Questions about first Tesla coil

Subject:  Questions about first Tesla coil
  Date:   Sat, 19 Apr 1997 19:56:13 -0400
  From:   "Owen Lawrence" <owen-at-iosphere-dot-net>
    To:   <tesla-at-pupman-dot-com>

        Hello, fellow Tesla coil builders!  I thought I'd finally
introduce myself
after enjoying the banter on this list for a few weeks now.  My name is
Owen Lawrence and I live near Ottawa.  Like any beginner I'm bustin'
with a
million questions, so I'll pose a few of them now.

        When I was in high school I built a coil but was never quite
happy with
the results.  However it was entirely my own design so I'm still proud
it.  I'll describe what I can remember of it and you can tell me how I
do better the next time.  I've always had this dream of doing it over
and getting it right, so here goes.  The primary transformer was a small
(brand new) ignition coil.  Starting with six volts DC I created pulses
using a relay wired to vibrate.  My four capacitors wired in parallel
made of approximately one square foot of aluminum foil glued to each
of a slightly larger sheet of 1/16" plexiglass.  Each capacitor was
an inch or two from the other.  By my calculations at the time I recall
assuming they were 50pF each, so 200pF in total.

        My spark gap was made from two small bolts.  I filed off the
threads near
the end of one of them to make it round, and sparked to the head of the
other bolt.  With a nut on the threads and a wire attached to the nut I
an adjustable spark gap.  Next, the primary winding was a cylinder of 14
guage bare copper wire, maybe a dozen turns at one foot diameter, height
about 8" or 9" inches in total.  In the middle of this winding was my
secondary, which was about (and I'm really guessing here since I never
count, even back then) 800 turns of 32 guage enamelled wire on a 2" PVC
pipe about 18" long, terminated with a 1/4" stove bolt.  I don't
at all how all of these parts were wired together.  Does it really

        In the end I was able to get sparks maybe 2" long.  I could draw
fine threads of sparks from anywhere along the secondary by holding my
finger near it.  I had hoped for much longer sparks.  Now I can see many
ways this coil could have been improved but it has been dismantled for
nearly two decades and I just want to learn what I can from it.  I know
was functioning as a Tesla coil because I borrowed an oscilloscope and
measured a frequency of roughly 32MHz with the typical damped envelope. 
You can't belive how excited I was in the middle of the night when I
applied power.  Well, maybe you can.

        So here's my question:  NOW it seems to me that 32MHz is much
too high a
frequency for this setup.  Is it the length of the secondary winding
really dictates the best frequency for a Tesla coil (assuming no
capacitance, that is)?  I've tried out some of the formulas I've found
the archives and a more likely frequency seems to be in the
of 2MHz.  Is it possible my coil was resonating at a much higher
If so, could I have improved perfomance by adding capacitance to the
circuit to lower the ring freqency to the same wavelength as my
coil?  (Sorry if I get the terms mixed up; correct me where I'm wrong
I'll learn eventually)  If my assumptions are not correct, can you tell
how you think this coil was performing?  

        I would like to build a new coil and would like to use an
ignition coil (I
actually have the original one!) for my primary step up transformer. 
things are limited in the amount of power they can output, so I was
wondering what sort of spark length I should expect from a Tesla coil
properly designed around this limitation.  If all goes well, I will use
experience to take the step up to a neon sign transformer and see where
that leads.  I also have an old microwave oven in the garage with a
transformer, whatever that means.  Can anything be done with this?  But
getting ahead of myself.

        As far as my primary winding goes, the dimensions I chose were
arbitrary.  It just seemed to look right so that's how I did it.  Would
have benefitted from a smaller diameter?  Many of the Tesla coil plans I
see these days (in magazines or whatever) seem to have a tightly wound
primary with a diameter only slightly larger than the secondary.  I
liked these because they always used insulated wire which severly limits
the choice of where it can be tapped.  I can't remember exactly who said
it, but since I've been devouring what I can find on the internet about
Tesla coils I read somebody's remark that they preferred the shape of
field produced by a spiral coil over that of a helix.  But that's where
ended.  What's the real explanation here?

        Another question:  I also own a high voltage power supply that I
from an Information Unlimited kit (30KVDC).  I bet some of you are
with it, yes?  Two 2N3055 transistors and a flyback transformer.  Is
any reason why this wouldn't be good for operation in a Tesla coil?  I
would definitely want to protect the diodes in the high voltage
though, because they're expensive.  I'm not gone on having a spark every
1/60th or 1/120th of a second, so if I have to wait while a capacitor
charges that's fine.  Whatever works best with the equipment at hand.

        Finally, I've seen some of the suggestions for building high
capacitors.  One problem mentioned was that for rolled capacitors once a
certain length is exceeded inductance starts to become a problem. 
this be reduced by tapping off each plate along its length?  When it's
rolled up the tabs from each plate could all be joined together so you'd
drawing off charge from the side of the plate rather than the end. 
end up with terminals at each end of the capacitor.  Excuse the artwork:

    |  +    +   +                                                 |
(dialectric inbetween)
    |   -   -      -                                               |

    roll it up:  -------------------------------->

          | +
        /   \
        |   |
        \   /
          | -

        I have other questions (like, I still don't understand
quenching, how much
does a pole pig cost, and why isn't there a tesla builders' usenet news
group) but these are the ones that have been nagging at me.  Hopefully I
won't be the only one that would benefit from answers to any of these
questions.  If you prefer to refer me to resources rather than answer
questions directly, that's fine.  Any response will be much appreciated. 
I'm very excited to have found such a cool bunch of people.  I've
got several offices of people oooh-ing and ahhhh-ing over your pictures
operating Tesla coils I've downloaded (my wife is the only holdout :)).
Thanks a million and I'll talk to you later!

  - Owen -