The reflex was an interesting radio receiver circuit that had a certain vogue amongst constructors of the 1920s and 1930s, until the advent of more efficient vacuum tubes. It enabled the builder to make at least one amplifier tube do double-duty, thus saving on tube count. That's the theory, anyway – in practice, the circuit could be a bit tricky.
Simply put, in a reflex circuit, a tube would be made to amplify not only radio frequencies, but audio frequencies as well. This was accomplished by feeding the audio output of the detector tube back through one of the radio-frequency amplifier tubes. In a superheterodyne arrangement, the audio might be amplified instead by one of the intermediate-frequency tubes. Since the two frequencies were vastly separated in value (one being in the supersonic range, the other in the audible range), and the tube was operated as an amplifier and not a mixer, there was little chance of interference between the two.
At the output of the reflexed tube, radio frequencies went on to the detector, and audio to the power output amplifier tube and loudspeaker. The circuit worked reasonably well, as long as everything stayed in adjustment, which usually required a skilled operator. Given, though, the peculiarities of early tubes and components, the circuit was subject to instability and oscillating, resulting in the circuit doing a poor job of its tasks. In later years, some manufacturers managed to produce moderately stable commercial reflex receivers; but as tubes (and circuits) got better, and cheaper, the circuit fell into disuse.
274