Grid-leak detector

Lore & Background

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. After 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.

The stage performs two functions: detection and amplification. The control grid and cathode operate as a diode. At small radio frequency signal 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. The varying direct current voltage of the grid acts to control the plate current, increasing the audio frequency output compared to a diode detector at small input signal levels.

Reader's Guide

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 grid leak resistance and the grid-to-cathode impedance.

Tetrode and pentode tubes provide significantly higher grid input impedance than triodes, resulting in less loading of the circuit providing the signal to the detector. 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. The circuit's ability to both detect and amplify in a single stage made it a practical choice for early battery-operated receivers, and it remained in use among hobbyists long after commercial manufacturers moved to other detector types.

Did You Know?

• A grid leak detector uses a capacitor (grid condenser) and a resistor (grid leak) to develop a varying DC grid voltage from an AM signal.
• The grid leak detector was invented by Lee De Forest around 1912.
• Edwin H. Armstrong described using a resistance across the grid condenser in 1915.
• Tetrode and pentode tubes produce significantly higher audio output at small signal levels than triodes in grid leak detector circuits.