12AF6 Broadcast Band Regenerative Radio
As you can tell, this radio looks pretty neat IMHO. I liked the way my last radio turned out, I thought that I would try to out do myself. This radio uses a single 12AF6 tube. This tube has an interesting feature, the plate circuit will run on 12 volts dc. These tubes are called "hybrid" because the radios that used them had tubes and transistor circuits. These tubes, while no high voltage was required, but were unable to produce much power. These tubes were fine for the RF circuits but the audio section was best left to transistors. Not needing high voltage for the plate circuits allowed manufacturers to leave out vibrators, transformers and high voltage filters. This radio demonstrates this feature.
This radio has a regenerative circuit that was popular from the 1920's until the 1960's. The one tube amplifies the signal, then passes a tiny bit back to the input to be sent through again. This increases the gain quite a bit. The circuit I used was a generic type. The parts values aren't super critical, nor is the exact tube. Use what you can get your hands on.
My one tuber tunes the standard broadcast band from 1700khz down to 550 khz. The low end of the band is empty around here so the emphasis was on the high end. In some designs, the 365 pf capacitor isn't big enough to cover the entire band. If the radio doesn't tune high enough, take a couple of turns off the coil. Add a couple if the opposite is true. (hint: It is easier to remove turns than to add them, so start with some extras.
The grid leak resistor is 10meg ohms. This is kind of a high value but are still available. Another builder had recommended using that high of a value. I didn't question it.
The center of activity with this radio is the coil. My coil has three windings. The main winding is wound with litz wire (40 strands of 44 gauge wire). This is the size of 28 gauge wire. The coil form is a 2 inch mailing tube. The outside diameter is about 2-1/8 inches. There are 66 turns on this coil. If you use a larger gauge wire, you will also need more turns. The coil form is 3-1/2 inches long.
The second winding is the tickler winding that connects back to the plate of the 12AF6. This is 8 turns of 30 gauge wire. If you build this radio and it doesn't work, try reversing the tickler winding. The third winding is the antenna coil. This is wound with 20 turns of 30 gauge wire. Another method of coupling is to connect a 50pf trimmer capacitor to the top of the main coil and the other end to the antenna. I have used the 3 winding coil on previous circuits and they worked well for me, so I kept with that design.
The front panel is made from a piece of 1/8 inch Garolite®. Garolite® can be cut on a table saw, but sometimes the edges tend to chip, so a fine tooth blade would probably work better. The base of the radio is a piece of stained 3/4 inch oak. This makes the radio look nicer on the inside. Using wood screws I mounted the components to the board. I then wired according to the diagram.
This radio requires 12 volts to operate. I decided to use a "wall wart" that delivers 12 volts at 500ma. That way I don't have to buy batteries. If I want to listen in the woods, I can connect this radio to my car battery. The toggle switch on the front is for power.
The connections on the radio front panel from top to bottom are: Antenna, Ground, and Phones. I will probably label these using a Brother labelmaker and some white on clear tape. The front panel is held to the box using some T-Nuts. Look at my other pages for details.
The radio is quite stable. I use Q-Dope, available from GC Electronics
to hold the coil windings in place. Don't use other types of glue
as they can destroy the performance of the radio. When operating a
regenerative radio, frequent adjustment of the regeneration control
is required when tuning the band. Try to keep the set out of oscillation
as there is no isolation between the tube and the antenna. You can
end up with a miniature transmitter that will disturb your neighbors
reception (if anyone except for me listens to AM radio anymore).
By the way, this guy in Germany likes these kind of sets too. Also check out Jeff Duntermann's 12 volt space charged tube resource.
73, Dave - N2DS