A grid-leak detector
is a combination diode rectifier
and audio amplifier
used as a detector
in vacuum tube A.M. radio
receivers. In the circuit, the grid of the detector -- usually a low-mu or medium-mu triode
-- is connected to the secondary of the final R.F. or I.F. transformer through a capacitor (100 µµF to 330 µµF -- 250 µµF being typical). This capacitor eliminates R.F. in the output circuit. (An R.F. choke in the plate circuit may also be used to eliminate any transient R.F. in the output circuit.) The grid is negatively biased through a grid leak resistor
(1 to 5 megohms -- 2.2 megohms being typical). This resistor may be parallel connected with the grid capacitor, or it may be connected directly to ground. As D.C.
current flows through the grid leak resistor, the control grid functions like the plate of a diode, which causes rectifying action to occur. Frequency variations in voltage across the bias resistor are amplified through the tube as in a normal audio amplifier.
A grid-leak detector has considerably greater sensitivity than a diode. The sensitivity is further increased by using a tetrode or pentode with a sharp cut-off control grid instead of a triode. The operation is equivalent to that of the triode circuit except controlled feedback is applied and controlled by adjustment of the screen-grid voltage.
All grid-leak detectors require a plate by-pass capacitor connected to ground to regulate plate current. For triodes, this capacitor should have a value of 0.001 µF to 0.002 µF. For tetrodes and pentodes, this capacitor should have a value of 250 µµF to 500 µµF. Tetrodes and pentodes grid-leak detectors also require a screen grid by-pass capacitor of at least 0.47 µF.
The heyday for grid-leak detectors was the 1920s when battery-operated, multi-dial T.R.F. radios using low-mu triodes with directly heated cathodes was the norm. When indirectly heated cathodes and A.C. powered receivers were introduced in 1927, most manufacturers switched to plate detectors, and later to diode detectors.
Although the regenerative grid-leak detector was one of the more sensitive detectors of its day, its many disadvantages limited it for use only in the simplest receivers. However, this did not stop some manufacturers from using regenerative detectors in their radios. Many of the cathedral radios and other table models made by Philco during the early 1930s used a regenerative detector fed by a superheterodyne converter tube. This allowed the detector to double as a sort of I.F. amplifier, thus saving money by eliminating the need for another tube.
Controlling volume levels
Most receivers with grid-leak detectors lack automatic volume control (A.V.C.)
biasing circuitry. As a result, R.F. signal levels must be adjusted to vary audio output levels. Manufacturers used various circuits to control volume levels:
- Battery-operated radios of the 1920s varied the filament voltages of the R.F. tubes to adjust volume;
- In early A.C. sets, and in many of the early-1930s Philco radios mentioned above, the volume control adjusted signal levels at the antenna input connection;
- In some early sets equipped with tetrode or pentode R.F. amplifiers, the volume control adjusts the screen grid voltage:
- In later radios, the volume control adjusts the cathode bias voltage of at least one R.F. amplifier.
Because R.F. signal levels are adjusted to change volume levels, constant manipulation of the volume control is required to find weak signals.
Use in F.M. receivers
Because the Foster-Seeley discriminator
-- a detector circuit used in many early F.M.
radios -- also responds to A.M. radio frequency signals, all A.M. R.F. signals must be squelched before reaching the discriminator. This is achieved with the use of a limiter
which, in tube circuits, usually takes the form of a grid-leak detector. A sharp-cutoff pentode with a high transconductance
rating -- such as types 6AC7
-- is used in this circuit.
(F.M. ratio detectors, which are not effected by A.M. R.F. signals, do not use limiter circuits.)