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I'm glad I am numbering these sets, or I would be lost! #51
takes a step back from the ultra-litz wire that I used on my #50 set. Number 51
is a single coil, two capacitor circuit. I have used this circuit on many of my sets.
The disadvantage of this circuit is the controls interact. But with careful adjustment
very good results can be obtained with this single coil set. The two tuning
capacitors may be juggled to get good selectivity and sensitivity. Increasing
the value of the antenna capacitor while decreasing the value of the detector
capacitor (so as to keep the station in tune), results in better sensitivity
at the expense of selectivity. I had a chance to get proficient at tuning
this type set in the 2003 crystal set contest.
I used 165 strand, 46 gauge litz wire
for this project. 32 feet (9.6 meters) of wire is needed to make this coil, including
the flying leads. Other litz wires can be used, such as 40/44 and even
660/46, if the coil dimensions are adjusted. I wound 42 turns on a wooden
coil form with a 1-3/4 inch hub (45 mm). After I wound 25 turns, I brought
out the wires for the tap, and then wound the rest of the turns. The inductance
measured 140 µH. If you use 365 pf capacitors, you may want to order
a little extra wire and wind 3 more turns on the coil. Since I had 540 pf
of capacitance to work with, I decided to use a slightly smaller coil.
The variable capacitors are surplus 270 pf types sold by
Leeds Radio in Brooklyn, NY. Send
Richard an e-mail and he will take care of you. He is another one of the
good guys. The knobs are some that I offer for sale here. They have a very
nice look. The skirt will take a label if you are so inclined. I used a
FO-215 diode made by ITT. I found them to
be better than the usual 1N34A. The use of the thumb nuts for the diode
connection is mostly for cosmetic reasons. I like the brass color on
the black styrene.
Making these little radios on a styrene or garolite base is
easy and the results are very nice. The rubber feet keep the wires and
screws from touching the table. I like using wood pieces to hold the
coil and front panel. They look nicer than the metal angle brackets.
The 2:1 vernier on the variable capacitors is helpful in tuning.
The fairly large litz gives this radio a nice performance boost. It
will make a nice hobby class entry for the upcoming crystal radio
reception contest. So get busy and make one!
73 and good crystal DX. Dave - N2DS
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