Grid-leak detector
The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube.
[1][2] Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.
Early applications of triode tubes (Audions) as detectors usually did not include a resistor in the grid circuit.
[7] 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 grid leak detector has been popular for many years with amateur radio operators and shortwave listeners who construct their own receivers.
The stage performs two functions: The control grid and cathode are operated as a diode while at the same time the control grid voltage exerts its usual influence on the electron stream from cathode to plate.
The resistor permits dc charge to "leak" from the capacitor[18] and is utilized in setting up the grid bias.
[19] At small carrier signal levels, typically not more than 0.1 volt,[20] the grid to cathode space exhibits non-linear resistance.
[23][24][25] In this region of operation, the demodulated signal is developed in series with the dynamic grid resistance
The coupling capacitor will acquire a dc charge due to the rectifying action of the cathode to grid path.
[31][32] The capacitor discharges through the resistor (thus grid leak) during the time that the carrier voltage is decreasing.
[35] The plate current is passed through a load impedance chosen to produce the desired amplification in conjunction with the tube characteristics.
[49] A tube requiring comparatively large grid voltage for plate current cutoff is of advantage (usually a low amplification factor triode).
[52] Tetrode and pentode tubes also produce significantly higher audio frequency output amplitude at small carrier input signal levels (around one volt or less) in grid leak detector applications than triodes.
[36] The radio frequency input impedance of the grid leak detector is dominated by the tube's grid input impedance, which can be on the order of 6000 ohms or less for triodes, depending on tube characteristics and signal frequency.
