Eighty-five years ago, this gentleman is teaching himself Morse code thanks to a one-tube code oscillator shown in the August 1939 issue of Popular Mechanics.

It used a single 12A7 tube, and for the filament, it used a “curtain burner” line cord to drop the voltage to 12 volts.  The article suggested learning the code by sending from the attached chart, and then having someone else, preferably someone already skilled at the code, to send faster and faster.