Homemade Single Tube Radio
There it is, my friends. My first tube radio built in 2003. It sure is a pretty one, if you ask me. My friend Jim Frederick inspired me on this one. I used what I had laying around for the most part. The schematic drawing represents the everyday parts that you can use to build this little radio. The coils are some old 1920's honeycomb coils that I bought last year at a hamfest. They are the same coils that I used on my Miller clone crystal radio. The coil holder is also the same. (hint: If you need small diameter dowel rods, visit your local chinese restaurant.) The variable capacitor was made by Benjamin, and the two bypass capacitors by Faradon. The grid leak resistor / capacitor are also old timers.
The antenna coil I used was 320 micro henries. This is kind of large. If you have a long antenna, you might consider making this coil much smaller in value. All three of the coils can be wound on a 3 inch form (about 4.5 inches long or so). The antenna winding might be around 20 turns or so in this case.
The audio transformer is a Stancor TAPC35. It has a 10k:2k ohm impedance. You can use a 10k to 8 ohm if you would like to use the more commonly available headphones.
The front panel is Garolite® and the base is 3/4 inch oak. I used a forstner bit to drill half inch holes in the bottom of the base for the terminal screws to go.
The regeneration pot works very smooth. If you find your set goes into oscillation at a low setting of the regen pot, reduce the number of turns on the tickler coil. You can also put a low value resistor (1k ohm or so) across the tickler to reduce the sensitivity. The plate current is about .4 ma, so I expect long life from my B batteries (5 nine volt batteries hooked together). If when you build the radio and it doesn't seem to work, don't panic. Chances are (50-50) that the tickler coil wires need to be swapped.
If you don't have a 3Q5 tube, a 1Q5 will also work. The filament
voltage would then be 1.5 volts. A 3Q4 or 3V4 are also good candidates.
The regeneration pot can be anything from a 50k to 2meg.
I make the labels with a Brother P-Touch labeler that is connected to my computer. They look very nice on my little radio. The white on black tape blends in well with the Garolite®.
Give this set a try. Build to your own tastes and if you can, make it look nice. It pays. 73, Dave - N2DS