One of the first prototype superheterodyne radio receivers built by inventor Edwin Armstrong. The superheterodyne, the circuit used in virtually all modern radios, was invented by Armstrong in 1918 while he worked in a US Army Signal Corps laboratory in Paris during World War 1. This is one of the receivers constructed at that laboratory, shown in a 1920 article in an amateur radio magazine. It is constructed in two parts. The lefthand section consists of the mixer and local oscillator. The oscillator uses an Armstrong "tickler" circuit. The two large knobs are the input tuning capacitor (left, labeled TUNING) and the local oscillator tuning capacitor (right, labeled HETERODYNE). The upper center multiposition switch controls the filament current, while the lower center knob controls the feedback coupling of the oscillator. The righthand section contains an IF transformer filter, 3 stages of RC-coupled IF amplification and a detector stage. The multiposition switch on this section controls the filament current of the 4 tubes, while the righthand knob (labeled AMPLIFICATION) controls the gain of the amplifier. All the tubes are VT triodes made by Western Electric. The circuit uses an IF of around 75 kHz.
Caption: "One of the first complete Armstrong amplifiers built in Armstrong's Paris laboratory. The cabinet at the left contains the tuning and heterodyne circuit and at the right the amplifying cabinet is shown"