Medium Wave One Tube Loop Radio
Ok - go ahead and scroll down and look at all the pictures first. I'll wait. It's kind of like having your desert (eye candy) first before the meat and potato's. Have you finished? Good. Let us continue.
I am proud to introduce you to my first loop tube radio. I had this in mind for a while and it took me a long time to get started. The radio design was simple, but how was I to make this into another show piece radio? Do I make it into a 1920's look, or a rock n' roll 50's radio? This one is both. The design, and parts are something that you might have seen in a 1950's Poptronics magazine. Then there are some hints that have that definite roaring 20's look. The loop itself is an example as well as the art deco Garolite® front panel. The wood base gives this radio a breadboard look. I sure did it all. This puppy kept me up nights thinking how I was going to build her. I like that curve that decorates the front panel. I used a large pizza pan as a guide.
The radio is a simple regenerative type using a 6418 subminiature tube. This is a pentode tube that has a 30 volt plate maximum rating. The filament runs on 1.25 volts at 10ma. With 18 volts on the plate and a low current filament, the batteries will last a long time. I built another radio using this same tube.
The most popular way of controlling the regenerative feedback in a pentode based radio is a variable resistor voltage divider connected to the screen grid. I decided to use a throttle capacitor this time. The main advantage is there is no need for a double pole power switch. Since all B+ currents flow through the tube, turning off the filament also shuts off the B battery current. I had experimented with throttle capacitor regeneration control before but never made a project this way. Before I built the final project, I used my breadboard to check the circuit. It worked great.
A standard pair of high impedance magnetic headphones are used to listen to the music. A crystal earphone may be used if a 4.7k ohm resistor is shunted across the earphone. Sound powered, or balanced armature headphones work even better.
A dual section, 270 pf variable capacitor is used to tune this radio. A single section capacitor of around 600 pf total will work. My radio tunes to about 1630 khz, due to the size of the capacitor and construction of the loop.
Now that I have discussed the more or less easy parts, it is now time to talk about the special feature of this little radio. That feature is the ability of this radio to work as a loop antenna for another radio. This means that this project is likely to have daily use around my house. Here is how it works:
I added a quarter inch stereo phone jack, which is on the lower right corner of the front panel. This jack has two cut-off switches built in. These would normally mute a speaker when the headphones were plugged in to the stereo. In this case, the switches remove the tickler coil turn from the radio circuit and this winding can now be used as the pick up winding to a radio. This is an automatic feature of this project. If this is something you don't want to do, I have provided a "lite version" schematic for your building pleasure.
The loop is the first part of my radio that I built. I used two sticks that are 25 inches (63,5cm) long. They are 1/2 x 3/4 inches (12mmx18mm) square. They are notched in the center so they fit together nicely. The mast is 3/4 inches square and is 20 inches long. There are two 4 inch (10cm) square pieces that hold the loop together. These can be made from about anything. I used HDPE pieces that were in my bin. There are 4 end pieces that have notches for the wire. I made these pieces from 1/16 inch (1,5mm) Garolite®. These pieces are 4 inches (10cm) by 1-3/4 inches (4,5cm) There should be 14 notches in your design. Four brass thumb nuts make the connections. If you make one, if it is slightly smaller, the tuning would be perfect. If you disconnect the loop, make sure your headphones are unplugged. This is in case you short certain wires, causing battery current to flow.
The 4 wires between the antenna and radio is Belden 8890 miniature test lead wire. It is rubber covered and very flexible. I made some little wire spacers to keep the wires neater and to reduce the detuning as the loop is turned. They add a lot to the looks too. Which reminds me, if you move your body near this loop, the radio will detune. Normally this could be thought of as a design flaw, but I think of it as a feature. If you put the loop radio into oscillation, and with a near by radio, beat this oscillation with a local station, you have a theremin. Just think, you can make music while listening to Limbaugh.
Happy building de N2DS!