It's Miller Time!!! — Dave's #16 Radio
Hello again crystal set fans. This is my rendition of the J.W. Miller crystal radio that was made in the fifties. I was fortunate to find one of these sets on ebay and even luckier that I was able to obtain it. You folks that are really into this diode radio thing will recognize this set. The Miller set was originally designed as a Hi-Fi AM radio tuner. There was no earphone connection, only a wire with an RCA connector to plug into a fifties amplifier.
My radio attempts to copy the circuit closely but using some of my own ideas. The main difference between the Miller and my circuit is the use of two of those honeycomb coils that I had in my junk box. You might recognize these coils from my #14 and #15 radios.
As usual, my radios aren't very well planned, but seem to work out in the end. This one is no exception. First, I try to use the materials that are around my house. The oak base was for a tube set, but I can make more of the bases. The Garolite® is 3/16 inch thick and was left over from my #3 project. I bought the vernier dial a little while ago not knowing where it would end up. The tuning is a little sloppy, but it serves well for this set. The capacitor I got in a trade deal. The shaft was too short to use with the vernier so I added an extension. As I was fitting the pieces together, I noticed that the vernier dial would be very close to the base board. I decided that the capacitor had to be raised. I had a small block that was trimmed and ready to be finished that fit perfect. As luck would have it, I had already drilled the holes for the metal brackets in the front of the Garolite®, but they were far enough apart so the piece of would fit in between the two brackets. So you see, not all my luck is bad, just most of it.
The wiring is quite straight forward. I used the fahnestock clips and solder lugs. I used some terminal strips for tie points and a little bare wire never hurts the looks. The coils have an extra winding. It is green in color. I could have removed it but I might have damaged the coils. Besides it really makes the coils look cool! The original design used ferrite core coils. The core is about 5/16 inches in diameter. I didn't count the windings as I wasn't going in this direction.
The coils are mounted at about 90 degrees from each other. This is to prevent coupling between the two coils. The coils are mounted to the base with a little assembly that I thought of. There are two pieces of wood, a half inch square and 1-3/4 inches long. I drilled a 9/64 inch hole through it for the wood screw that attaches the little block to the base. I then drilled two 3/16 inch holes partially through the wood, perpendicular to the mounting hole. I then took two inch and a half, 3/16 inch diameter dowel rods and placed them in the holes. After the measurements were worked out, I stained all the pieces to match the base. I really like how they turned out, if I do say so myself.
These two 240 micro henry coils have to be fairly close in value for this set to work right. Mine are well within 1% of each other. I did this using my inductance meter. Another method would be if you had a grid dip oscillator hanging around, plug in each coil and tune a close by radio until you hear the oscillator. If they tune real close, you are all set. Otherwise when you are winding the coils keep good track of how many turns you wound.
Next, I had to build the bifilar coil for this project. I cut a 3 inch piece of quarter inch dowel and drilled one small hole through the dowel at one end. Then, another hole 2-1/2 inches away from the first. I wound about 50 turns with two strands of 30 gauge insulated wire, side by side. The center connection of the coil is made by taking the two ends on one side of the coil and connecting them together. If your radio doesn't work and you have checked out everything else, try reversing the coil connections.
After the set is built and you are receiving stations, tune to the highest frequency station you can hear and adjust the trimmer capacitors for maximum. This step is important for maximizing the signal heard.
The results? Well it sure looks great, but it is low on performance compared to my other sets. I found the same to be true with the Miller set. In all fairness, the Miller set was designed for use with an amplifier. It was built for selectivity, not sensitivity. I am working on a multi-position switching system (up to 6 receivers). This will let me quickly switch from set to set. I will then be able to give you all a better idea of performance. I will say that this is probably an "urban" receiver. If you have plenty of strong stations and you need to separate them, I would say that this is your set.
This radio uses a band-pass circuit made up of the dual gang variable capacitor, the 240 uH coils and the bifilar coil. This gives much better selectivity than the single tuned circuits that are commonly used.
I built another Miller style crystal set.
More information on the Miller set can be found at Scott's site. Best wishes from -- Dave N2DS