One Tube Loop Radio
W3LKOM K0MRADS! Last week we build big MiG fighter jets for the glorious workers party of Soviet Union. This week we build one tube radio with left over MiG parts.
Now how did I get this idea? These ideas used to come to me late Saturday nights during alcohol therapy with Ranger Bob. But since those days are over, I only have myself to blame. For a long time I have seen projects using tubes on the German Röhrenbude (tube shack) web site. These tubes always looked so cool and different, I had to get some and try my little project. Since I like building small tube radios, I decided that was the way to go.
Since these tubes operate just like the American types, I had to do something special to show this was no ordinary tube. The idea came to me to display this tube as the centerpiece of the radio. The loop antenna was chosen as a compliment to the tube display.
This circuit is the same as my previous loop radio project. That radio was very successful and I knew that this circuit would be a good basis. The tube is a Russian type 1J24B or so it translates. Unfortunately there is little information on this in English. But fortunately for me, there was enough in German to give me the incentive to give it a try.
The loop antenna has two windings. The main detector winding, which is tuned to the
frequency desired, and a one turn loop of wire that acts as the tickler or regenerative feedback
winding. The feedback is controlled by a throttle capacitor, just as they did in the 1920's.
Power is supplied by a single AA battery at 1.5 volts. The current draw of this tube is around 13 ma at 1.2 volts. A 22 ohm dropping resistor is added to drop a small amount of voltage, so the tube operates as it should. The plate supply is a pair of 9 volt batteries wired in series for 18 volts. My experiments show this to be the right voltage for this circuit.
My little radio is built as many of my other sets are on an oak wooden base. There are four 1 inch (24mm) spacers to hold the chassis above this base. The chassis is made from a piece of 1/4 inch (6 mm) HDPE. I already had it cut for this base so I used it. Other non-metallic materials may also be used.
The panel is my usual 1/8 inch (3mm) Garolite® sheet cut to fit. I like working with
this material. The panel is 11-1/4 inches (28,5cm) wide. It was this wide so that the three
major sections, the tuning capacitor, tube and contols would fit evenly across the front.
Everything is wired on the front panel, except for the battery holder and loop. The construction was very easy, except for drilling that big hole. The hole is 2-1/8 inch (54 mm) in diameter. This was the largest drill I had. It is actually a forstner bit. If you use one of these, make sure your drill press can handle the load and be prepared to spend a while drilling. Next time, I'll use my scroll saw. :)
The tube is mounted by it's wire leads. The plate connection is on top of the tube. I spread the other wires and used seven 8-32 screws and hardware, including some attractive brass acorn nuts. I was at first going to use plain terminal strips but this project needed something more.
The loop is the most difficult and very time consuming part of this project. There are many steps, lots of materials, but not a lot of cost. The construction is roughly in the order of this discussion.
I started just as with my other loop radio by using two oak sticks cut to 25 inches (63,5 cm) long. A notch is made in the middle of each stick so that the pieces will make a cross.
There are notches cut 3/4 inch deep in each end to accept the 1/16 inch (1,5mm) thick Garolite® wire spreaders. The size of the spreaders are 4 x 1-3/4 inch (10x4,4 cm). On each spreader there are 14 notches, spaced 1/4 inch between them, except for the middle where there is a 1/2 inch space. After I wound the loop, I found that only 13 notches were really needed. To mount the spreaders, I used a single flat head screw and nut. I drilled the holes while holding the spreader in place. The fit was nice and snug.
The mast is a 3/4 inch (18mm) square, 22 inch long piece of oak wood. I notched the top so it would fit in the cross. I drilled a 3/8 inch (9 mm) hole in the bottom to accept a piece of dowel. This provides a way to rotate the loop for the best reception. You also need to drill 6 or 8 small holes to feed the ends of the wire that will connect to the terminal block. For the big winding, I looped the wire through two holes. The tickler winding just went through one hole. I also cut small decorative notches so the wire wouldn't hit the wood too much causing a distortion in the winding. You have to fit the loop together and see where the wires will hit to get the position. After this basic work is done, then stain and otherwise finish your wood.
You will need two 4 inch (10cm) squares of Garolite® (or other material) to make the center pieces of the loop. There are 10 small flat head screws on the front and 5 on the back. Their positions are staggered so that none of the screws will hit the ones coming from the other side.
A small piece of Garolite® is used to make the 4 terminals that connect the loop to the radio. After you assemble the loop, then the wire has to be strung. I used 22 AWG magnet wire, but 24 will also work well. String the big winding first, then the tickler last. Then when that is done, the loop is ready to use.
Not too much alignment needed here. You may have to fiddle with the fixed capacitor that is across the variable throttle capacitor to get the regeneration the way you want it.
I would like to make a few comments on loop antennas and the tuning thereof. I learned from a gentleman that the distributed capacitance of a coil increases as the diameter of the coil is increased. This answered my long standing question on how come such a large value of variable capacitor was needed to tune the entire band.
The tuning capacitor as a maximum and minimum range. If you divide the maximum range into the minimum you get a certain ratio. (A 15-365 pF capacitor would have a ratio of about 24. However if you add some capacitance from the grid leak detector plus a bunch of distributed capacitance from the loop antenna, the overall ratio falls, and there is no way to tune the whole band.
But if you increase the capacitance to over 500 pF and hopefully the minimum capacitance stays low, you have enough tuning ratio to cover the whole band. If you can, try to find a 600 pF variable (I used 540 pF) to insure easy tuning of the whole band. Also remember that the loop windings have to change by an even 1 turn at a time. It is only by luck that I get them working as they should.
This is the first article on using the small Russian tubes to make a regen radio that I have seen in English. Don't let the strange Russian symbols keep you from having a little fun here. 73 and happy radio building ~ Dave, N2DS