Dave's #6 Crystal Radio — Elgrande
I call my #6 set the El Grande. This is an update of my fifth receiver. I found the knobs, the tap switch points and arm at Play Things Of Past. Gary has lots of old radio parts there and at reasonable prices! Because of the nature of his inventory, it does take some searching and fiddling to find what you want, but you are richly rewarded.
I tried to make this radio look authentic. I am sure that those H.H. Smith binding posts weren't around in the twenties, but I like them a lot. Otherwise someone might think that this is truly an old crystal set. (The diode on the inside gives it away)
This crystal set has two coils, two variable capacitors, making up two tuned circuits. The coils are wound on a two inch mailing tube available at Staples. Before winding the coil, I sprayed some Parks brand shellac over the coil form. This will keep out the moisture and gives the coil a nice shine. I bought the shellac at Lowes. The antenna side coil has 120 turns with taps about every 13 turns. The 8 taps and two ends are connected to the tap switch points. This coil is a little over three inches (75mm) in length on the seven inch (175mm) coil form. This coil is wound with 23 gauge wire.
The tap switch has a 1-1/4 inch arm. So the contact points are also 1-1/4 inches from the arm pivot. I laid out the switch by first putting masking tape on the surface. Then I measured for the arm pivot hole. I took a protractor and measured the angles. The contacts are at 15 degree increments. By drawing a vertical line, I made the top two contact points 7-1/2 degrees from center. You might want to practice on a spare piece of Garolite® before you make the big cut. My first switch had the "Dave Let terman tooth gap" in the center.
Since I live out in the sticks, far away from the big 10-50kw stations, I decided to go with tight coupling from the primary to the secondary windings. The detector coil starts about a quarter inch (6 mm) away from the antenna coil.
The detector coil is wound with my ole favorite litz wire. As before, my litz wire is 40 strands of 44 gauge wire with a nylon covering. There is a lot of wire in a one pound spool, so you will see me using a lot of this! There are 65 turns wound on the form with the diode tap at 50 turns.
There are four thumb nuts located just below the four binding posts. By using jumpers, you can configure the antenna tuner so the capacitor is in parallel or series with the antenna coil, or you can take the variable capacitor out of the circuit. The settings of the jumpers will depend on the antenna you are using and the frequency that you are tuning. Use the trial and error method to find which is best for you.
The two binding posts on the left are for the headphone connections, and the next one is the ground connection, and the binding post on the right is for the antenna.
Since I had the multi ganged variable capacitors, I decided that I would parallel two gangs in the antenna circuit. This gives me a larger capacitance range. To increase the tuning range in the detector circuit, I put a 200pf fixed capacitor between the stator that connects to the coil and a spare stator. This increases the maximum 440 pf somewhat to tune the whole broadcast band but doesn't make the tuning impossible. The third gang of each capacitor is left unconnected.
Speaking of more crystal sets... I want to build at least one shortwave
set. Also on the to-do list is a crystal set breadboard. This will
consist of variable capacitors, and diode detectors all tied to Jones
barrier strips. This will allow for easier experimentation.
So stay tuned... (pun intended)