Wikipedia: "The first practical prominent device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplif

Wikipedia: "The first practical prominent device that could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912." It takes quite a bit of time to develop an electronic instrument, and the realization it could be done must have come some time after amplifiers became more widely used. So 1920 isn't unrealistic, IMO.