#47 Crystal Set — "The Big One"
The nearly 12x 14 inch (30 x 35 cm) oak base this radio is mounted on gives you an idea of why this is named The Big One. This radio is not my design. I was given the chance to build this radio as a commissioned project. What an honor!
This set is based on the Australian Mystery Set, first designed in the early thirties. This in combination with another circuit from Australia, rounds out this design. This is not my first mystery set. My #3 set was also a mystery set.
I can't take credit for that beautiful coil wound with 1000+ strand 46 gauge litz wire. It is wound on a 6 inch styrene form. The coil is supported with four styrene legs that I machined. The two coil windings are bifilar wound in the middle. This means that the two coil wires are wound in tandem. Other coil details will be made available at a later time.
The capacitor is a very high grade 500 pf type. There are ceramic insulators and the plates are silver plated. It doesn't get any better than this. Along with the smoothest vernier tuning system makes this the nicest tuning set I have ever built.
There is a 5 to 20 pf variable capacitor that couples the top of the detector coil to the diode. A single pole switch allows for switching between diodes. Very sweet! The output is meant to drive an audio transformer. Basically, the higher the primary impedance and the more iron there is in the transformer, the better the sensitivity and selectivity. Of course the secondary has to be matched to the headphones that will be used. This radio deserves no less than a set of sound powered headphones.
The only other item worth mentioning is this type of radio is susceptible to shortwave interference. I placed a spare 240 uh coil in series with the antenna connection and the interference went away. It is a pleasure to tune this dx receiver!
73 and good crystal DX. Dave - N2DS