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Pentode

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube (thermionic valve), which was invented by the Dutchman Bernhard D.H. Tellegen in 1926.[1] Pentodes (termed "triple-grid amplifiers" in some early literature[2]) are closely related to beam tetrodes, and an improvement over conventional tetrodes, which were themselves a development of triodes.


Types of pentodes

Variable transconductance ("vari-mu", "remote-cutoff" or "super-control") tubes in general are those with a non-uniform grid wire spacing to allow them to handle a wide range of input signal levels without excessive cross-modulation distortion, and so useful in Radio frequency stages where Automatic gain control is applied to the pentode. Examples include: 1T4, 6K7, 6BA6, and the EF83 (while perhaps the EF85/6BY7, and certainly the 6JH6, could be described as "semiremote-cutoff" pentodes).
Sharp-cutoff ("high slope" or ordinary) pentodes have the more ordinary uniform spacing of grid wires, and so mutual conductance decreases in an essentially uniform manner with increasing negative bias, and has a more abrupt cutoff. These pentodes are more suitable for audio amplifiers. Examples include: EF37A, EF86/6267, 1N5GT, 6AU6A, 6J7GT. Often in the European valve naming scheme for pentodes an even number indicated a sharp-cutoff device while odd indicated remote-cutoff; the EF37 was an exception to this general trend, perhaps due to its history as an update to the EF36 ("The Mullard EF36, EF37 and EF37A" at the National Valve Museum).
Power output pentodes, also referred to as "Beam Pentodes" [3], include the EL34/6CA7, 6K6GT and the EL84/6BQ5. Sometimes the word "pentode" was avoided, in names such as "beam power amplifier" or "beam power tube", at a time when the pentode patent was an important issue.

Note: "triode-pentodes" are not a different type of pentode, but an envelope containing both a triode and a pentode, such as an ECF80 or ECL86.

Advantages over the tetrode

A tetrode could supply sufficient power to a speaker or transmitter, and offered a larger amplification factor than the earlier triode. However, the positively charged screen grid can collect the secondary electrons emitted from the anode, which can cause increased current toward the screen grid, and cause the anode current to decrease with increasing anode voltage over part of the Ia/Va characteristic.

A pentode, as introduced by Tellegen, has an additional electrode, or third grid, called the suppressor grid that solves the problem of secondary emission. The suppressor grid does this by being held at a low potential, usually either grounded or connected to the cathode. Secondary emission still occurs, but the electrons that come from it can no longer reach the screen grid as they have less energy than the primary electrons, and, hence, cannot pass the grounded suppressor grid. The electrons from the secondary emission are re-collected by the anode.

Pentodes, therefore, can have higher current outputs and a wider output voltage swing; the anode/plate can even be at a lower voltage than the screen grid yet still amplify well [4].

Comparisons with the triode
Example pentode current/voltage characteristics for differing control grid voltages.

Pentodes (and tetrodes) tend to have a much lower feedback capacitance, due to the screening effect of the second grid.
Pentodes tend to have a higher noise,
Triodes have a lower internal anode resistance, and hence higher damping factor when used in audio output circuits, compared with pentodes, when negative feedback is absent. That also reduces the potential voltage amplification obtainable from a triode compared with a pentode of the same transconductance, and usually means a more efficient output stage can be made using pentodes, with a lower power drive signal.
Pentodes are almost unaffected by changes in supply voltage, and can thus operate with more poorly stabilised supplies than triodes. An examination of the characteristics shown here will show that the plate current hardly changes as plate voltage varies.


Usage
A General Electric 12AE10 double pentode

Pentode valves were first used in consumer-type radio receivers. A well-known pentode type, the EF50, was designed before the start of the World War II, and was extensively used in radar sets and other military electronic equipment. The pentode contributed to the electronic preponderance of the Allies. After World War II, pentodes were widely used in TV receivers, particularly the successor to the EF50, the EF80. Vacuum tubes were replaced by transistors during the 1960s. However, they continue to be used in certain applications, including high-power radio transmitters and (because of their well-known valve sound) in high-end and professional audio applications, microphone preamplifiers and electric guitar amplifiers. Large stockpiles in countries of the former Soviet Union have provided a continuing supply of such devices, some designed for other purposes but adapted to audio use, such as the GU-50 transmitter tube.

Triode-strapped pentode circuits

A pentode (or, less commonly, a tetrode) can have its screen grid (grid 2) connected to the anode (plate) and the resulting "triode-strapped" (or "triode-connected") device has characteristics very similar to a triode (lower anode resistance, lower noise, more drive voltage required). This is sometimes provided as an option in audiophile pentode amplifier circuits, to give the sought-after "sonic qualities" of a triode power amplifier. There are situations where this arrangement is unsafe, for example when doing so (without a series resistor) could exceed the screen grid's power or voltage rating, but remains a valuable option due to the difficulty in obtaining good modern power triodes.

See also

Beam tetrode
Pentode transistor
Valve audio amplifier – technical
List of vacuum tubes

References

^ G. Holst and B.D.H. Tellegen, "Means for amplifying electrical oscillations", US Patent 1945040, January 1934.
^ "RCA Receiving Tube Manual, 1940"; p118
^ "Sylvania Receiving Tubes Technical Manual, 14th Edition" p 143
^ "RCA Receiving Tube Manual, 1940"; p8

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