A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s. History Early applications of triode tubes (Audions) as detectors usually did not include a resistor in the grid circuit. First use of a resistance in the grid circuit of a vacuum tube detector circuit may have been by Sewall Cabot in 1906. Cabot wrote that he made a pencil mark to discharge the grid condenser, after finding that touching the grid terminal of the tube would cause the detector to resume operation after having stopped. Edwin H. Armstrong, in 1915, describes the use of "a resistance of several hundred thousand ohms placed across the grid condenser" for the purpose of discharging the grid condenser. The heyday for grid leak detectors was the 1920s, when battery operated, multiple dial tuned radio frequency receivers using low amplification factor triodes with directly heated cathodes were the contemporary technology. The Zenith Models 11, 12, and 14 are examples of these kinds of radios. When screen-grid tubes became available for new designs in 1927, most manufacturers switched to plate detectors, and later to diode detectors. The grid leak detector has been popular for many years with amateur radio operators and shortwave listeners who construct their own receivers. Functional overview The stage performs two functions: Detection: The control grid and cathode operate as a diode. At small radio frequency signal (carrier) amplitudes, square-law detection takes place due to non-linear curvature of the grid current versus grid voltage characteristic. Detection transitions at larger carrier amplitudes to linear detection behavior due to unilateral conduction from the cathode to grid. Amplification: The varying direct current (dc) voltage of the grid acts to control the plate current. The voltage of the recovered modulating signal is increased in the plate circuit, resulting in the grid leak detector producing greater audio frequency output than a diode detector, at small input signal levels. The plate current includes the radio frequency component of the received signal, which is made use of in regenerative receiver designs. Operation The control grid and cathode are operated as a diode while at the same