Adjustable Cap Details

From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Thursday, January 22, 1998 12:31 PM
To:  Tesla List
Subject:  Adjustable Cap Details

> > 
> > ----------
> > From:  Rene Caldera [SMTP:Rene-at-compuserve-dot-com]
> > Sent:  Wednesday, January 21, 1998 12:15 AM
> > To:  Tesla
> > Subject:  Adj. Pri. Cap
> > 
> > I'm thinking of building an adjustable primary cap(plate type) so that I
> > can use it to tune the main circuit instead of the primary coil.
> > 
> > Can anyone tell me why this wouldn't be a good idea(besides the extra work
> > involved).
> > 
> > Thanks
> > 
> > Rene Caldera

The best way to accomplish this is to place a rather small value
variable cap in parallel with your regular fixed cap. Don't try
to make the variable part TOO large, as it is easy to use the
primary taps for the rough tuning, and just use the variable
cap to do the really fine tuning.

A decent variable cap can be made using a 5 gallon plastic bucket,
one that has a removeable lid. Since the bucket is round, cut out 
two round plates from sheet aluminum. The sheet must be thick
enough to have mechanical strength to retain its flatness. 
The size of the plates must be such that the bottom one fits snugly
into the bottom of the bucket. The top one is going to be have a
length of 1" PVC pipe or broom handle or other good insulating material
attached to it as a handle. This handle will go through the center of
the bucket cover and is used to raise and lower the top plate.

The terminal for the bottom plate can come out the side of the bucket.
The terminal for the top plate can be at the top of the bucket lid,
and attach to the plate via a length of flexible wire. Since you
want the bottom of the moveable plate to be perfectly flat, you can
make that plate a circle with a little tab at the rim, and bend the tab
up so that you can punch a hole and attach a screw to clamp the wire.
Mounting the handle is easy if you make a few right angle brackets,
attach them to the rod, and epoxy them to the center of the plate.

You want to ensure that the plates never come closer than about 1/4",
so strategically attach several small pieces of 1/4" thick 
plexiglass at strategic points on the bottom plate using epoxy.

You will be putting in transformer oil or other insulating oil. You
should put in at least a couple of inches of oil, more if you happen
to have it. Realize that as you attempt to move the upper plate, it
will act like a piston. You MUST have some means for the oil to move
as you move the upper plate. You can accomplish this by cutting 
notches about 1"x1" at regular intervals around the rim. I have a big
press that can cut 1" diameter holes, and that is what I use.

One last detail is to have a means to lock the handle in place.
You can make a collar, or just drill holes through the handle and
insert a peg on the appropriate hole. Use your ingenuity!

A cautionary note: It IS possible to adjust such a cap on-the-fly
while the coil is actually running. It is also fairly dangerous to
do so. Exercise caution!! A safer approach is to adjust the cap only
when the circuit is not energized.

Hope this helps.
Fr. Tom McGahee