Plate detector (radio)


Plate detector (radio)

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.

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 coresponding 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 ciruits, 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 contol 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

* [http://www.nostalgiaair.org/PagesByModel/864/M0012864.pdf Schematics of Packard Bell models 35A and 65.] Two typical superheterodyne radios with a triode plate detector. Sold by Packard Bell in the early 1930s.
* [http://www.nostalgiaair.org/PagesByModel/372/M0017372.pdf Schematic of "Silvertone" models 6114 and 6115.] A typical T.R.F. radio with a pentode plate detector. Sold by Sears Roebuck in 1939.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Detector (radio) — Passband modulation v · d · e Analog modulation AM · …   Wikipedia

  • radio — /ray dee oh /, n., pl. radios, adj., v., radioed, radioing. n. 1. wireless telegraphy or telephony: speeches broadcast by radio. 2. an apparatus for receiving or transmitting radio broadcasts. 3. a message transmitted by radio. adj. 4. pertaining …   Universalium

  • Grid-leak detector — 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… …   Wikipedia

  • Invention of radio — Great Radio Controversy redirects here. For the album by the band Tesla, see The Great Radio Controversy. Contents 1 Physics of wireless signalling 2 Theory of electromagnetism …   Wikipedia

  • Vehicle registration plate — A vehicle registration plate is a metal or plastic plate attached to a motor vehicle or trailer for official identification purposes. The registration identifier is a numeric or alphanumeric code that uniquely identifies the vehicle within the… …   Wikipedia

  • Magnetic detector — 1902 1918: Marconi s Wireless Magnetic Detector (London) The magnetic detector was one of the first practical devices able to make radio signals audible through a pair of headphones. It was not as sensitive as some detectors but despite that, it… …   Wikipedia

  • Mass spectrometry — (MS) is an analytical technique that measures the mass to charge ratio of charged particles.[1] It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical… …   Wikipedia

  • Measuring instrument — Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea …   Wikipedia

  • Inductively coupled plasma mass spectrometry — ICP MS Instrument Acronym ICP MS Classification Mass spectrometry Analytes atomic and polyatomic species in plasma, with exceptions; usually inte …   Wikipedia

  • Timeline of Russian inventions and technology records — The Hall of Space Technology in the Tsiolkovsky State Museum of the History of Cosmonautics, Kaluga, Russia. The exhibition includes the models and replicas of the following Russian inventions: the first satellite, Sputnik 1 (a ball under the… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.