Tube coil design improvements

To: tesla-at-pupman-dot-com
Subject: Tube coil design improvements
From: "Simon Winder" <swinder-at-microsoft-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Mon, 28 Feb 2000 18:57:35 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Steve,

thanks for your suggestions about improving my tesla coil design last year.
I have made a number of changes and I thought I'd tell you about them and
ask maybe a few additional questions. Also I'm copying this to the list.

I implemented all the things you suggested with regard to improving the
efficiency of the design and getting rid of parasitics. I added a lot more
filtering/bypass and now I observe a pure sine wave on my scope when the
thing is running rather than a mess. I no longer generate lots of mains
noise - no clicks or buzzing from my stereo, and I no longer crash my scope
or my apartment's central heating control unit ;-). Also I have not managed
to get the anodes to glow, even after 5 minutes of operation.

Unfortunately I have not managed to increase the output at all (above about
5"). I changed over to 1/4 wave with a vertical coil. The primary is 9"
diameter with 10/12/14/16/20 turns tapped for experminentation. I used hv
test leads for the feedback coil. I experimented with lots of feedback
configurations. The secondary is the same 3" diameter form with 444 turns
16" long. I tried it with/without a top load. Resonance occurs at 1080kHz
without the top load, and 840kHz with it. My primary caps have varied
between 200pF and 440pF depending on the number of primary turns I am using.
I have experimented with a number of combinations of tank LC ratios. My
tuning capacitor allows me to pinpoint the best resonance quite well.

In general my primary inductance has increased because of the larger
diameter. Unfortunately, this develops a much higher tank voltage and I
can't turn the power up above 50% without arcing between the plates of the
tuning capacitor (5kV rated). BTW my anode supply is 1KV followed by a 1.4uF
voltage doubler. Do you think a larger capacitor and smaller number of
primary turns would help? I hear that people try to keep the primary turns
high to "match the tube output impedance" but I don't see why this should
work, as surely the tube Ra should match the discharge impedance transformed
into the tank, not the reactance of the primary.

Do you think there is an advantage to going to a lower frequency of
operation?
Do you think that the fairly loose coupling between primary and secondary is
a problem?
How does the tank LC ratio affect the operation?
My power supply transformer runs completely cold. How can I transfer more
power?

Any advice would be appreciated. This is a complex science!

Simon