Tetrode

The four electrodes in order from the centre are: a thermionic cathode, first and second grids, and a plate (called anode in British English).

Tetrodes were widely used in many consumer electronic devices such as radios, televisions, and audio systems until transistors replaced valves in the 1960s and 70s.

Beam tetrodes have remained in use until quite recently in power applications such as audio amplifiers and radio transmitters.

A current through the heater or filament heats the cathode, which causes it to emit electrons by thermionic emission.

In the course of his research into the action of the audion triode tube invented by Edwin Howard Armstrong and Lee de Forest, Irving Langmuir found that the action of the heated thermionic cathode was to create a space charge, or cloud of electrons, around the cathode.

This had two advantageous effects, both related to the influence of the electric fields of the other electrodes (anode and control grid) on the electrons of the space charge.

[2] Second, the transconductance (rate of change of anode current with respect to control grid voltage) of the tube was increased.

It is intended for service as a power amplifier driver where the potentials are obtained directly from a 12V automobile battery."

[6] Another important application of the space-charge tetrode was as an electrometer tube for detecting and measuring extremely small currents.

This type of tetrode was used in many imaginative ways in the period before the appearance of the screen-grid valve revolutionised receiver design.

The anode current in the valve, and hence the RF output amplitude, is modulated by the voltage on G1, which is derived from a carbon microphone.

The principle of the modern superheterodyne (or superhet) receiver (originally named the super-sonic heterodyne receiver, because the intermediate frequency was at an ultrasonic frequency) was invented in France by Lucien Levy in 1917[15] (p 66), though credit is usually also given to Edwin Armstrong.

[16] A somewhat complicated technique, it went out of favor when screen-grid tetrodes made tuned radio frequency (TRF) receivers practical.

[citation needed] However the superheterodyne principle resurfaced in the early 1930s when their other advantages, such as greater selectivity became appreciated, and almost all modern receivers operate on this principle but with a higher IF frequency (sometimes higher than the original RF) with amplifiers (such as the tetrode) having surpassed the triode's limitation in amplifying high (radio) frequency signals.

But for economy, those two functions could also be combined in a single bi-grid tetrode which would both oscillate and frequency-mix the RF signal from the antenna.

In today's receivers, based on inexpensive semiconductor technology (transistors), there is no cost benefit in combining the two functions in one active device.

Round at MOV and Bernard Tellegen at Phillips developed improved screen grid tubes.

[26] Neutralizing circuits were not required for a well designed screen grid tube RF amplifier stage.

[30] To take full advantage of the very low grid-anode capacitance, the shielding between anode and grid circuits was observed in the construction of the radio.

The S625 valve was mounted in a grounded, plane, metal shield aligned to correspond with the position of the internal screen grid.

They were commonly used in the design of radio-frequency amplification stage(s) of radio receivers from late 1927 through 1931, then were superseded by the pentode tube.

Where the anode voltage is less than that of the screen grid, there is a distinctive negative resistance characteristic, called the dynatron region[32] or tetrode kink.

The approximately constant-current region of low slope at anode voltages greater than the screen grid voltage is also markedly different from that of the triode, and provides the useful region of operation of the screen grid tube as an amplifier.

[33] The low slope is highly desirable, since it greatly enhances the voltage gain which the device can produce.

Early screen-grid valves had amplification factors (i.e. the product of transconductance and anode slope resistance, Ra) fifty times or more greater than that of comparable triode.

The beam tetrode was patented in Britain in 1933 by three EMI engineers, Isaac Shoenberg, Cabot Bull and Sidney Rodda.

[37] The High Vacuum Valve company of London, England (Hivac) introduced a line of power output tetrodes in August 1935 that utilized J. H. Owen Harries' critical distance effect to eliminate the dynatron region of the anode voltage - anode current characteristic.

[38] A range of tetrodes of this type were introduced, aimed at the domestic receiver market, some having filaments rated for two volts direct current, intended for low-power battery-operated sets; others having indirectly heated cathodes with heaters rated for four volts or higher for mains operation.