Modern Radio Laboratories ® /Alfred P. Morgan Mash-up

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Making the coils.
Taking the mystery out of what wire goes where.

Alfred P. Morgan coil sketch
Elmer Osterhoudt and Alfred P. Morgan both give instructions on how to wind the coils and both are hard to comprehend. Morgan advises to just buy the coils, but gives instructions on how to wind a coil on a Bakelite coil form.

Elmer wants you to follow the diagram on the left, drawn in his typical style of drawing with no perspective. Are you going to buy blank coil forms and wire, and try to wind a coil based on this perplexing drawing? Better just buy the coils already made from MRL. This lousy drawing is good marketing.

The superior drawing in Morgan's book is still confusing. Above, on the far right drawing, pins 1 and 2 on the coil are closest to you, and pins 2 and 3 are the fat pins. Once you see that one fat pin is in the rear, the drawing becomes clearer.
The circle in Elmer's drawing is the socket. If you mentally tilt the circle and compare it with the Morgan socket, "P" is pin 1.

These drawings, from two articles in Radio Builders Manual published in 1934, show the same pin connections as in Morgan's drawing. (Ignore the letters and follow the coil connections to the pins.) Note that the coils are rotated a quarter turn when compared with the Morgan drawing.

Grid Coil
Tickler Coil
This is so confusing to me that I edited the drawing. Each coil is connected to a fat pin and a thin pin and both are wound in the same direction. It doesn't matter which direction as long as they are both the same. The tickler can be on the top or the bottom. You can't just pick any pin to start, or you may end up with the grid and tickler coils reversed.

Alfred P. Morgan Coil
Alfred P. Morgan Coil
The Morgan coil drawing simplified.

4 prong coil pin locations
Another way to visualize the connections.
The picture on the right shows the bottom of the coil, not the view looking down the inside of the coil.

To complete the confusion, the numbers on a four pin socket don't match up with the numbers in Morgan's drawings. If I had never seen a real coil, I'm not sure I could make one based on these drawings.

This explains the guys who say, "I built one of these when I was 10 years old. I've spent my career in electronics thanks to Alfred P. Morgan." These kids were winding radio coils from incomprehensible instructions. It is no wonder they were successful in life. (Actually, one guy who built the Morgan set told me he became a Geologist, so the "career in electronics" theory is not rock solid. Get it? "rock" solid. Never mind.)

The coils are "standard" in the pin configuration. (The only reason I state this is that Morgan said to buy the coils, and if there wasn't some standard at the time, the radio wouldn't work.) A 1935 Bud Radio Products catalog states that their coils can be used in any circuit specifying a four prong coil, so this configuration dates back at least to 1935, and Elmer Osterhoudt had been making them prior to 1932.

There are two small pins and two large pins. Logically, you'd think that the big coil would connect to the big pins and the small coil would connect to the small pins, but that's not the standard. Each coil is connected to a small pin and a large pin.

So why is that? A four pin vacuum tube has the filament on the fat pins. If a coil was wound across the fat pins and you accidentally plugged it into a tube socket, it would heat up while putting a dead short across the source of the filament voltage. There would be a contest inside the radio to see which would turn into a puff of smoke first; the transformer, the coil, or the wires going to the tube socket.

4 prong coil pinout
The pin connections of the two coils (compare to the drawings). Wind both coils in the same direction. For example, if you wind the primary coil down from the top, starting on pin 1 and ending on pin 2, then also wind the tickler down from the top, starting on pin 3 and end on pin 4. Both coils must be wound in the same direction or the radio won't oscillate. Leave 1/8" between the two windings.
Here's another way to visualize it.

Another Mystery Solved

coil pinout designations
The logic behind the pin out designations on a coil is that the coil used a tube socket. When you look at a schematic or pictorial of an old radio, the designations on the coil don't seem to make much sense unless you see the coil together with a vacuum tube, as shown above. F plus and minus are the connections to the filament battery on the vacuum tube, but the coil socket has the same labels.

NOTE: The pinout of the coil above does not agree with what we have found to be the "standard" coil. P and F+ seem to go to the smaller, or "tickler" coil instead of the larger coil.

MRL coils
A set of MRL coils.
Elmer put the tickler winding on the top, Alfred Morgan put his on the bottom. It doesn't really matter as long as you wind the primary and tickler coils in the same direction.

The MRL coils wind upward, the Morgan coils wind downward. In other words, Elmer made a hole near the bottom of the coil form, pushed a wire through it and into pin 1. He then wound the coil up towards the top. Morgan did the opposite; he made a hole in the top of the coil form, pushed a wire through it and into pin 1, and then wound it towards the bottom.
BUD coils
A set of BUD coils from the 1920s.
Though both of these sets are color coded, there was no industry standard for the colors. The fact that the red (and possibly the green) coils are similar is either a coincidence, or Elmer adopted a color scheme from another manufacturer.
OCTO coils
"OCTO" brand four pin coils. Red, blue, brown and green. The red and brown probably tune similarly to
the red and brown BUD coils, but not the blue and green. The tickler winding are on top, like the MRL coils.
 Photo by Emil Sarlija

Here is MRL / Morgan comparison data for the AM Broadcast Band coils. Two coils are needed to cover the band.



Primary Coil turns Wire
Tickler Coil turns Wire
COIL 1          
MRL 10 - 140 84 28 14 28
Morgan 10 - 365 70 30 20 30
COIL 2          
MRL 10 - 140 170 34 25 34
Morgan 10 - 365 150 34 45 34
Notice Morgan uses less turns on the primary and more on the secondary, but he used a variable capacitor that is double the value Osterhoudt used. Osterhoudt tells us the first MRL coil will tune from 950 kHz to 2100 kHz and the second coil tunes from 436 kHz to 1000 kHz. The forms are 1 3/8" in diameter. 

Coil winding jig

winding the coil
winding the coil
To wind the coil you need tension on the wire. This can be done by passing the wire through a hole in a clothespin (an idea from one of Elmer's handbooks).

Scrape the insulation from the end of the wire. Drill or melt (with a hot pin) a hole in the top of the coil form, pass the wire through the clothespin, into the hole, and into pin 1.

Heat the pin with a soldering iron. Solder will wick into the hole, soldering the wire to the pin.

When the coil is wound, hold it in place with a piece of tape. Drill or melt a hole in the form, scrape the insulation from the end of the wire, and pass the wire through the hole and into pin 2.

- Click on image -
A quick coil winder
MRL coil winder from HB-6 "How To Make Coils."

After you wind the main coil, wind the tickler coil in the same manner. I found that trying to get the bottom tickler winding wire into pin 4 was almost impossible due to how close the hole in the bottom of the coil form was to the pin. The fix for this is to hold the winding in place with a piece of tape and leave about a foot of wire on the end. Scrape off the insulation at the appropriate place. Push the wire through the hole in the form, bring it out the top, then loop it around and insert it into the pin. Pull it tight and solder.
MRL 1 tube clone
The completed coil. It tunes from below the broadcast band up to about 1100 kHz. Two coils are needed to cover the entire band.

MRL Colis
The second coil, on the left. It tunes from around 900 kHz to 1750 kHz. Next to it is an MRL coil that covers about the same range. I try not to handle the Elmer Osterhoudt MRL coils too much. The coil on the left is wound on an MRL form made by Paul Nelson in 2015.