Dave's #58 Crystal, Now A Tube Radio
Have you been wondering what could be a new use for that old oscillation transformer you had left over from your spark days? Well, look what I have cooked up in my kitchen. A shortwave crystal set using some old spark radio components.
While innocently rummaging through eBay I stumbled upon this transformer and capacitor. Since not too many people understood what this stuff was (including the seller), I took the opportunity to grab these treasures. I knew that coil would make a good tuned circuit for shortwave, but I didn't know how close it would be to what I wanted. I ended up with a radio that would tune the 49, 41 and 31 meter shortwave bands.
The standout component in this radio is that coil or as they called it in the old days, an oscillation transformer. It was a device added to a ham radio operators spark station, in an attempt to keep his signal in the 200 meter range. Have you ever keyed a spark rig? I did at the AWA Museum. It is quite a thrill. I sent my call sign on the key. (I hope the transmitter wasn't connected to an antenna.) Anyway, I'm getting a little off the track here, but you should expect that from me by now.
In keeping with the old time look, I went with a cat whisker detector for this old radio. I'm casting my own pyrite detectors too.
As you see in the pictures, the coil has several clips hanging off of it. They are for ground, antenna, detector tap and one wire to the stator of the variable capacitor. All these taps can be moved to provide the best signal sensitivity and selectivity. The further up the coil from ground, the louder the stations are (to a point) but the selectivity gets worse. It is fun experimenting with the coil taps.
The variable capacitor was in poor condition when I received it. One of the insulators was gone, turned to powder. It was an easy job to take a little piece of styrene and make another insulator. Then after a session with the steel wool, the capacitor was presentable. That dial scale was another eBay treasure.
The wood base is solid oak, stained and poly coated. Everything mounted easily and those white ceramic stand off insulators give the rig a nice finished look.
I do cheat by placing a 1N34A diode in the circuit, in place of the pyrite detector. After I get the radio tuned, then I take out the diode and poke around the pyrite for the hot spots. This was a fun radio to make. I hope you like it too.
Guten Empfang wünscht Dave - N2DS
My #58 shortwave crystal set is now the #58-1625 shortwave radio! Long live the firebottles!
I have found that no matter how well you make a shortwave crystal set, they really don't work like their MW brothers. I think that you will agree that the set did look great. I was very proud of it. Tube regenerative radios work better than crystal sets because of the Q-multiplication effect of a regen set. This is especially important with shortwave radios because there is no way that a shortwave radio could tune this sharp.
So I did a one evening conversion of this crystal radio to a one tube set. This radio uses a single 1625 tube. The 1625 was developed around the time of the Second World War. It has been a workhorse of the Amateur Radio Operator for a long time after.
Since I only had one coil to work with and didn't want to add a separate tickler coil, I tapped the oscillation transformer. That method works well with tubes that have a cathode. I took out the detector stand and added the tube socket with the other components. I just happen to have a nice red knob that looks great on this radio. That is the regeneration pot.
Power is provided by a 12 volt at 500 ma wall wart power supply. Extra filtering is needed for the section that really needs the dc, the plate and screen circuit. So I added a 560 ohm resistor and a 1000 uf capacitor. It is quiet now. These values aren't critical. Just use what you have.
The 12 volts is safe. You should be careful about handling the tube after the supply has been on for a while. It is as hot as a 6 watt light bulb.
This set will never be a contest winner for reception, but it could be a winner in an art contest.
Dave - N2DS