My Homebrew SuperHet Receiver Build

I have built many Regenerative radios and decided to give building a SuperHet radio a try. My SuperHet radio uses hand wound coils in the Antenna transformer, two IF transformers, the Local Oscillator, and the Beat Frequency Oscillator. It covers the 80M or 3.5Mhz band. The Antenna transformer and Local Oscillator coil used in the Frequency Converter or Mixer Stage are pluggable, allowing them to be swapped out to cover other bands. In the future, I hope to create a pluggable Antenna transformer and Local Oscillator coil for the standard AM broadcast band. The blue radio in front has the same number of IF stages as my homebrew SuperHet. I set it there to give you an idea of scale.

SuperHeterodyne Radio Theory 

Invented by engineer Edwin Armstrong, a superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. Virtually all modern analog radio receivers use the superheterodyne principle.

My Inspiration

I had some trouble finding plans for a vacuum tube superheterodyne receiver on the Internet. I put the question to an antique radio forum, I often frequent. Another forum member suggested I acquire a copy of the now out of print "The Impoverished Radio Experimenter Volume 5", a small booklet that completely covered the topic of a homebrew vacuum tube superheterodyne receiver. This book was my inspiration as it covered the build of such a receiver and included a whole alignment and troubleshooting section. 

The Impoverished Radio Experimenter Volume 5 on Amazon:

Circuit Theory

Above is the schematic of the homebrew SuperHet AM (amplitude modulated) receiver I am to build. As you can see, it consists of a Frequency Converter or "Mixer" Stage, an IF (Intermediate Frequency) Amplifier Stage, and a Detector Stage. This radio only has one IF amplifier stage where most modern radios have two. Adding another IF amplifier stage would increase the size of an already rather bulky radio. Tuning is accomplished by adjusting the frequency of the LO or Local Oscillator, the result is always a 400kHz amplitude modulated signal, feeding the single IF Amplifier Stage. The IF transformer before and after the 6SK7 tube "pass" the 400kHz signal, while acting as a direct path to ground for all other frequencies. One triode of the 6C8 acts as the Detector which strips the RF (Radio Frequency) carrier, leaving the AF (Audio Frequency). The second triode acts as a BFO or Beat Frequency Oscillator that, when enabled, allows the detection of CW or Carrier Wave transmissions. 

Layout

The base of my SuperHet receiver is a 12" x 20" small wooden "craft" pallet, I obtained from our local craft store. During one of my trips with my wife to the craft store, I noticed that they had a complete selection of small wooden boxes and pallets that are ideal for homebrew receiver  construction. I first drew a 1:1 scale layout of the placement of all components on the craft pallet, then I drew the wiring between the components. It's important to get the layout nailed down, prior to construction. 

Shielding 

The IF transformers and BFO helical coils need to be contained in non-ferrous shielding, so as to prevent crosstalk between one another, and to avoid interference to any nearby electronics. I chose to build the shielding enclosures of cardboard clad in aluminum foil. I carefully cut pieces from an old Chewy (online pet supplier) cardboard shipping box, using an X-Acto knife, and a framing square as a straight edge to cut along.  

A 2" x 2" "window" needs to be cut in the cardboard divider, or septum, that separates the two IF coils, within the shielded enclosure. The "window" needs be centered and at the same height as helical coils, allowing for maximum coupling between them. 

I proceeded to cut the remaining cardboard pieces, needed for the shielding.  

Each shield enclosure was taped together using masking tape, to prepare for gluing. 

Elmer's glue was applied to each seam of the shield enclosure. I then used my index finger to remove the excess glue, forming a nice bead. 

Each shield enclosure requires "feet" so that it can be mounted to the wooden base. I used a couple blocks of wood to keep them in place, during the gluing progress.

Each cardboard shield enclosure must be clad in a non-ferrous metal, in this case aluminum foil. I used an X-Acto knife to cut the required aluminum foil panels, to be affixed to the shield enclosures. 

Elmer's glue was then applied to the sides of the ​cardboard shield enclosure.

The aluminum foil panels were then affixed to the cardboard shield enclosure. 

Here is the finished BFO coil enclosure. 

I then tested to ensure that there was continuity between aluminum foil panels. 

Aluminum foil straps are required to make contact with the foil panel, affixed to the wooden base. 

I built two more cardboard shield enclosures, one for each IF transformer. 

The divider, or septum, that separates the two IF coils within the shielded enclosure, needs to be clad in aluminum foil as well. The foil over the 2"x 2" window needs to be cut. 

Then folded back and glued on the back side. 

The divider needs to be centered then glued in place. The 2" x 2" windows needs to be toward the top of the cardboard shield enclosure. 

Aluminum foil straps attached to the divider are required to make contact with the aluminum foil panel, affixed to the wooden base. ​

I then used Windex window cleaner, and an automotive chamois to remove glue residue from the foil clad cardboard shield enclosures.

Coil Winding

I built a Home Brew Helical Coil Winder Jig, to assist with this project. 

It even has a magnetically triggered counter, that counts the number of coil turns.

Click on the link below, to find out more about my coil winder:
Home Brew Helical Coil Winder Jig

Winding the coil is as simple as turning the crank, while guiding the wire on the cardboard tube. 

Once finished winding the coil, I placed a piece of masking tape to hold the varnished coil in place. I then applied a thin bead of E6000 clear adhesive, to hold the coil windings in place.   

I scrapped the varnish from the end of the wires. I then used my Mega 328 Component Tester, to measure the Inductance and Resistance of each coil, in order to check for consistency between them. 

Based on my calculations, each IF coil requires 260 turns of #30 magnet wire on a 1 5/8 diameter cardboard tube.  The BFO coil requires 260 turns of #30 magnet wire, with a tap at the 195th turn. Each coil starts 3/4 down from the top of the cardboard tube. Four IF coils and one BFO coil were needed. 

An Antenna coil is also required. I wound 13.5 turns of #22 magnet wire on a 1 1/4 inch four pin tube socket. At this point a loop for a tap is created followed by another 13.5 turns. The magnet wire is held in place with E6000 clear adhesive. After the ends of the magnet wire were scrapped clean of varnish, they were threaded thru the hollow pins. 

A layer of masking tape was applied, then another seven turns of #30 wire was wound on the Antenna coil form, once again held in place with E6000 clear adhesive. 

The ground connections of the primary and secondary winding of the Antenna coil are soldered to the same pin. The antenna input lead, secondary center tap, and secondary connection all get soldered to their own individual pins.  

There should be continuity between each pin of the Antenna coil. 

In addition, a Local Oscillator coil is needed for the Frequency Converter Stage. This is also #22 magnet wire on a 1 1/4 inch four pin tube socket. The Local Oscillator coil requires multiple taps. I wound two turns, starting at the bottom of the tube socket, and put a loop in for a tap. Then another two turns and another loop for a tap. Again, another two turns and another loop for a tap.  At this point seven and a half turns then a loop for a tap. Finally, eleven and a half turns were wound. 

The wires at the start and end of the Local Oscillator coil winding were soldered to individual tube socket pins. The third socket pin was soldered to a wire that goes to the tap at turn number four. The fourth socket pin was soldered to a wire that goes to the tap at seven and a half turns, past the tap at turn six.  

There should be continuity between each pin of the Local Oscillator coil. 

Wiring

Unfortunately, due to an issue with my camera, I lost many pictures of the construction of my SuperHet receiver. So the Saga continues with the wiring of my SuperHet.

Prior to wiring, I dry fit all of the components to the wooden base. I then removed the components and gave the wooden base a two-tone white and grey paint job. I then remounted the components, once the aluminum foil panels were glued to the areas where the cardboard shield enclosures were to be mounted. The white end caps on the aluminum panels hold the coils in place inside the cardboard shield enclosures.  In order to neaten the wiring, I ran the tube filament wiring on underside of the wooden base, then drilled holes by each tube socket to run it to each tube socket. The other side of the tube filament wiring is terminated with Fahnestock clips. 

I found it easiest to mount the tube sockets, used for connections to the Antenna transformer and BFO coils, upside down when soldering the wires to the terminals. 

All resistors and capacitors were tested with my Mega328 Component Tester, before being soldered in place. 

After the tube filament wires were connected to each tube socket from the underside, I ran the black wires used for the ground connections on the top of the wooden base.  All the ground connections terminate at a central Fahnestock clip, located on the side of the base, where the filament connections terminate. I also wired each component connected to ground to the appropriate connections on the tube sockets. 

Next, I ran the red wires for the 90 Volt B+ Voltage and the green and yellow wires used for coupling between the Frequency Converter, IF Amplifier, and Detector Stages. All 90 Volt B+ Voltages terminate at a ​central Fahnestock clip, located on the side of the base, where the filament connections terminate.

Wiring was then ran for the two front control panels. It is a lot easier to run the wiring, prior to installing them. 

Time to wire the BFO/IF Gain control panel. 

As you can see, the back of the control panel is lined with aluminum foil that is connected to ground. ​

Time to wire the Antenna/Tuner control panel. 

The back of this control panel is also lined with aluminum foil that is connected to ground. 

Small 6-70pf trimmer capacitors were soldered across the terminal strips that connect to the IF transformers and BFO coils. These will be used to adjust the resonant frequency. 

Labels were applied next to the Fahnestock clips, close to the BFO/IF Gain control panel, to indicate the required input voltages, needed to power my SuperHet.

Labels were also applied next to the ​Fahnestock clips, close the Antenna/Tuner control panel, to indicate the Antenna and Ground connections. 

Variable capacitors on the Antenna/Tuner control panel were labeled, as to their function. In addition, a label was added to the top of this control panel, to indicate the purpose of this contraption. I painted a silver line on each dial, to indicate its position.  

The above picture was taken while testing my SuperHet, hence the alligator clips. Labels were added to the BFO/IF Gain control panel as well, below the IF Gain potentiometer, BFO Pitch variable capacitor, and BFO on/off switch. Once again, a silver line was painted on the BFO Pitch dial to indicate its position. The Fahnestock clips, that connect to the high impedance head phones, were also labeled.

Coil Installation

Time to mount the four IF and one BFO coils. The four IF coils are the same, while the BFO coil has a center tap. The coils cylindrical forms are pushed down on the circled white plastic caps, that are affixed to the wooden base. 

Each IF coil lead was threaded through a white PVC insulator. Then the ends of each IF coil leads were scraped clean of varnish and soldered to the terminal strip. Finally, the white PVC insulators were secured to the wooden base, with E6000 clear adhesive.

A resistance check was done at each terminal strip, after the IF coil was soldered to it. It should read between 10 to 15 ohms, if the IF coil has a good connection. 

The BFO coil leads were also threaded through a white PVC insulator. Then the ends of each the coil leads were scraped clean of varnish and soldered to the terminal strip. The white PVC insulators where secured to the wooden base with E6000 clear adhesive. Resistance checks were also done at the terminal trip, to ensure the BFO coil has a good connection. 

Here is a picture of the IF and BFO coils wired in place. In addition, I plugged in the Antenna and Local Oscillator coils. 

Cardboard Shield Enclosure Installation

Next, the cardboard shield enclosures were installed. Each one is fastened to the wooden base with four wood screws. 

A washer was placed under the head of each wood screw, to prevent it from pulling through the base of each cardboard shield enclosure.

I had to shim each cardboard shield enclosure with washers, in order to ensure that it makes contact with the aluminum foil on the wooden base. 

Each cardboard shield enclosure was checked for continuity with the aluminum foil, on the wooden base. 

Testing and Alignment

The book "The Impoverished Radio Experimenter Volume 5" includes a handy schematic that shows all the proper voltages for each stage. At this point, none of the tubes were installed.

I used a Heathkit Model HP-23A Power Supply, to power my homebrew SuperHet.

I created a custom wiring harness to power my homebrew SuperHet. The HP-23A Power Supply has 6.3 VAC, to power the filaments. I used the -130 Volt DC output for the 90 Volt input voltage, just in reverse polarity. With load, the -130 Volt DC Output was close to -100 Volts DC. A jumper was also required between two points on the HP-23A's output socket, for it to power on. 

I also connected a homebrew audio amplifier, with high input impedance, to the "Phones" connection on the BFO/IF Gain control panel.  

After one last check of the wiring, I installed the 6C8 tube in the Detector Stage.

In order to test the Detector Stage, I removed the grid cap connection from the 6C8 tube, applied power and turned on the external amplifier. Next, I shut off the BFO switch. You know this stage is working if you touch the metal top of this tube and hear AC hum from the speaker. Don't worry,  there is no voltage on the metal top of this tube. If no sound, perform voltage measurements at the tube socket, and compare with the Approximate Voltages schematic. Troubleshoot, if voltage measurements differ from the Approximate Voltages schematic.

I then Installed the 6SK7 in its socket, applied power, and perform voltage measurements at the socket of this tube. I then compared these voltage measurements with the Approximate Voltages schematic. At this point I would troubleshoot, if voltage measurements differ from Approximate Voltages schematic.

Alignment

I used my old Eico Model 324 Signal Generator for the alignment process.  

​For my homebrew SuperHet, alignment is the process of: 
- Tuning all IF(Intermediate Frequency) transformers to the same frequency, in this case 400kHz.
- Adjusting the BFO(Beat Frequency Oscillator) Trimmer capacitor, so that it "Zero Beats" with the BFO capacitor's plates half meshed.
​- Adjusting the LO(Local Oscillator) for the correct frequency range, in this case 3.5MHz to 4.0MHz.
- Adjusting the Antenna Trimmer capacitor, connected to the Antenna transformer, for maximum volume, with the Antenna Peek capacitor's plates half meshed.

The dial on my Eico Model 324 Signal Generator is not too accurate, I used my YAESU FRG-7700 to validate the frequency output of the Model 324.​

IF(Intermediate Frequency) Transformer Alignment

I set my Eico Model 324 Signal Generator to a 400kHz internally modulated AM signal, then set the signal generator output lead near pin 4 of the 6SK7 tube. Once my homebrew Superhet and high impedance amplifier was powered on, I heard the faint sound of the signal generator. At this point, using an alignment tool, I adjusted the trimmer capacitor on the output side of the second IF transformer for maximum volume. I circled the trimmer capacitor I adjusted in the above picture. 

Next, the trimmer capacitor on the input side of the second IF Transformer was adjusted for maximum volume. The yellow lead on the left side of the tube is connected to the signal generator.

Finally, the trimmer capacitor connected to the grid of the 6SK7 tube was adjusted for maximum volume. I then proceeded to "tweak" all three trimmer capacitors again for maximum volume. 

BFO (Beat Frequency Oscillator) Alignment

This step is performed with power applied to my homebrew SuperHet and an high input impedance audio amplifier powered on and connected to the "Phones" connections on the BFO/IF Gain control panel.  In addition, I set my Eico Model 324 Signal Generator to a 400kHz carrier signal only with no Amplitude Modulation, then set the signal generator output lead near pin 4 of the 6SK7 tube. I move the BFO Switch to the on position and set the BFO Pitch capacitor so that its plates were half meshed.  I then adjusted the BFO trimmer capacitor for "Zero Beat". When adjusting the BFO trimmer, you should hear the audio pitch from the SuperHet get lower and lower, disappear then get higher again. The point at which the audio disappears is Zero Beat.

LO (Local Oscillator) Adjustment

The LO(Local Oscillator) trimmer capacitor must be adjusted, to allow the "Tune" variable capacitor to cover the correct frequency range, in this case 3.5MHz to 4.0MHz. 

Cut the power to the SuperHet and install the 6SA7 tube.  

This step is performed with power applied to the homebrew SuperHet and a high input impedance audio amplifier powered on and connected to the "Phones" connections on the BFO/IF Gain control panel. 

Once the SuperHet is warmed up, perform voltage measurements at the socket of the 6SA7 tube. Compare these voltage measurements to the Approximate Voltages schematic. Troubleshoot if voltage measurements differ from Approximate Voltages schematic.

During this adjustment, the Tune variable capacitor's plates should be half meshed. 

​I set my Eico Model 324 Signal Generator to a 3.75MHz internally modulated AM signal, then clipped the yellow signal generator output lead to the top of the Antenna Coil. Sorry about the blurry picture.

I mounted the LO (Local Oscillator) trimmer capacitor to the Local Oscillator coil's tube socket, circled in the above picture. This trimmer capacitor is  adjusted, until you hear a loud tone from the frequency generator. This sets the Tune capacitor to pickup signals at 3.75MHz. Now with the Tune variable capacitor's plates fully meshed my SuperHet should pickup signals at 3.5MHz. With the Tune variable capacitor's plates unmeshed, my SuperHet should pickup signals at 4.0MHz.

At this point, you can adjust the last trimmer capacitor, connected to the first IF transformer's input coil, for maximum volume. The trimmer is circled in the above picture. 

Antenna Trimmer Adjustment

This step is performed with power applied to the homebrew SuperHet and an high input impedance audio amplifier powered on and connected to the "Phones" connections on the BFO/IF Gain control panel. 

During this adjustment, the Antenna Peak and Tune variable capacitor's plates should be half meshed. 

I set my Eico Model 324 Signal Generator to a 3.75MHz internally modulated AM signal, then clipped the yellow signal generator output lead to the top of the Antenna Coil.

The Antenna trimmer capacitor is also adjusted for maximum volume. This will allow the Antenna Peak variable capacitor to be adjusted for resonance between 3.5MHz and 4.0MHz.

Secure Wiring

The final assembly step of my homebrew SuperHet, was to secure the longer runs of wiring to the wooden base. I used tape to hold the wires in position while I applied a dab to E6000 clear adhesive to hold them in place. For shorter wiring runs, I just bent the wires into orthogonal positions, so that the wiring was pleasing to the eye. 

Conclusion

My homebrew SuperHet was two years in the making. It took time to build the Helical Coil Winder, wind the required IF, BFO, Antenna, and Local Oscillator coils, and build the required cardboard shield enclosures while trying to work and maintain a family life. It was well worth it and I proud of my radio creation. This whole journey started with the book "The Impoverished Radio Experimenter Volume 5" which inspired me to build a SuperHet of my own. 

As an addendum to this project, I will research how my homebrew SuperHet can be converted to the Standard AM Broadcast band (525kHz to1705kHz) as I typically cannot find much going on in the 80 Meter band (3.5Mhz to 4.0Mhz), this receivers intended frequency operation, day or night. 

My Homebrew SuperHet in Action!