Superheterodyne Radio Uses One Tube

Introduction

Hi Friends. This project starts me in a new direction. First it was crystal sets, then regenerative radios and now it is time for my first superhet radio. I built a nice multi-tube superhet radio when I was in high school. But this is a new effort for the new century. I hope this little project will give you some ideas for a similar set.

The Idea

Nothing in this project is real original,  but I did do a couple of modifications from the circuits I saw. The fellow that  started this, as far as I can tell is Jack Bryant, KE4ID with his  one tube superhet contest radio project. That was my basis for this set. I started drawing the circuit. I usually don't release too many details early in a project but this time is different. I did get some good suggestions from my fellow radio friends, Ben Tongue and Mike Peebles. I had most of the design down but these two fine fellows gave me some extra ideas that were helpful in this design.

This radio is built with the One Active Device (1AD) contest in mind. My two previous entries were done with two regenerative contest receivers. My 6SK7 and Acorn sets where the projects. They worked very well. But as time goes on, different ideas have to be tried. Jack and the others reported good results with these radios, especially when operated close to local stations. I don't have that problem, but I just had to try it.

The Circuit

The central part of this project is the 6BA7 vacuum tube. Using research of others, I decided that due the higher gain over other similar tubes, that this was the way to go. The other major section of this radio is the detector circuit. Looking closely, it looks like a  fixed frequency crystal set. That's what it is alright. The Bogen T725 provides a closer match between the detector diode / IF transformer and the headphones. An optional  tap selector switch was added so that changing headphones would be easier.

Better selectivity can be realized if you exchange the Bogen T725 for a transformer for an Überformer. The Überformer has an input impedance 5 times the Bogen and provides a much lighter load to the IF transformer secondary. The benny resistor (18k) will have to be changed  to a higher value, about 180k ohms.

The input coil is one of my standard spider web coils with an inductance of about 225 microhenries. I used a spider coil form with a two inch (50mm) hub and a  4-1/2 inch (108mm) outside diameter. The coil is 1/8 inch thick HDPE material. I wound 54 turns of 165/46 litz. The number of turns was based on using a  440 pf variable capacitor. This coil was wound with some extra turns and then  during testing, a couple of turns were removed until the entire band plus a tiny bit  covered the entire dial.

There is a front panel 20 pf trimmer that is used to adjust for any tracking error. In my original design, this adjustment was a screwdriver adjustment. But since tracking isn't perfect across the dial, this occasional adjustment will keep the  radio peaked.  Details of the adjustment of this coil is described below in the alignment section.

Another tuned circuit in my superhet radio is the local oscillator. This coil inductance depends mostly on the variable tuning capacitor value. The value of my coil turned out to be 98µh. This was determined by a handy online superhet oscillator tracking calculator. If you are using somewhat different parts, you  are on your own. The accuracy of the tracking is dependant on how close your coil is to the right value. The coil is tapped with the tickler tap meant to be connected  to the cathode. Be careful to pick the right coil. The 4 lug coils are sometimes  meant for the tickler to be placed in the oscillator triode plate, such as with a 1A7 tube.  This type won't work in this circuit.

Since the Intermediate Frequency (IF) is 455khz, the oscillator works always at a frequency 455khz above the incoming signal. There is a trimmer adjustment capacitor that is wired across the main variable capacitor. There is also a padder capacitor in series with the main variable capacitor and the trimmer. The padder adjusts the maximum capacitance that the local oscillator coil will work with. The alignment procedure will also be discussed below.

The IF can was taken out of an old radio. It is an air wound transformer with two adjustable capacitors for peaking. This "IF can" resonates at 455khz. I selected the  last IF can as that would provide the best load to the detector. If you don't have one of those IF cans laying around, try Gerd at Reinhöffer Electronics. Ask for the 47.02 model (the 45.02 will also work if you want a smaller size).  The power supply requires 6.3 volts at 0.3 amps and around 170 volts DC at 10 milliamps. I used a supply built in a little blue box that I made for my projects.

This radio has an automatic gain control just like the big boys.  The AGC is fed from the detector through a 4.7 meg ohm resistor over to the bottom  of the antenna coil. The volume can also be reduced by moving the antenna  tuning unit away from the radio. Although the dynamic range of this circuit  isn't what it is in multi stage radios, it still can help cut the fading somewhat.

The part of this set that is not shown in pictures or the schematic is the circuit for the antenna tuner. This other unit is placed near this receiver and provides an antenna input. This dual tuning system is essential for good dx reception.  A good example of a front end circuit to use is my antenna tuner. You can also use an existing crystal set that you might have laying around and place that near the input coil on this radio.

That little blue box in some of the pictures is a digital dial frequency counter. It is sold as a kit and the box has to be added separately. It runs on a 9 volt battery. You can program the 455 kHz offset so the dial reads the received frequency. This is a great help for knowing where you are listening. It takes an easy connection directly to the tube cathode via a short length of shielded cable. If you are building only radio, you can build it in and power it with a rectified and filtered line off the heater voltage. But you know me. I can't stop at just one superhet.

Construction

My first superhet is  built on a chassis and panel made from 1/8 inch (3mm) Garolite®.Garolite® is classed as a plastic, but I believe it is made from paper and resin. That does make it a plastic. It is a hard material and is easy to machine. A metal chassis or just about anything else will work fine. I like the black panel look.  The chassis size is a large 9 inch (22cm) by 6-1/2 inches (15,6cm).

Your construction will depend a lot on how your main tuning capacitor mounts. My capacitor is mounted on 1-1/4 inch (30mm) "jack screws" that have a male and female threads on the ends.

The chassis stands on a piece of 3/4 inch (18mm) oak board that has been stained to my liking. I used 1 inch (24mm) stand offs between the wood base and the Garolite® chassis. This has worked well in two of my most recent projects. A base is not really needed as long as there are standoffs from the chassis to act as feet.
I drilled all the holes, using forstner bits for the larger holes. A small drill press does the job. A scroll saw and sander were used to make the chassis and panel.

I used three, 4 lug terminal strips in this project. The locations turned out to be pretty good. The terminal strip on the left is for the detector portion. The top one is for the B+ voltages, while the strip on the right is for the cold end of the antenna coil and the connections going to the pot as well as the oscillator grid fixed capacitor.  It is also important to notice the positioning of the tube socket. The heater terminals (pins 4 and 5) face down in the picture. They have the yellow wires connected to them. Building in this way will make the wiring shorter.

All three of the trimmer and padder capacitors should be situated for  easy adjustment.

Alignment

Now comes the fun part. The first thing to do is to get the input coil right. You want this LC circuit to tune the entire MW band, from 530 to 1710khz. I started with a coil that I knew had too many turns. I used my signal generator and a close by radio to make this coil adjustment. Start with the front panel antenna trimmer at mid position.  The radio should not be powered. In my case, I did this before I wired the set.

Set your radio and generator for 530 kHz and see where the tuning capacitor adjusts to when you hear the signal change in the nearby radio. Then move up to 1710khz and check that. You may find that you can't tune that high. If you want, reduce the  trimmer capacitor and try again.

You may have a lot of capacitor left over at the bottom of the dial. If so,  remove a turn and try again. Look at the top and bottom frequencies. Adjust the trimmer until the band covers nearly all the dial range of the tuning capacitor. I make the distance equal at the top and bottom to the end of the capacitor range. If there is  too much out of the band space, just take another turn off the coil and try again. Don't try to cut this adjustment too close. When you are done, adjust your generator and radio to 600 kHz and temporarily mark that position on the dial. Do the same at 1500 kHz. These will be your two tracking alignment points.

As I said, the antenna circuit can be done before the set is completed.  Now, on to the alignment of this superhet after you have found it to work. With the radio powered up, the IF can should be aligned first. Connect your signal  generator across the input coil and tune the capacitor all the way to the bottom of the band. While listening to a modulated signal at 455khz, adjust the IF can for the loudest signal. As the signal gets louder, reduce output of your generator.

Lightly couple a signal into the receiver from your modulated signal  generator. Start at your 600 khz mark and tune the oscillator padder until you hear the signal. Rock the dial and padder to make sure you have the loudest signal, as the temporary marks might not be exactly on. Next, move up to the 1500 kHz mark and adjust the oscillator trimmer until you hear the signal. Rock the dial and readjust the oscillator trimmer for the  loudest signal. Repeat again at 600 khz and back at 1500. If everything went ok, you are aligned.

Operation

Connect your antenna and ground to the antenna tuning unit. You will have to adjust the tuner and radio tuning each time you want to change frequencies. The gain control is usually run at full volume, but can be reduced if too loud. There is not too  much to the operation of this radio. Perhaps that is a letdown after all the effort you put into building it. As a final note, please be careful of the voltages present on this set when powered up. If you are a beginner, you should get some one on one advice before building an AC powered set. This radio should not be built by a beginner but someone that has been around higher voltages. I hope to offer a battery operated version of this set in the future.

Happy building de N2DS!