Plate detector (radio)

Plate detector circuits were most commonly used from the 1920s until the start of World War II. In 1927, the advent of screen grid tubes permitted much more radio frequency amplification before the detector stage than previously practically possible.^[3][4] The previously used grid leak detector was less suited to the higher radio frequency signal level than the plate detector. Diode detectors also became popular during the later 1920s because, unlike plate detector circuits, they could also provide automatic gain control voltage (A.V.C.) for the radio frequency amplifier stages of the receiver. However, the dual-diode/triode and dual-diode/pentode tubes commonly used for detection/A.V.C. 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.

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Negative bias is applied to the grid to bring the plate current almost to cutoff.^[5] The grid is connected directly to the secondary of a radio frequency or intermediate frequency transformer. An incoming signal will cause the plate current to increase much more during the positive 180 degrees of the carrier frequency cycle than it decreases during the negative 180 degrees. The plate current variation will include the original modulation frequencies. The plate current is passed through a plate load impedance chosen to produce the desired amplification in conjunction with the tube characteristics.^[1] A capacitor of low impedance at the carrier frequency and high impedance at audio frequencies is provided between the tube plate and cathode, to minimize amplification of the carrier frequency and remove carrier frequency variations from the recovered modulation waveform.^[6] The allowable peak 100% modulated input signal voltage is limited to the magnitude of the bias voltage, corresponding to an unmodulated carrier peak voltage of half the bias voltage magnitude.^[1]

Either fixed bias or cathode bias may be used for the plate detector. When cathode bias is implemented, a capacitor of low impedance at the carrier frequency and high impedance at audio frequencies bypasses the cathode resistor.^[1] Cathode bias reduces the amplification obtainable.^[1]

Plate detector circuits usually do not produce A.V.C. voltage for the radio frequency (R.F.) stages of the receiver. In these receivers, volume control is often accomplished by providing variable cathode bias of one or more stages prior to the detector. A potentiometer is used to implement the variable cathode bias. The most common connection of the potentiometer (typically 4 kΩ to 15 kΩ linear taper) is as follows:

• One end of the potentiometer is connected to the antenna coupling component;
• The wiper is connected to ground (in A.C. receivers) or B minus (in A.C./D.C. receivers);
• The other end of the potentiometer is connected to the cathode of at least one R.F. amplifier (in T.R.F. receivers) or the cathode of the converter and/or the I.F. amplifier (in superheterodyne receivers).

To set a limit on the ability of the volume control to reduce the bias on the stages that it controls, the potentiometer is often equipped with a mechanical rotation limit facility that prevents the resistance from being reduced below a specific amount.

• 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 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 transformer primary or first tuned circuit (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.

• '01A, 1H4G, 6C6, 6J7, 6SJ7, 12F5, 12J5, 12J7, 12SF5, 12SJ7, 24, 24A, 27, 30, 36, 37, 56, 57, 76, 77, 201A, 301A