Re: Retuning with bigger cap?

To: tesla-at-pupman-dot-com
Subject: Re: Retuning with bigger cap?
From: "Marco Denicolai"<Marco.Denicolai-at-tellabs.fi> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Mon, 16 Nov 1998 11:36:42 -0700
Approved: terryf-at-verinet-dot-com

Give us a COMPLETE description of your system (tank supply, spark gap,
etc.) and then we can maybe help you.

As well, did you MEASURE resonance frequency of primary and secondary
SEPARATED or you just calculated it?

I did like this:

1. measured (signal generator + oscilloscope) resonance freq. of just
secondary with topload.
2. measured (same setup) resonance freq. of just primary cap and primary: I
setup the primary tap while measuring to obtain the same frequency I
measured on the secondary.
3. marked the primary tap position, assembled the whole TC and then tested
(on the running TC) small variations of the tap position against spark
length.
4. it seemed like the original tap position was already very near to the
optimal position.

How did you perform your tuning?





tesla-at-pupman-dot-com on 16.11.98 03:58:33

To:   tesla-at-pupman-dot-com
cc:    (bcc: Marco Denicolai/MARTIS)
Subject:  Retuning with bigger cap?




Original Poster: Sam Laur <slaur-at-sekunda.pp.utu.fi>
I had those small caps in series a while back, I got them tuned to a bit
better performance. The chain is 13 caps long, for 7.7 nF; the primary is
about 10 turns of 0.75mm2 stranded on a 18 cm coil form, and the secondary
is the same, 10.7 cm form with 1260 turns 0.315mm enameled wire. And the
top
terminal is just a 5.25" hard disk platter.
So, in tune, ground strike stayed about the same, 15-17 cm, but air
discharge
got noticeably thicker, noisier and longer at 10-12 cm. I even measured
the waveform on an analog oscilloscope, I was standing about two meters
away
from the coil and holding on to the probe tip. Voila - a cheap antenna :-)
But, it seemed to oscillate at about 310 kHz, double the secondary resonant
frequency. Strange.
Then I reasoned that the primary resonant frequency should exactly double,
if
I exactly quadruple the capacitance, so I made three more strings of 13
caps,
for a total of 30.8 nF for all four strings. But now the air discharge, or
what's left of it, is about 1-2 cm and ground strike is 5 cm. No matter
what
I do with the primary, 4 to 15 turns, it doesn't get any better than that.
The strangest thing is that when I measure the waveform with the bigger cap
and 10 turns in the primary (original setup), it seems to be a
superposition
of 155 and 310 kHz signals a bit out of phase in relation to each other,
I would have expected a cleanish 155 kHz decaying sine if it were in tune.
What on earth is happening here?