> Larger topload = lower tuning on primary
> Smaller topload = higher tuning on primary
Same as above. A larger topload decreases the resonance, and vice versa.
> Increasing gap = lowering frequency
> Reducing gap = raising frequency
No, not at all. The resonant rise happens during one gap firing. Changing
the break rate just changes the amount and speed that energy gets to the
primary coil. It has NOTHING to do with resonance.
> 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
> 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
> 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?
A number of things - topload is too large, even if it is resonating at the
right freq. Power supply is too small, but I dont believe that for one
second - 12/120 is really big :)... The most likely is that it is de-tuned.
> Is it possible to have a tank cap too big for a coil, regardless of PSU
> current? Inductance?
With an AC system, yes. The cap is designed to charge at a certain rate,
according to the xfmr output. If the cap is far too big, it wont charge
fully, and the gap won't fire properly.
> Shouldn't closing the gap allow for higher tuning on the primary?
No, as I said firing rate has nothing to do with resonance. Resonant rise
occurs during a single gap firing when the arc is 'lit', not over a series
> Is it possible that the primary "sweet spot" is so small that I am
> it? I only have taps every turn.
Yes, quite probably. Some coils need to be tuned to an inch before they
preform. Try sliding the tap along until the max is reached.
> Would using a smaller topload help?
> Isn't a 15/120 more than adequate for the 14.3nf caps?
with a 12/120 I would go for much more than 14.3nF. With a 10/100 I am using
35nF and would like to use about 40-40nF
> 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!!
:) good luck
> Thanks all,
> Jonathan Peakall