Dave's #2 Crystal Set
Hello again crystal radio fans. Just can't get enough of these crystal sets, huh? Ha ha, me too. The solder had barely cooled from my first crystal set, when I started in on the second.
Again, I am using Gollum's spider web coil for my first crystal set. This time I used a box that is large enough for the two variable capacitors with their old looking knobs. The box is wooden with an oak finish. I bought the box at Bed Bath and Beyond. They have several sizes in stock. The box doesn't come with a top, so I stopped by Michael's and found some thin plywood that I could make a top from. This is where all the components are mounted. I used a Greenlee one half inch chassis punch to make the holes, but you have to be very careful so as not to split the plywood. If you can find a good black plastic, that would be better. I am using those same cool H.H. Smith all-metal binding posts. This time all I need is 4, antenna, ground and two for the headphones.
The coil I used the litz wire as in my first set. I decided that I would try a Budweiser beer coaster as a coil form. I left the diameter (slightly over 4 inches) as it was, and cut 13 slots in the coil form. Other brands of beer coaster can be used as long as the size is right. For a non-alcoholic crystal set, try an O'douls coaster. I then weaved the wire as shown here. Dave likes to save money whenever he can!
Update! I have made some changes to this set. I changed the rotary switch to a 8 point tap switch and wound a new spider web coil. The coil has a primary winding for the antenna circuit with a variable capacitor in series with it. The secondary has a variable tuning capacitor and a diode tap.
The coil form is 4-¼ inches (10.7 cm) in diameter. I like these Budweiser coasters better than the real spider web coil forms. I cut 11 slits in the spider form. Each slit should be 1-½ inches (3.75 cm)long. I wound the antenna circuit with 30 gauge kynar coated, silver plated wire. This wire was left over from my wire wrapping days. 28 or 30 gauge magnet wire should be good. I wound 40 turns, tapping every 5 turns. At each tap, I carefully cut and stripped the wire. I twisted it together and continued my winding. When I was done, I took pieces of the same wire, and cut, stripped and soldered it to the windings. Another way might be to not cut the wire, but remove some insulation, and wind a piece of stripped wire around the coil winding. But after I soldered the wires, I used some heat shrink tubing to make the connections neater.
I then wound the detector winding using litz wire. I used the 40 strand, 44 gauge wire with the nylon coating. I wound 30 turns with a diode tap at 24 turns. I wound the antenna winding first, then the detector winding as this winding might to be pruned somewhat. The windings will fill the spider form all the way out.
The first coil worked ok, but just didn't have that "feel". If you are using a real short antenna, I would say that the original would be alright. Don't take my word for it. Try them both and see for yourself.
The tap switch plate was installed to cover up the half inch hole that the rotary switch held. I could have used the rotary switch but tap switches do look pretty nice. This switch has 8 points and the arm is slightly over one inch. I used the black Garolite® that I had available. It would have been nicer if the piece was a quarter inch larger. But it does work well and looks fine. The switch is made up of decorative thumb screws and the arm is 3/8 inch brass. A stiff spring keeps the arm tension just right.
73 and happy building from Dave Schmarder, N2DS