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Re: [TCML] Trans-cap uf matching, over/under?

I have been fond of using under resonant capacitor values, especially for small transformers. The transformer fires so fast that I don't think that there is much potential for it to get hurt. It also produces a pleasing tone that I prefer, to the rat-a-tat-tat sound of a coil with a larger capacitor load on the transformer. Be prepared to use a lot more turns on your primary with those smaller capacitor values. That being said, I have found that a 15/30 combined with a .0075 capacitor works very well. I like to use a 4" x 20" coil with 1400 turns on it, and about 20 turns on the primary. My turns are closely spaced 8 ga solid copper bare ground wire. A 13" to 15" o.d. toroid will use up all the turns on the primary. I have seen sparks in the 30" range using such a setup, combined with a simple fan cooled single spark gap set to 7/32"

If you go to this link, you will see a small coil I had on ebay last year. It had a less-than-resonant capacitor value on it, but I don't remember the exact capacitance. I incorrectly stated that it had phenolic primary holders, when it has polyethylene blocks.

Dave Goodfellow

Brian Hall wrote:
I have a 15kv/30ma NST so the ideal capacitance is .0053 uf, but since I don't want it to match impedance with the transformer and risk frying out a leg, I have read that 1.3 to 1.4 x that capacitance is desirable, aka LTR (larger than resonant ... impedance)

So, that would give me a range of .0068 to .0074 uf for the tank capacitor (at around 30 to 40 kv)

My question is how would the performance be affected if I were to use 0.011 uf (a hair over 2 x resonant impedance)?
And what would happen if I go a bit under it, such as .0033 uf?  What effect would that have on the TC circuit or even the hv transformer?  Would it hurt anyting?

Lastly, if I were to connect 2 NSTs of the same value in parallell, such as two 15/30 NSTs, to basically get one 15/60 power supply, would the calculation for the impedance of the 2 NSTs of Z = E / I (to find the cap value) be the same as if I had a single 15/60 NST? if it were, then .011 uf would be a hair above resonance with the transformer, but not as high as 1.3 x above it ...

---------------------------------- Brian Hall

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