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re: de-tuning or what?

Original poster: "Peter Lawrence by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>

      you ask some good questions.
> I've had several other responses commenting that  possibility.  I don't
> understand the mechanism  on how heated spark gaps cause my problem.

The energy that is initially in the primary cap gets transfered to the
secondary cap (which is the torroid topload plus some distributed capacitance
in the secondary coil itself). If that energy is not lost through sparks
and streamers then it continues to oscillate between the primary cap and
the secondary cap (at a frequency K (<1) times than the "resonance" frequency
of the primary L-C and the secondary L-C, where K comes from the degree of
inductive coupling between the primary and secondary).

The key here is "if not lost through sparks and streamers", you want to force
the energy to be lost in this way, rather than continuing to oscilate between
the two systems. The way this happens is if the spark gap ceases to conduct,
in which case the energy is stuck in the secondary system and if voltage is
enough for breakout then the energy turns into sparks and streamers, yahoo!

A spark gap that is "hot" will keep on conducting and result in continued
oscillations between the two systems, and eventually the energy is lost
as resistive heating of the coils and in the spark gap rather than as
sparks and streamers. "hot" spark gaps keep on conducting because the ionized
air between them says ionized longer, and hot electrode surfaces emit
electrons easier.

A spark gap that is "cold" will stop conducting when most of the energy
has been transfered to the secondary system, because at that time there is
very little voltage across the spark gap, and when it is cold there will
be very little ionized air between the electrodes and the electrodes them
selves will not be very emissive.

You will very often read about "quench" and the importance of spark gap
quench, this is what they are talking about. "First notch" quench refers
to the gap ceasing to conduct after the first time the energy flows into
the secondary, "second notch" refers to after the energy has oscillated
back down to the primary and back up to the secondary a second time, etc.

Somewhat contrary to the hot verses cold "explanation" is my own personal
observation that Tungsten-Carbide spark gaps seem to quench better when
hot than when cold - at least I have to close down my gap as it heats up
rather than open it up as is the case with Brass and Stainless Steel. This
seems so odd that I'm still looking for alternate explanations, like since
my gap structure is made out of acrylic that it is warping as it heats up
and when it warps perhaps the gap is getting farther appart requiring me
to crank in on the screws... Someday I'll get in there with my dial calipers
and measure before and after heating up and see...

> I am using polypropylene cap. Other coilers are using the same type as mine
> and don't seem to have such problems.

Then quite probably, as many others have also noted, it is your spark gap and
not your caps that are the problem. As others have noted, there are a variety
of solutions, more gaps in series, blowing cooling air across the gaps, using
tungsten or tungsten-carbide electrodes. In the long run if you are using
too much power (> 1000 Watts) you will have to resort to a rotary gap but
thats "a whole 'nother set of problems"...

-Peter Lawrence.