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Re Design of my first coil

>Original Poster: "Samuel Rosset" <samr-at-chez-dot-com> 
>
>I'm about to build my first coil, but before starting, I would like to tell
>you how I plan to build it, and I would really like to have some feedback
>from you, experienced coilers.
>  
>1) I found a NST 12KV 30ma that I'm planing to use. According to the
>formula I found on a TC site, the matching capacitance would be 0.0066 uF
>(micro farad). I planned to build either a SW cap or a alu-glass cap. But
>this morning, my physics teacher showed me an "object" (It's most likely a
>cap, but who knows...) here is what is written on it: Spargue, 500MMFD
>20KV. 20KV??? It will work fine for my coil, but what does 500 MMFD mean???
>0.5 uF (If it's that, it is way too big)?? 0.0005 uF???(better; I would
>need 10 or 12 of these) Can I use these???

500 MMFD is 500 micro micro Farads, or 500pF, or .0005uF.  That's about
10 times too small.

>2) Spark Gap : Satic Gap with 5 gaps. total spacing : 0.25'' (with a
>computer fan to get rid of the heat.) Do I need more gaps???

That will work.  Just make sure that the fan is directed to push/pull air
through the ARC CHANNELS of the gaps.  This is what helps the quenching
and is independant of the mere cooling of the assembly.

>3)Primary: copper wire (0.25'' diameter) in a flat spirale. Inner Turn
>diameter : 6.5''. Spacing between turns 0.25''. 12 turns should be What I
>need, but I plan to have 15 turn for tuning... Main question : Is the flat
>spiral the best or should I build a cone???

My first coil also used a flat primary and a 4.25" secondary.  The best
thing I ever did was move to a 15 degree conical primary.  MUCH better
performance.  Apparently with 4" or smaller primaries, the coupling is
marginal with flat spirals.  With 6" and up, coupling is adequate with
flat.

>4)Secondary: PVC pipe. Winding diameter 4.5''. winding length 18''. Now, I
>read somewehere that I should use a wire AWG 24. My question is : What is
>the diameter of such wire and where could I find 1000 feet (old transformer
>etc???)

The 18" winding length is slightly on the short side for a 4.5" secondary
and may limit the length of your sparks, particularly if you opt for the
conical primary.  When dealing with secondary diameters of 4.5" and
below, most folks aim for the high end of the 4-5:1 H:D ratio.  It's
unlikely that you could find enough wire in an old transformer, I think
most folks just have to buy spools of it.  #24 wire is .0201" dia.

>I have thousands of other questions, but here are the basic ones. Oh! a
>last one. What would happen if the capacitance of the main tank is way too
>big (let's say 100 times too big???)

If it's 100x too big to achieve the same resonant frequency as the
secondary, then you'll get no sparks from the secondary, and you might
even find that your spark gap won't fire.

Regards, Gary Lau
Waltham, MA USA