How To Build A Crystal Or Regenerative Radio

​My Radio Project!

I have always been fascinated by antique radios. The ability of a crystal radio to pickup radio stations without any external power source and the warm glow of vacuum tubes in a tube radio. In the 21st century, it is harder to find good inexpensive examples of crystal and tube radios, even on e-bay, so I decided to build my own. There are plenty of schematics out on the Internet you can use to build either a crystal or vacuum tube radio.

​Crystal Radio Project

Here is an example of a crystal radio I built, originally the parts were from a Proof Slinky Crystal Radio kit I purchased from Amazon. I made it more nostalgic by adding a wooden base and Fastenal clips for connections. In addition I replaced the 1N914 Silicon diode that came with the kit for a more sensitive 1N35 Germanium diode. A crystal radio uses the RF (Radio Frequency) energy rectified by the diode detector to drive a high impedance ear phone.

Proof Slinky Crystal Radio Kit on Amazon

​The Battery Box Project

You need the proper A and B voltages before you can power a vacuum tube radio. The A battery is used to power the filament or heater in the tube. It is usually low voltage but requires a pretty hefty amount of current to keep the filament glowing. The B battery is used for the plate voltage, is it usually high voltage but the current draw is very low. For my projects I decided that I would need between 1.5 and 3 volts for the A voltages and 22.5 or 45 volts for my B voltages. Now, you could build an expensive AC power supply to provide these voltages but I thought it would be simpler and safer to run the tube radio off of batteries. Also, I decided to make the battery power supply modular so that I can use it with different vacuum tube projects I constructed so I created "The Battery Box". The wood box I used was an unfinished jewelry box available at a local craft store. I us two D cell batteries with a tap between them to provide the 1.5 and 3 volt A voltages. But how do I get the B voltages of 22.5 and 45 volts from a normal battery? Simple, I take five 9 volt batteries and wire them in series with a tap after the third 9 volt battery for 22.5 volts. I also include an SPST switch in line with the negative connection for both A and B connections so as be able to cut power when needed.

​Audio Amplifier

The crystal and tube regenerative radios can only drive a high impedance ear phone. In order to better hear I created an audio amplifier using a 386 audio amplifier integrated circuit.

​Audio Amplifier Kits on Amazon

​First Tube Regenerative Receiver Project

 chose to build a regenerative receiver as my first vacuum tube project. A regenerative receiver or "autodyne" was invented in 1914 by Edwin Armstrong. It consists of an amplifying vacuum tube with its output connected back to its input through a positive feedback loop thereby amplifying the incoming radio signal by a large factor. The high gain factor also increases the radio's selectivity or the ability to reject interfering signals. This type of receiver made a great first project as it requires very few components. The Regeneration knob controls the amount of positive feedback fed back into the amplifier and needs to be adjusted after you tune in a new broadcast. The receiver employs a common 1T4 Pentode tube that is still cheap and easily obtainable.

Here is a picture of the “guts” of my first regenerative receiver. As mentioned the radio consists of very few parts.

​First Transistor Regenerative Receiver Project

Bipolar transistors work equally well in a regenerative receiver. Because the regenerative circuit is driving a 386 integrated circuit audio amplifier I was able to connect this circuit to a low impedance speaker.

As you can see from the backside of my first transistor regenerative receiver, only a 9v battery is required to power this circuit. I used a printed circuit board purchased from Radio Shack to mount all of the components.

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The only external connections to the transistor regenerative receiver are ground and antenna.

​Tube Regenerative Receiver With Vario Coupler Project

Here is a picture of the front of my second tube regenerative receiver. This one has the ability to receive both standard AM broadcast band and a part of the shortwave spectrum depending on the position of the band select switch. The receiver employs a common 1T4 Pentode tube that is still cheap and easily obtainable.

From the back side you will see some differences from my first tube regenerative receiver, instead of a variable resistor to control regeneration I employ a vario coupler. The outside coil is part of the tuned LC circuit used to receive a signal at a specific frequency. The internal "tickler coil" is movable and connected to a dowel rod attached to a knob at the front of the radio. It generates the positive feedback needed to increase the gain of the tube circuit. By changing the ticker coil's orientation relative to the coil that is part of the LC circuit, the amount of positive feedback is changed.

Here is a back view of my Tube Regenerative Receiver With Vario Coupler.

Variable Capacitors on eBay

​Regenerative Receiver with Vintage Vacuum Tube

The third regenerative receiver I built uses a vintage Type 30 Triode tube and a vario couple to provide positive to increase amplification. I don't know when the triode tube I used was made, but I understand from online research that tubes with two digit designation extended back to the 1920s.

From the back you can clearly see the Type 30 tube in the porcelain base. This regenerative radio has the fewest discrete components of any regenerative radio I built.

Here is a top view of the vario couple I built for positive feedback to increase signal amplification.

Current Radio Project

As you can see my work is never done. Here is a picture of a new regenerative tube radio I am building that employs one 1T4 Pentode tube for RF application and another 1T4 Pentode tube for AF application.

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Where do I find Crystal and Regenerative radio schematics on the Internet?

By far, Dave's Homemade Radios site has the most extensive collection of Crystal and Regenerative radio schematics.

Dave's Homemade Radios

The Xtal Set Society, in my opinion, is the leading authority on the art of building crystal radios.

Xtal Set Society

​Where can I find Vacuum Tubes in the 21st century?

E-bay is a good source to find vintage vacuum tubes. Most sellers offer “buy it now” options if you don’t want to wait for the bidding war. There are many companies on the web that sell vacuum tubes as well. Remember, musicians and audiophiles still cherish the mellow sound given off from an amplifying vacuum tube so there are still plenty of sources for said device. In addition I have had good luck with the vendors below:

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Antique Electronic Supply

TubeDepot.com

​Conclusion

Building Crystal and Regenerative radios has really been a satisfying hobby for me. If you have an interest and have basic electronic, soldering, and construction skills I recommend trying it yourself.