Grid Leak Detection

Grid-leak detector in electronics:

What it is: This refers to a specific electronic circuit used in early radio receivers (mostly before the 1930s) to demodulate amplitude-modulated (AM) radio signals. It used a vacuum tube and exploited the non-linear behavior of its grid to extract the information signal from the carrier wave.
Grid current: In this context, grid current refers to the flow of electrons between the cathode and the grid of the vacuum tube. In normal operation, the grid voltage is negative, repelling electrons. However, in a grid-leak detector, the incoming AM signal causes the grid voltage to swing positive during peaks, allowing grid current to flow briefly.
What happens:
- The grid current charges a capacitor connected to the grid, building up a DC voltage proportional to the average amplitude of the signal.
- This DC voltage biases the tube, affecting its output (plate current.)
- The demodulated and amplified signal appears across the output stage of the circuit.

Grid current in micro-amps plotted against positive voltage bias applied to the grid (0.3, 1.0, 1.5 and 2 volts.)

Audio output in millivolts versus RF input in millivolts for grid rectification (grid leak) and plate rectification.

Audio output 50, 100, 150, 200, 250, 300, 350, 400, 450, 500 mV.

RF input: 0, 20, 40, 60, 80, 100, 120, 140, 160 mV.

Possible modern version with shockley diode. JFET gate not biased forward enough to start conducting.

Possible modern version with BJT. If it does operate in grid leak detection mode the collector current should fall with increasing RF input.