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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
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