Plate detector (radio)
Encyclopedia
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.
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 modulate
d 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 pF to 0.002 µF for triodes, or 250 pF 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.
(typically 500 kΩ to 2 MΩ 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:
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.
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.
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
circuit used in A.M.
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
radios. This circuit employs a tube with an indirectly heated cathode
Hot cathode
In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
, typically a medium-mu triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, or a tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
or pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
with a sharp cut-off
Cut-off (electronics)
In electronics, the term Cut-off identifies a state of negligible electrical conduction which is proper of several types of electronic components when a control parameter , is lowered or increased past a value called conduction threshold or simply...
control grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
. Rectification of R.F.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signals occurs in the plate of the detector tube. This differs from a grid-leak detector
Grid-leak detector
A grid-leak detector is a combination diode rectifier and audio amplifier used as a detector in vacuum tube AM radio receivers.The grid leak is the small current through the grid of a vacuum tube caused by the small negative EMF which is present in all vacuum tubes as a result of a space charge...
, 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.)
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
bias to the R.F. amplifier
Valve RF amplifier
A valve RF amplifier or tube amplifier , is a device for electrically amplifying the power of an electrical signal, typically radio frequency signals....
tubes.
Overview
Plate detectors are used in both T.R.F.Tuned radio frequency receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
and superheterodyne receivers. The grid is connected directly to the secondary of the final R.F. or I.F.
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
transformer. The cathode is connected to ground through a circuit consisting of a parallel-connected bias resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
(usually 10 kΩ to 50 kΩ) and bypass capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(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 modulate
Modulate
Modulate is Bob Mould's fifth solo album, released in 2002. Although a few tracks on his previous release, The Last Dog and Pony Show, had featured tape loops and samples, Mould shocked his fans with such a dramatic embrace of electronica...
d 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 pF to 0.002 µF for triodes, or 250 pF 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
Double diode triode
A double diode triode is a type of electronic vacuum tube once widely used in radio receivers. The tube has a triode for amplification, along with two diodes used as detectors, or Automatic gain control...
and dual-diode/pentode tubes commonly used in these circuits had bulk wholesale
Wholesale
Wholesaling, jobbing, or distributing is defined as the sale of goods or merchandise to retailers, to industrial, commercial, institutional, or other professional business users, or to other wholesalers and related subordinated services...
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
Great Depression
The Great Depression was a severe worldwide economic depression in the decade preceding World War II. The timing of the Great Depression varied across nations, but in most countries it started in about 1929 and lasted until the late 1930s or early 1940s...
.
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 potentiometerPotentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
(typically 500 kΩ to 2 MΩ 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
External links
- Schematics of Packard Bell models 35A and 65. Two typical superheterodyne radios with a triode plate detector. Sold by Packard BellPackard BellPackard Bell is a Dutch computer manufacturer and a subsidiary of Acer. The name was previously used by Packard Bell, an American radio manufacturer founded in 1926. In 1986, Israeli investors bought the name for a newly formed personal computer manufacturer. Originally the company produced...
in the early 1930s. - Schematic of "Silvertone" models 6114 and 6115. A typical T.R.F. radio with a pentode plate detector. Sold by Sears Roebuck in 1939.