It's Miller Time Again! Dave's #52 Radio
Hi Friends and neighbors. It is April and this is only my first crystal set of the year. Shame on me. I had a lot happening over the winter and my creativity wasn't anywhere to be found. My creativity got a jump start a few days ago when I received this wonderful box in the mail from a friend.
The red wood on this box is padauk. This is an exotic African wood and red is it's natural color. The wood on the top is lacquered tiger maple and the finish is as smooth as new baby's rump.
Now to the radio part. This radio is a dual coil set. There are two 240 uh coils wound with on a wood spider form. The form should be coated with a poly material to keep the moisture out. This radio is fashioned after a crystal radio tuner made in the 1950's by J. W. Miller. My #16 set was my first "Miller Radio", and a picture of the original set is at the bottom of the page. My #16 was a very special radio to me, just as this one is becoming.
I will describe the circuit the best I know how. The mystery to me is the front end components. That is the 300 uh coil and 22 pf capacitor. The original Miller set used somewhat different components. These parts seem to work very well in this radio. The 300 uh coil is a "low Q" variety. If you want more selectivity, reduce the 22 pf capacitor down to 15 pf.
Next there are two tuned circuits, separated by a small capacitor. This is the bandpass tuning circuit. It differs from the Miller and my #16 as I elected to use a small capacitor between the circuits instead of a bifilar wound choke. I had first tried a 10 pf capacitor between the sections but I found that aligning the two tuned circuits to track was impossible. I had his problem with another circuit, so I knew what to do. I made a "gimmick" capacitor with two pieces of 30 gauge, kynar coated wire. The wires are only about 1.5 inches (38mm) long. If you use thicker insulation wire, you may need more twists. I believe the capacitance is somewhere around 2 to 5 pf.
The detector part of the radio is also somewhat modified from the Miller radio. Both sets use a capacitor to couple the diode back to the second tuned circuit. The value is low enough to limit the loading on the tank circuit. Several of my other sets use this system. The 27 mh choke works very well here as in some of my other sets.
The original output was intended for connection to an audio amplifier (tube type). This can be done with my circuit as it stands. I did design this for use with a ceramic earphone. By using a transformer, sound powered phones can also be used.
It is very important that the two litz 240 µH coils be made as close as possible in inductance to each other. If you have forms the same size and are careful to put the same number of turns on the coil, you should be ok. Also, the two coils should not be close enough to couple to each other. I put mine at 90 degree angles so as to minimize the interaction.
There are small trimmer capacitors on the variable capacitor. (I only use
two gangs of that three gang capacitor). If yours doesn't have these trimmers,
you can add your own, with about 20 pF on each capacitor.
To align this set, tune in a station that is fairly loud at the top end of the dial. Have the trimmers set at mid position (about 2 or so turns out if you have built in screw trimmers). Then tune one of the trimmers and rock the main tuning back and forth until the loudest signal is heard. A DVM across the earphone will work as a tuning indicator.
How does this work? It is the best "Miller" type set I have tried so far. The big litz coils help bring in the stations. This is not a DX radio though. Other circuits, such as my #50 set are much more suited for DX. This radio is easy to operate as it has only one tuning dial. The sound from this set is great. The beauty of this radio is its single dial tuning. It is simple as a superhet radio to tune.
Take a look at my first Miller style crystal set.
I hope you enjoyed seeing and reading about this set as much as I have had fun making it. 73 and good crystal DX. Dave - N2DS