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RE: Tuning an OLTC ... basics ...

Original poster: "Lau, Gary" <gary.lau@xxxxxx>

> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Monday, April 25, 2005 11:22 AM
> To: tesla@xxxxxxxxxx
> Subject: Re: Tuning an OLTC ... basics ...
> Original poster: Davetracer@xxxxxxx
> I have an old fashioned, spark gap Tesla Coil ...

Then perhaps you should have changed the subject line?

> I wandered by WinTesla and the Java calculators. Here's the bare
> 12,000 volts, 60 ma, which would like a cap of 0.0135 (13.5 nf) for Z

The mains-resonant or "Z-match" cap value has no special functional
benefit (it used to be thought to extract the maximum power from an
NST).  The thinking now is to use a so-called LTR, or Larger Than
Resonant cap value.  The benefit is that it will be less likely to cause
damage should you open the gap too wide.  Also, assuming that the power
supply is capable of fully charging the cap, a larger "bang" will
produce longer sparks.  I'd recommend a value of 1.5 to 2 times the
mains-resonant value.

> Primary winding is what you'd expect, 10 turns, flat. The primary will
> "twanggg" at 240 Khz if I hang a 0.0241 mfd (24.1 nf) cap on it.
> Secondary is 12.5 inches of #25 wire, for 589 feet. 3.5" diameter.
> 1/4 wave length is 354 feet, which makes me wonder if they calculated
> right (grin).
> H/D aspect is 3.57 and likely number of turns is 643.

You do realize that the wire length only affects how much wire you need,
and nothing to do with 1/4 wavelengths, right?

> The secondary would like to twang at 693.72 khz, except if I put a
> on it,
> which will bring it down to 240.40 khz, which will match the primary.
> A couple questions:
> How far do you generally have to move the primary "clip" to get an
> A quarter turn? An eighth?

It depends on how many total primary turns you have.  I have something
like 16 turns, so +/- 1 turn has only a small effect.

> I'm running with tinfoil on plastic now for caps and clearly don't
> enough capacitance. My 'scope shows a strong frequency being emitted
> 800 Khz, with all sorts of lovely harmonics. Is it getting time to
> up some MMC's? Or should I go beer bottles (grin)!

If you can, it's always a good time to switch to MMC's.  The 800KHz on
your scope - is this from capacitively coupling the scope probe to a
running, sparking coil?  Seems way too high!

> I do have a scope, can I use it to help me tune this puppy in? I do
> have a signal generator, alas, and although I hear rumours that there
> directions in how to build one from some 555 timers somewhere, I have
> yet found that!

It you don't have a signal generator, I don't think you can use a scope
for pri-sec tuning.  Terry's 555-based tuner is at
http://hot-streamer.com/TeslaCoils/Misc/TCT/.  I built one, and I
recommend it.

> Do you think I should load this into my Jeep and drive up to Terry's
> shamelessly offer him a bribe?
> Many thanks for helping this relative newbie,
>      Dave Small

Regards, Gary Lau