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Experimenter's Section

A Winding Machine for Spaced-Turn Chokes
December 1931 QST Article

December 1931 QST

December 1931 QST  Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from QST, published December 1915 - present (visit ARRL for info). All copyrights hereby acknowledged.

There are still a lot of people who wind their own coils, whether it be for an amateur radio rig or for work in the lab. I know I've wound many a coil around a drill bit or wooden dowel. This simple coil winding machine that appeared in a 1931 edition of QST magazine would be a handy addition to anyone's bag of tricks, especially if find yourself winding single-layer coils that have a fixed space between the windings. The home stores like Lowes and Home Depot sell small pieces of oak that would be perfect for this kind of project. A little stain and a coat of varnish would give it a real vintage look. Use your soldering iron to burn your name onto the base.

A Winding Machine for Spaced-Turn Chokes

By W. H. Heathcote, ZT6X

Space-wound chokes are made easily if one has a screw-cutting lathe, but these expensive items seldom form part of a ham's equipment. The following description of a machine for winding spaced chokes will, I trust, be of assistance to hams not in possession of lathes. Most of the material will be found in the junk box of the average ham, but even if all the material has to be purchased the cost would be negligible. Since spacing the windings decreases the distributed capacity of a choke and - more important - raises the breakdown voltage at the end turns where the voltage per turn is always highest in a transmitter of any power, the time spent in making the machine is well worth while.

Referring to Fig. 1, it will be observed that a traverse motion of the choke form along the horizontal rod is obtained when the handle is turned in a clockwise direction. The nut soldered to the circular plate through which the threaded spacing rod screws moves the choke form along, the number of turns per inch being dependent upon the number of threads per inch on the spacing rod.

A Winding Machine for Spaced-Turn Chokes - RF Cafe

Fig. 1

A piece of wood approximately 16" x 4" x I" will serve nicely as a baseboard, and to this two blocks of wood to carry the bearings are screwed; these are 2 3/4" (in height) by 3" x 1/2". One is fixed about a half inch from the edge of the board and the other 8 inches away from the first one. Another block of wood 2 1/22 inches in height, also 3" x 1/2", is mounted 4 inches distant from the second block, and to this last block the circular plate is fixed. The main shaft is a piece of rod 12" in length threaded a half inch at one end and 1 1/2" at the other. The purpose of the sleeve (see Fig.  1) is to enable the choke form to be inserted and removed with a minimum of trouble. If the main shaft is released from the socket on which the spacing rod is soldered, it is only necessary to unscrew the wing nuts on the collar and the main shaft can be instantly withdrawn, thus releasing the choke form. The sleeve is 4" in length and of sufficient diameter to allow the main shaft to pass through freely. A collar about 3/8" in length is soldered over the end of the sleeve nearest the conical disc. Without this collar the tubing is likely to cut into the conical disc if it is made of hard rubber or other soft material, especially if a thin tube is used for the sleeve.

Coil winding machine dead center - RF Cafe

Fig. 2

The bearings are both 1/2" in length. No.1 can be a piece similar to that used for the sleeve. No.2 will have to be large enough in diameter for the sleeve to pass through. The bearings are soldered to brass "saddles" and screwed to their respective bearing blocks.

The spacing rods are four inches in length. An assortment of rods with different thread pitches will allow a choice of different spacings between turns. One end of each rod is soldered to a socket (which may be made from an old binding post) as shown in Fig. 2. Care should be taken to see that sufficient space is left, after soldering the rods to the sockets, for the main bearing shaft to screw firmly in the socket. A simple way to insure this is to screw a piece of wood halfway through the socket, place it upright in a vise and after centering the spacing rod in the socket run the solder into the surrounding cavity.

The circular plate is a disc of 10 gauge brass 3" in diameter. Holes slightly larger in diameter than the spacing rods are bored a half inch from the center. Through the center bore a hole to enable the plate to be held in position by means of the small bolt and wing nut on the block of wood on the baseboard. Solder nuts corresponding to the gauges of the spacing rods over the holes already bored for that purpose.

The handle is very simple and needs little description. A nut is soldered on the side nearest the sleeve. The one shown on the outer side acts as a locknut. The collar is about 1/4" in thickness with holes bored and tapped at opposite sides for the wingnuts. The guide (Fig. 2) can be made either of wood or hard rubber. Notches are made with a file every 1/8" or 1/4" along the top for holding the wire steady when the winder is in operation. The conical discs can be made of tin, hard rubber or wood.

The machine as described above will only make a winding 3" in length. Two chokes could be wound and placed in series if it were necessary to wind a choke on a form greater than two inches in diameter. Hard rubber or fibre tubing cut into 4-inch lengths is used by the writer as choke forms. After the shaft, discs and choke form have been placed in position the wing nuts on the collar are tightened up and on turning the handle pressure on the sleeve will center and tighten up the form, after which the locknut at the handle end of the rod can be fitted.

When winding chokes of say 100 turns, to decrease the distributed capacity it is a good idea to use a spacing rod with about 50 turns to the inch and after winding 25 turns remove the wire to the next notch cut on the top of the guide. The same procedure is followed after each 25 turns wound, the result being a spaced choke with additional spaces between sections.

A number of improvements will suggest themselves to hams; in fact I have made several myself, but to make things as clear as possible I have shown and described the machine originally built.


Many amateurs do not realize that there is often a real advantage in the use of tuned filter circuits for power supplies, since the amount of inductance and capacity necessary for a given degree of filtering is much less than that required in the more common filter arrangements. Here is some interesting information from Franklin Offner, W8AJZ-W9FTO:

"A few days ago I was working with W3AH, measuring the ripple voltage from various condenser-choke combinations. The ripple voltage was measured with a one-ma. rectifier-type voltmeter in series with a 2-μfd. condenser. Our results, though only an indication of what can be done, lead me to believe that hams can do a lot better than merely pile on the microfarads and henrys in single or multiple section filters.

Lowpass filter schematic - RF Cafe

Fig. 3

"We tried various combinations of the following: a 1-μfd. condenser, a 2-μfd. condenser, an RCA double 30-henry 80-mil choke and a couple of Stromberg-Carlson 250-mil 4-henry chokes (43¢ each) all used simply because they were available. With the best combination of chokes in the brute-force arrangement using 1 μfd. on the input and 2 on the output, the ripple was around 6 volts, from a 550-volt (each side) transformer, full wave, at 100-mil load. Then the circuit of Fig. 3 was hit upon, and the ripple voltage output from this combination was only 0.8 volts.

Evidently the two 4-henry chokes and the 1-μfd. condenser were series resonant, since either adding to or subtracting from capacity or inductance caused a large increase in ripple output. It is probable that by varying the values of the condenser and choke in the series resonant portion better filtering would be obtained and also by more careful adjustment of the inductance of the choke, "A," possibly by using one with variable air gap, in order to make the combination resonate at exactly 120 cycles. Obviously the choke used at "A" may be of low current-carrying capacity, since it carries no d.c. This point is the big advantage of this circuit over one using tuned traps in series with the output - that is, the chokes carry no d.c. and therefore their inductance does not vary with the load drawn.

Feeder switching schematic - RF Cafe

Fig. 4

"We intend to do more work on this circuit, and would appreciate hearing from anyone else trying it."

Feeder Switching

In the October Experimenters' Section two diagrams were shown for switching feeder condensers from series to parallel, in one of which, Fig. 7, a connection was unfortunately omitted. The right-hand feeder should be connected to the right-hand switch blade; if this is not done the diagram will not work when the switch is thrown to the" parallel" position.

Several letters were received from readers who caught this mistake, with Clem Wolford, W8ENH, and Robert A. McConnell, W8FJ, both suggesting a switching arrangement which is quite the simplest we have seen. Fig. 4 is the diagram. The switch is a double-pole single-throw affair, the condensers being in series when it is open and in parallel when closed.



Posted March 12, 2020                                        
(updated from original post on 10/24/2012)

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