The first ever AM transmission occurred in 1906. The first commercial AM station was KDKA in Pittsburgh which began operation in 1920.
De Forest's first commercial Audion receiver, the RJ6, came out in 1914. The Audion tube was always mounted upside down, with its
delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.
Below is an example of single tube triode grid-leak receiver from 1920, the first type of amplifying radio receiver. In the grid
leak circuit, electrons attracted to the grid during the positive half cycles of the radio signal charge the
grid capacitor with a negative voltage of a few volts, biasing the grid near
its cutoff voltage, so the tube conducts only during the positive half-cycles, rectifying the radio carrier.
Fifteen years later, Hallicrafters comes out the the SX-9.
Five years after that, we have the SX-28. That's pretty impressive.
The SX-9 schematic. There's a lot of stuff that carried over into the SX-28.
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Last modified: Monday, 1 Jun 2026