Eighty years ago this month, the April 1941 issue of Popular Science showed this novel little one-tube receiver. Even though it’s basically a crystal set, it qualifies as a one-tube radio since it uses a rectifier tube, a 25Z5, in place of the crystal.

The article points out two things that might not be obvious. First of all, even though the set is running on AC power, there is no hint of 60 cycle hum. The electric power does nothing but heat the filament, and the heated cathode and plate serve as the detector, with no electrical connection to the power. The magazine also notes that it’s safe to use a ground connection, since there’s no direct connection.

Even though the filament normally runs on 25 volts, it should be under 6 volts in this circuit. An 80 watt resistor is used to drop the voltage, although I bet an incandescent light bulb could be used.