RE: The 1500t secondary myth

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>Someone mentioned the other day that Marc's coil tended
>to give extraordinary spark length results.  I looked at Paul's site but
>didn't see evidence of this.

First of all, I haven't seen any evidence either. John Freau's "1.7 times
square root of power at 120bps" is still the efficiency standard for Tesla
coils as far as I'm concerned.

(BTW- I always think of the 1.7 in this equation as the "Freau number", sort
of like the Reynolds number in fluid mechanics. I don't think it is a proper
dimensionless number, but what the hell.)

In the case of a spark gap coil, the lower frequency will let you use more
primary turns hence less gap losses. If you were using a high frequency
resonator before, with the same spark gap and capacitor, swapping it for a
low frequency one might cause a huge boost in spark output. I remember
Richie B. telling me about the dramatic improvement he got when swapping his
secondary for one with more turns, the sparks more than doubled in length.
In Zo terms, he went from 20k to 40k.

Then again, I know Richie used top quality components and workmanship so I
doubt his primary losses would have been excessive in either case. I think
it was down to the Zo matching the streamers better.

Marc's 3000 turn coil is similar to my OLTC II which had 260mH and 31pF for
a Zo of about 90k and Fo=66kHz. I got my high Zo by using a large diameter
secondary (10" dia by 30" high, 1500 turns) rather than Marc's 3000 turns.
And, the resonator of the OLTC II seemed to perform fine, at 100 bps it gets
very close to the performance predicted by John Freau's equation. (39"
measured vs. 41" predicted)

Steve Ward's block busting monster DRSSTC-2 has a fairly high Zo too, I make
it 63k ohms from simulation. But my FANTC simulation of it came out with a
resonant frequency of 56kHz whereas Steve measured 45kHz. (I assumed 90%
winding fill, giving 2564 turns of 26AWG in the 45.3" wound length.) He may
have had a higher fill.) So the true Zo may only be about (45/56)*63k= 50k.

maybe in the light of this we should revise the recommended range of Zo to
"between 40k and 80k" or something?

Steve C.