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Continuously variable primary

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.


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?