Re: New coil

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

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi John,

Congrats on first light on this coil!

Tesla list wrote:

>Original poster: "John Richardson" <jprich-at-up-dot-net>
>Hello,
>
>
>Upon first fire up, I tapped at turn 14 1/4, as I usually start from the
>outside in when first firing.  This happens to be the number of turns I
>wound up with before running out of a full roll of tubing.

Yep, that's how most of us do it. With the small toroid, should tune in 
at  15.9 turns (which you don't have). On this number I guessed at the 
secondary turns at 1480 and used a primary cap size of .0188uF. So if 
that's the case, you would need more turns than you currently have to get 
this coil properly tuned.

>   Instant multiple
>streamers, maybe four or five at a time, with the longest consistently
>hitting the 36" mark.  Within a minute or so, a capacitor lead arced to the
>muffin fan, killing it.  I set up another fan, and proceeded.  Within
>another minute, I heard a sound like a rifle shot every 15 seconds or so.
>Turns out the last capacitor in the bank was arcing to a bolt which joins
>this same cap to a lead to the primary.  The arc had to jump over an inch,
>and I figured that since both the last capacitor and the wire going to the
>primary SHOULD be at the same potential, I wouldn't have a problem.
>Why did
>this happen?  Even though there is a hole in this cap, there was no loss in
>performance.

Sounds like you killed the cap. I've killed one MMC cap (a long time ago in 
a galaxy far far away). It blasted a whole in side of the cap. Sounds like 
what our describing. It's possible that you overvolted the cap? The cap 
bank rating is 16kV, the transformers peak potential is 16.97kV. Your bank 
is kind of "on the edge", but this can become aggrivated by the spark gap 
distance. Did you set the gap by checking the NST's ability to arc across 
the gap (before hooking up to coil)?

In any event, the cap is dead, and now your down to 7 caps 
(.0214uF=assuming 0.15uF is value of single cap). Cap bank rating drops 
down to 14kV (you'll need to decrease the gap for the lower voltage). Or 
get more MMC's. I personally would try to get 2 strings of 12 caps for a 
voltage rating of double the NST (and this is modest, not greatly 
overrated). Also, that bank would yield .025uF and tune about 14 turns.
I know this sounds overboard for MMC's, but I'm part of the croud that 
would rather over-rate the cap rather than hope all goes well.

One thing in your favor here, the larger cap value of .0214uF (7 cap 
string) would put you in at about 15 turns (still need another turn of 
primary to make this). For what it's worth, my latest MMC of .025uF has 3 
strings of 18 caps (54 caps), which has a bank rating of 36kV (and I still 
worry, but no cap deaths).

>I am obviously going to need a larger topload, and was
>thinking of going to a 6x30 for the final configuration, judging from the
>length and amount of streamers I presently have.  Do you guys think I should
>add a few more turns to the primary, taking into consideration the larger
>topload and future dedicated 15/60 NST?

Yes, for the topload change in C. Now with the .0214uF cap bank, you need 
15.6 turns. So more turns would probably be good whatever direction you decide.

Take care,
Bart