A plate detector is a vacuum tube detector circuit used in A.M. radios. This circuit employs a tube with an indirectly heated cathode, typically a medium-mu triode, or a tetrode or pentode with a sharp cut-off control grid. Rectification of R.F. signals occurs in the plate of the detector tube. This differs from a grid-leak detector, which achieves rectification in the control grid. It also differs from the diode detector circuit commonly used to provide both R.F. rectification and automatic volume control (A.V.C.) bias to the R.F. amplifier tubes.
Overview
Plate detectors are used in both T.R.F. and superheterodyne receivers. The grid is connected directly to the secondary of the final R.F. or I.F. transformer. The cathode is connected to ground through a circuit consisting of a parallel-connected bias resistor (usually 10 KΩ to 50 KΩ) and bypass capacitor (usually 0.25 µF to 0.5 µF). When sufficient negative bias is applied to the grid, the plate current is pushed almost to the cut-off point. When a modulated R.F. signal is applied to the grid under these conditions, a corresponding increase in plate current occurs. As the signal amplitude varies, the plate current also varies, causing the plate to act as a diode detector while the tube as a whole also acts as an audio amplifier. A plate bypass capacitor (usually 500 µµF to 0.002 µF for triodes, or 250 µµF to 0.001 µF for tetrodes and pentodes) is used to regulate plate current.
Like most A.F. amplifiers in radios, the plate voltage is usually less than 60 volts. When a tetrode or pentode is used, the screen grid voltage is usually about one-half the plate voltage.
Plate detector circuits were commonly used from the introduction of indirectly heated cathode tubes in the late 1920s until the start of World War II. As R.F. tubes became more sensitive, grid-leak detectors (which are more sensitive than plate detectors) became less practical. Diode detectors were popular because, unlike plate detector circuits, they could also provide A.V.C. bias. However, the dual-diode/triode and dual-diode/pentode tubes commonly used in these circuits had bulk wholesale costs that were as much as twice the cost of the tubes commonly used as plate detectors. This made plate detector circuits more practical for low-priced radios sold during the depths of the Great Depression.
Because an indirectly heated cathode is required for this circuit to operate, it is not used in battery-operated radios.
Controlling volume levels
Plate detector circuits usually lack an A.V.C. bias circuit. In receivers equipped with A.V.C., volume levels are adjusted by a potentiometer (typically 500 KΩ to 2 megohms audio taper) that controls audio signal levels at the control grid of the A.F. amplifier. In receivers not equipped with A.V.C., the most common connection of the volume control potentiometer (typically 4 KΩ to 15 KΩ linear taper) is as follows:
The low side of the potentiometer is connected to the antenna connection at the antenna input coil;
The center wiper is connected to ground (in A.C. receivers) or B- (in A.C./D.C. receivers);
The high side is connected to the cathode of at least one R.F. amplifier (in T.R.F. receivers) or to the converter and/or the I.F. amplifier (in superheterodyne receivers).
To assure that proper cathode bias is maintained, many non-A.V.C. volume controls are usually equipped with a "stop" that maintains a small amount of resistance between the center wiper and the high end connection.
Other volume control circuits in non-A.V.C. receivers include:
A potentiometer (typically 500 KΩ audio taper) where the high end and center wiper are connected as above, but where the low end is connected to the control grid of audio output tube. (In this circuit, the potentiometer replaces the bias resistor for the output tube's control grid);
A linear taper potentiometer that adjusts the screen grid voltages of the set's R.F. amplifiers (if they are tetrodes or pentodes);
A linear taper potentiometer connected to the antenna (high end), ground (low end) and the antenna input coil (center wiper).
Because the volume control in non-A.V.C. receivers adjusts R.F. signal levels rather than A.F. signal levels, the volume control must be manipulated while tuning the radio in order to find weak signals.
Tubes commonly used as plate detectors
6C6
6J7
6SJ7
12F5
12J5
12J7
12SF5
12SJ7
24 and 24-A
27
36
37
56
57
76
77
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