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.
First use of a resistance to discharge the grid condenser in 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 discarging 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 indirectly heated cathodes and alternating current powered receivers were introduced 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.
The stage performs two functions:
- Detection: At small radio frequency signal (carrier) amplitudes, detection is due to non-linear curvature of the grid current versus grid voltage characteristic. Detection transitions at larger carrier amplitudes to diode detection due to unilateral conduction from the cathode to grid.
- Amplification: 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. Additionally, the plate current includes the radio frequency component of the received signal, which is made use of in regenerative receiver designs.
In the circuit, a capacitor (the grid condenser) couples a radio frequency signal (the carrier) to the control grid of an electron tube. The capacitor also facilitates development of direct current (dc) voltage on the grid. The impedance of the capacitor must be small at the carrier frequency and high at the modulating frequencies.
A resistor (the grid leak) is connected either in parallel with the capacitor or from the grid to the cathode. The resistor permits dc charge to "leak" from the capacitor.
In many grid leak detector implementations, the grid leak resistor provides a dc path from the grid to the positive side of the cathode or the positive side of the "A" battery; providing a positive fixed bias voltage at the grid determined by the dc grid current and the resistance of the grid leak. For maximum detector sensitivity, the value of the resistor is chosen to place the bias at the optimum point of curvature of the grid current versus grid voltage curve.
For minimum audio distortion, the time constant of the resistor and capacitor is chosen to be shorter than the period of the highest modulating frequency that is to be reproduced.
At small carrier signal levels, the grid to cathode space exhibits non-linear resistance. Grid current occurs during 360 degrees of the carrier frequency cycle. The grid current increases more during the positive excursions of the carrier voltage than it decreases during the negative excursions, due to the parabolic grid current versus grid voltage curve in this region. This asymmetrical grid current develops a dc grid voltage that includes the modulation frequencies.
At carrier signal levels large enough to make conduction from cathode to grid cease during the negative excursions of the carrier, the detection action is substantially that of a diode detector. Grid current occurs only on the positive peaks of the carrier frequency cycle. The coupling capacitor will acquire a dc charge due to the rectifying action of the cathode to grid path. The capacitor discharges through the resistor (thus grid leak) during the time that the carrier voltage is decreasing. The dc grid voltage will vary with the modulation envelope of an amplitude modulated signal.
The plate current is passed through a load impedance chosen to produce the desired amplification in conjunction with the tube characteristics. In non-regenerative receivers, a capacitor of low impedance at the carrier frequency is connected from the plate to cathode to prevent amplification of the carrier frequency.
- The grid leak detector potentially offers greater economy than use of separate diode and amplifier tubes.
- At small input signal levels, the circuit produces higher output amplitude than a simple diode detector.
One potential disadvantage of the grid leak detector, primarily in non-regenerative circuits, is that of the load it can present to the preceding circuit. 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. Other disadvantages are that it can produce more distortion and is much less suitable for input signal voltages over a volt or two  than some other detectors.
- H. A. Robinson, "The Operating Characteristics of Vacuum Tube Detectors", Part 1. QST, vol. XIV, no. 8, p. 23, Aug. 1930
- S. Cabot, "Detection - Grid or Plate", QST, vol. XI, no. 3, p. 30, Mar. 1927
- E. H. Armstrong, "Some Recent Developments in the Audion Receiver", Proceedings of the Institute of Radio Engineers, vol. 3, no. 3, pp. 215-247, Sept. 1915
- Schematics of Zenith models 11, 12 and 14. Three battery-operated Zenith grid leak models of the 1920s.
- K. R. Sturley, Radio Receiver Design (Part 1), New York: John Wiley and Sons, 1947, p. 377
- The Radio Amateur's Handbook (55 ed.). The American Radio Relay League. 1978. p. 241.
- J. H. Reyner, "Grid Rectification. A Critical Examination of the Method", Experimental Wireless, vol. 1, no. 9, pp. 512-520, Jun. 1924
- W. L. Everitt, Communication Engineering, 2nd ed. New York: McGraw-Hill, 1937, p. 418
- L.P. Smith, "Detector Action in High Vacuum Tubes", QST, vol. X, no. 12, pp. 14-17, Dec. 1926
- E.E. Zepler, The Technique of Radio Design, 2nd ed. New York: John Wiley and Sons, 1951, p. 333
- Landee, Davis, Albrecht, Electronic Designers' Handbook, New York: McGraw-Hill, 1957, p. 7-108
- Landee et al., p. 7-107
- W. L. Everitt, p. 421
- Signal Corps U.S. Army, The Principles Underlying Radio Communication. 2nd ed. Washington, DC: U.S.G.P.O., 1922, p. 476
- K. R. Sturley, pp. 379-380
- E. E. Zepler, p. 132
- H. A. Robinson, p. 25
- Rutland, David (September 1994), Behind the Front Panel: The Design & Development of 1920's Radios, Wren, ISBN 978-1885391001
- Schematic of Philco model 84 A superheterodyne cathedral radio from 1933 that uses a regenerative detector. (Note: The capacitor for the detector's control grid is the "tickler coil" winding on the IF transformer.)