RE: Coil Design for Review

The design looks fundamentally good, but when I calculate your primary tank
resonance, I get 165KHz, a little higher than your measured loaded secondary
Fres.  I think in operation, your secondary will want to run slightly lower
than it's static Fres.  So you can either get a little more tank C, a couple
more primary turns, or a smaller topload.  Personally I'd opt for the bigger
tank C (easier said than bought).

With a single static spark gap of 0.34", I measured a breakdown voltage of
24KV (with a 15KV NST).  I can't say for sure that the voltage is linear if
you break the gaps down into several and add them up, but your total gap of
0.39" may just be on the wide side for a 12KV NST.

I also used the same Fair Radio Sales cap and got very good results.  See

Regards, Gary Lau
Waltham, MA USA

Original Poster: pmathus-at-learningco-dot-com 

     Kind Tesla Gurus,
     Could you take a look at my TC design and tell me if you think it'll
     I've started work on the sparkgap and ordered the cap, but the rest is 
     changeable at this point. 
     Is my sparkgap too wide?  Am i going to tap out the primary trying to
     Most importantly, will this make big, scary sparks?
     thanks, and any comments welcomed.
     paul mathus
     transformer: 2 banked 12kv 30ma NSTs, for 12kv 60ma total.
     sparkgap: static RQ/TCBOR type gap, fan cooled, 13 gaps of .03 each for

     total gap of .39"
     tank capacitor: Fair Radio .01 mfd 100kvdc cap
     primary: flat, 15 turns of 1/4" refrigerator tubing at 1/4" spacing, 
     inside diameter 8", outside diameter 22.5", 60ft total. inductance 
     92.9 uh 
     secondary: 6" x 24", 22 gauge magnet wire, 37 turns/inch.  approx 900 
     turns. resonant freq 304 kHz unloaded, 154 kHz loaded with toroid 
     Toroid: 30" x 6" aluminum ducting, C(pf)=32