Schmarder's Super Spider Sizzlitzer Set — #49
Hi crystal radio fans. This is my #49 set. This set is a modified Australian Mystery Set (a.k.a. Telefunken). There is an important twist in this design. A ganged capacitor has been added for extra selectivity along with bigger 165/46 litz wire. This combination makes the set stand up and say G'Day.
As usual this set isn't my idea to begin with, although my first Australian set (#3) also had the dual gang capacitor feature. Mike Tuggle built one as shown on Darryl Boyd's excellent website. No one builds them better than Mike T. The set looked so interesting that I had to build my version. Since I am a spider coil fan, that is the way I decided to make this one.
The coil is made on a 6 inch (15 cm) wooden form. I have a few prototype wooden forms available if you are interested. The hub is 2-1/2 inches (6.3 cm) in diameter. The tuning coil section, L1 consists of 45 turns of 165/46 litz. L2, the detector winding is 20 turns of the same litz. To make this coil, first cut off 20 feet of litz. This is for the detector winding. With the rest of the litz, start winding the coil. After 12 turns are wound on the form, wind 20 more turns with both wires side by side as a bifilar coil. Leave the ends long enough to make your connections. Then continue winding the rest of L1 until you have 45 turns. There will be 65 turns total with the two windings. About 60 feet total of wire is required. The detector coil isn't too critical in the number of turns used. When connecting the coil, the bottom ends of each winding are towards the outside of the coil.
The tuning capacitor is a dual ganged 365 pf variable capacitor. Just about any junk box capacitor should work. But as with other sets, the capacitor with the ceramic insulators will work better. The audio transformer is a Bogen T-725. It is possible to operate this set with only a set of high impedance headphones but your selectivity will be better by using the transformer. There will be less circuit loading. You can add as many output binding posts you want, or even an impedance selector switch.
I have been testing this set for the last couple of weeks and found it to be impressive for a single tuning set. The vernier drive that I added helps. When you have to cover 115 channels at 10 khz apart in a 180 degree capacitor rotation, it is somewhat difficult for easy tuning. By adding a 6:1 vernier tuning system, you cover those same 115 channels in 1080 degrees. That is about 10 degrees between each station. Actually, since these variable capacitors tune frequency in a non linear fashion, the number of degrees between stations does vary.
I did add a part that might seem unusual. That is the 27 mH coil from the frame of the Bogen transformer to ground. Without the transformer frame grounded, there is an audio hum that is bothersome. If I were to directly ground the frame, I would get rid of the hum but the radio would not operate as intended. That is because of the high capacitance that exists between the transformer frame and the windings. Since the windings have rf energy on them, at both ends, I thought that a little choke would keep the rf from being grounded, but the low frequency audio would be shunted to ground. It worked very well. Those 27 mH chokes are nearly magical!
The diode that I used is an old ITT diode labeled as a FO-215. It has been discovered that they work better than the average germanium diode. If you don't have one of these, a 1N34A germanium diode will work fine. The wood base is 7x10 inches and is red oak. Remember that you control your destiny and that making your crystal set with your personal touches is a right enjoyed by all of us in the free world. Enjoy your creation!
73 and good crystal DX. Dave - N2DS