How to Build a One Tube Crystal Radio

Build it and they will come.

Building a crystal radio for a Boy Scout merit badge or science fair project is not only a fun activity but a learning experience. Constructing a black box out of paper, wire and a piece of fool's gold that captures electromagnetic waves and converts them into intelligible sound is an amazing accomplishment. Add a triode vacuum tube to run a speaker and the radio can be heard next door or far away.

Things You'll Need

• Crystal radio set
• Interstage transformer
• Vacuum tube (OA1, 30 or 864)
• Vacuum tube socket
• Rheostat (1,000 ohm--adjust size to fit required tube filament voltage)
• Miscellaneous hardware, wiring
• Headphones and/or speaker (horn type)
• 12-V DC battery
• 45-V DC battery
• Alligator clips (4)
• Solder
• Soldering gun or iron
• Wire cutters
• Needle nose pliers

Instructions

1. Purchase a simple crystal radio, prebuilt or a DIY kit. Assemble the kit.You can eliminate fussing with the "cat whisker" mount for the galena crystal (lead sulfide) and replace it with a diode. The object here is to have a working crystal radio. Attach the antenna system and a ground. Tune in to a strong local station. You can also use a schematic diagram available online. Some of the parts are homemade, such as the tank circuit that is the heart of the radio. Note that one radio uses a variable capacitor (365 pF) to tune the incoming radio signal, and the other uses a stationary capacitor (,001 uF) and variable coil. A tank circuit refers to a coil/capacitor combination that will resonate at one frequency selectively until the coil or capacitor is changed physically by tuning it. Then it will resonate at another signal's frequency. In this manner, one can tune the whole band for many frequencies. Attach the diode and earphones with an antenna and ground on the other end, and the crystal will create audible sound when tuned properly to a station.

2. Follow the diagram circuit and add in the interstage transformer. Use wire cutters to strip the primary leads and wrap each lead across the tank circuit. Use needle-nose pliers to wrap the wires in place. Solder on the connectors, adding a small amount of solder until it flows. Proceed to the other side of the interstage transformer, the secondary, and wrap the leads to the correct tube socket pins using pliers. Add batteries, rheostat and speaker. Wrap and solder in place. Cut off any excess wire. Add the vacuum tube last. Rather than invest in a switch, clip the batteries into the circuit with alligator clips. Adjust the filament voltage when the set is turned on. Use a volt-ohm meter to measure the filament voltage. Simply place the leads on the vacuum tube filament pins and read the voltage on the proper scale. Be sure your rheostat is at max resistance then slowly tweak it toward the center position. The voltage reading should match that recommended for the tube filament.

3. Test the unit by clipping in the batteries and adjusting the filament voltage. Wait for the vacuum tube to warm up before adjusting the radio's front tank circuit. The variable capacitor or coil will serve as a tuning dial. Test the unit with headphones first then add an output transformer to match the impedance to a horn-type speaker. Replace the headphones with an audio output transformer with the primary leads replacing the headphones (~2000 ohms) and the secondary leads connected to the horn speaker of matching impedance (~2, 4 or 8 ohms). Check your spare parts box for a transformer from an old vacuum tube radio that fits the appropriate impedance requirements.

Tips & Warnings

• Make sure the crystal radio is working prior to installing the tube.

• Disconnect both batteries from the radio when not in use.

• Keep the 45-V DC supply away from the tube filament connections.

• Hot solder can burn exposed skin. Be careful.

References

• Old Headphones: Circuits for Crystal Radios