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Re: Conical secondaries
Original poster: "John Williams by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jwilliams-at-edm-dot-net>
>Does any one know anything about conical secondaries? I'm asuming there was
>some point in Tesla making them.
I suppose actually having made one might count?
First thing you ought to know, winding one is a screaming pain in
I found a really marvelous form a very long time ago in a surplus yard.
It was pretty obviously from a reasearch project of some kind and is fiberglass
of the sort that is so hard and airless in the lay up that it rings like a
fine china when struck.
I sanded and smoothed and coated the surface in preparation of the
winding and took it to the home of an tesla coil expert I was aquainted with
and he set up his winding jig and went to spinning the wire onto it.
And that lovely layer of wire went up about a third of the way along
the coil and then when the tension on the previous windings got high enough
the windings popped over each other and wound up in a tangled necklace
around the narrow end.
Did I mention that they are a royal pain the the butt to wind?
He tried it twice more before giving up in frustration.
I took it back and another friend and I set up a braced platform with
a lazy susan bearing, screwed the thing down and set about doing about ten
or twenty turns on it over a layer of varathane, letting it dry enough to hold
the windings and then putting on another ten or so turns on another layer
of wet varathane. We also painted the windings as we went.
I did mention that winding one of these is like having a case of
hemerroids you can't get rid of?
When it was done I baked it under an infrared lamp while a small
motor turned the lazy susan rig. I applied about ten layers of varathane to
the thing and then sanded it with grit as small as 1500 and then buffed it
out with rubbing compound and finally with gleam toothpaste.
The friend who helped me wind it had a machine shop in his
garage and there we fabricated the internal barrier plates, three of half
inch acrylic, and the top plate of black mycarta - a kind of paper phrenolic.
The stand off is turned delrin that holds up a 12 x 3 spun aluminum
In the end it was easily the prettiest coil I ever made.
Sometimes things that are a serious pain in the rump are worth
Unfortunately, we were using a set of surplus doorknob capacitors
that developed a resistive component when they warmed that led to
the tank circuit going into a multioscillation mode and it made the
secondary crawl with dearsonval breakdown patterns and then one day
I was tired and incautious and did not dress the ground wire properly.
The windings at the base burned through to the ground cable and
now I have the job of cleaning it up and starting the rewind process
all over again.
One of these days I will. I keep telling myself.
It is sitting near me as I type this. Little carbonized spots on it
that have to be dug out and cleaned and backfilled... and of course the
blow through that is the serious problem.
But I know what burned it and how and the cap has been
replaced with a commercial pulse capacitor made for the job that
cost me an arm and a leg.
It will live again.
Why build one?
Well, honestly, one of the big reasons is that they just
look nifty. This is a small coil that is about twenty inches wide
at the base and seven or so wide at the top. It is about twenty
two inches tall. Wound and polished and dressed out sitting
in the helical flat primary it just plain looks mean.
Technically, conical secondaries have a higher coupling
than cylindrical coils do as a rule. Maybe a little to much. And,
there is also the possibility that internal flash though tends to
be inhibited in such a geometry because the magnetic field internal
to the coil comes to a kind of choke point at the top.
At least that is what I was told when I first got into
coiling a very long time ago. I was also told more recently that
conical coils have a higher rate of internal flash over, and so
the three barrier plates.
From what I have seen not a whole lot is actually
known about how they really behave because not a lot of
people are crazy enough to make one.
I did mention that they are a real pain in the....
...yeah I did didn't I?
Somewhere I have pictures of this beastie if anyone
is interested. Sadly none of when it was running. I could
scan what I have and wedge it onto my web site someplace.
Someday it will live again!