Re: Variable Cap In Secondary?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
> 
> Hi,
> 
> I have looked into using a 15kV 500pF vacuum cap to do tuning but the
> voltage or current always gets too high before any real tuning would be
> achieved.  All the models tend to point to either a very high voltage cap
> (at the expense of output voltage in the secondary) or many caps in
> parallel in the primary.  Just no clear good way to do it.  Them caps run
> about $700 a pop unless you can get them second hand...
> 
> Cheers,
> 
>         Terry

	The variable capacitor required to fine tune the "average" primary
would be enormous, and doubt that any ever existed.  The use of a series
variable inductor is far more practical, and easy to implement.  I have
used a small roller coil out of an old WW2 transmitter in series with
the primary of my small coil.  It is turned by means of nylon cord
wrapped around a pulley (I don't want to get my nose too close to the
thing).  The tuning range is perhaps 10% from the nominal 280 kHz, quite
enough to give evidence of a peak 	WHEN THERE AREN'T MANY STREAMERS, or
when the primary voltage of the NST is set so low that there are only a
few sparks per second.  However, at full power after breakous occurred
the tuning hardly mattered.  Changing the inductance like this affects
the coupling, but probably not enough to matter.

Ed