Re: Off-axis primary inductance

Terry, all,

> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
> At 07:05 AM 12/2/98 -0700, you wrote:
> >On Tue, 1 Dec 1998, Tesla List wrote:
> >>  Actually if you guys took a good look at the photos of Dr. Tesla's
> >>  colorado spring coil you would see that he to used an off-axis 
> >>  tunable inductor to fine tune his primary and secondary circuits...
> Where can I find more information about this method of tuning? Or a basic
> description of how I can try this on my coil? It sounds like an
> interesting thing to do. And it would probably help me tune it. :)
> Travis
> <<<< Hi Travis,
>     Perhaps I can simply answer this question.  All they are talking
about is
> using a second inductor in the primary LC circuit to tune the coil.  There
> is the primary inductor but any other inductance simply adds to the primary
> coils inductance and can tune the primary frequency.  This tuning inductor
> can be anywhere in the primary circuit.
>     There are some brush inductors used in HAM stuff but the current in the
> primary circuit is so high they usuall burn up.  A simple coil with taps is
> all it really takes.  Of course, you can only add inductance and not reduce
> it with this method but since the primary is usually tapable too this isn't
> a big deal.  A second tuning coil in the primary is really nice on fancy
> coils that have pretty fixed primary coils that can't be tuned any other
> way.  If you see a coil that does not have an adjustable primary, it
> probably has another inductor hiding somewhere or the primary cap is
> adjustable in some way. - Terry >>>>

I'm intending to take this idea one step further by using a variable 
reluctance coil. I have constructed two "cores" , both heavily gapped 
internally by recovering 3C8x ferrite cores from old computer SMPS, 
smashing them to bits and stuffing the chips into PVC and acrylic 
piping. The largest of the two is about a foot long, 4.5" diameter 
and weighs a bundle and has an Al value somewhere around 300nH/turn^2 
if memory serves. By sliding this core in and out of a modest winding 
I have a variable inductance. This might also be useful for adjustable
primary ballasting with a stiff power transformer. The power losses 
should be considerably less than the gap. I did experimentally check a 
primary with one of these cores for losses and it looked better on 
the scope than an equivalent inductance and much larger air wound 
primary, gap included.