Continuously variable primary

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

I'll post on hot-streamer/temp a mostly-complete drawing of what I 
have in mind for a c.v. primary.  File TCHTUNE.JPG.  Comments 
welcomed, of course.

http://hot-streamer.com/temp/TCHTUNE.jpg

The whole thing is to fit into one of my 18 gal. plastic 
food-buckets.  In the drawing, the "near disk" fits against the 
bucket's inner ridge.  The view is looking into the bottom of the 
bucket, which will be "down" when it's all put together.  The primary 
capacitor-string is to be wrapped around the outside of the bucket 
(to constitute an additional turn), and the secondary coil is then to 
be plunked down on top of the assembly.  The spiral coil shown in the 
drawing ends up close to in-plane with the bottom of the secondary.

The coil is mounted on the "far disk" and it spirals toward the 
viewer (which is ultimately downward when it's all turned over, of 
course), spaced gradually by increasing quantities of 0.02"-thick 
washers under the fastening points.  As a consequence, as the rotor 
assembly turns, it always clears inner coil turns by at least 0.16 
inch.  The rotor assembly slides back & forth on the drive disk and 
at the same time the contact-carrier part of it hinges up & down as 
required, to maintain contact with the coil.

To wind the coil in the first place, I'll mount flat onto the "far 
disk" a set of the little plastic tubing-clips marketed by 
McMaster-Carr; that's what the starred holes are for.  The rest of 
the clip-holes are drilled to clear 4-40. Then, I bend the tubing as 
required & snap it into the clips.  Then, using a marking pen, I'll 
put a dot on the under-side of the tubing thru each of those 4-40 
holes.  Then carefully remove the coil and remove & put aside the 
clips.  Then drill & tap for 4-40 at all the marked places on the 
coil.  Then re-mount the coil on 4-40 machine screws, spiraling it 
upward (in the drawing) with the 0.02"-thick washers.

The contacts are 9 segments of phosphor-bronze spring-wire (each 
handily already-curved as received in a large coil).  Each will have 
a tang bent up, to pass through matching holes in the support plate, 
and all are to be  clamped in place with 2 screws + a small 
cross-plate (shown dashed).  The soldered connection to them is made 
at the tangs, with 3/4"-wide copper braid,  The braid is to pass 
through a hole (not shown) in the drive disk and thence to coil 
around the driving shaft (also not shown) to allow for the 3 or so 
turns of rotation.

The business-ends of the contacts are kept in place by two nylon 
screws, which also serve to guide the in & out motion of the rotor 
assembly, by virtue of their being positioned between coil turns.

In addition to the variable turns, I'll have 3 fixed external turns, 
also of 1/4" tubing.  I'm shooting for tuning over a range of ~70-110 
KHz, to accomodate the two 12"-diameter secondaries that I have, 
which resonate at ~100 and ~125 KHz.

So, everyone...what have I overlooked?

KCH