December 1931 QST
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
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
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 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
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
"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.
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.
"We intend to do more work on this circuit, and would appreciate
hearing from anyone else trying it." FEEDER
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.
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