Eighty years ago this month, the July 1936 issue of Popular Science contained the plans for this code practice oscillator.  The main feature was that it required neither headphones nor a battery, since it had sufficient volume to drive a speaker, and ran off household power.

As shown in the schematic, it used a single dual tube, a 12A7, with one half serving as rectifier and the other half as oscillator.  The 12 volts for the filament was supplied with a curtain burner line cord.

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