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Pentode symbol
Electrodes, from top to bottom:
anode (plate)
suppressor grid
screen grid
control grid
heater (filament)

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube (thermionic valve), which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. [1] The pentode consists of an evacuated glass envelope containing five electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode (called a "triple-grid amplifier" in some early literature [2] ) was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted [3] [4] [5] by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.


Types of pentodes

Advantages over the tetrode

The simple tetrode or screen-grid tube offered a larger amplification factor, more power and a higher frequency capability than the earlier triode. However, in the tetrode secondary electrons knocked out of the anode (plate) by the electrons from the cathode striking it (a process called secondary emission) can flow to the screen grid due to its relatively high potential. This current of electrons leaving the anode reduces the net anode current Ia. As the anode voltage Va is increased, the electrons from the cathode hit the anode with more energy, knocking out more secondary electrons, increasing this current of electrons leaving the anode. The result is that in the tetrode the anode current Ia is found to decrease with increasing anode voltage Va, over part of the characteristic curve. This property (ΔVaIa < 0) is called negative resistance. It can cause the tetrode to become unstable, leading to parasitic oscillations in the output, called dynatron oscillations in some circumstances.

The pentode, as introduced by Tellegen, has an additional electrode, or third grid, called the suppressor grid, located between the screen grid and the anode, which solves the problem of secondary emission. The suppressor grid is given a low potential, it is usually either grounded or connected to the cathode. Secondary emission electrons from the anode are repelled by the negative potential on the suppressor grid, so they can't reach the screen grid but return to the anode. The primary electrons from the cathode have a higher kinetic energy, so they can still pass through the suppressor grid and reach 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. [9]

Comparisons with the triode


A General Electric 12AE10 double pentode

Pentode tubes were first used in consumer-type radio receivers. A well-known pentode type, the EF50, was designed before the start of 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.

The Colossus computer and the Manchester Baby used large numbers of EF36 pentode tubes. [10] [11] [12] [13] Later on, the 7AK7 tube was expressly developed for use in computer equipment. [14]

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 can have its screen grid (grid 2) connected to the anode (plate), in which case it reverts to an ordinary triode with commensurate characteristics (lower anode resistance, lower mu, lower noise, more drive voltage required). The device is then said to be "triode-strapped" or "triode-connected". This is sometimes provided as an option in audiophile pentode amplifier circuits, to give the sought-after "sonic qualities" of a triode power amplifier. A resistor may be included in series with the screen grid to avoid exceeding the screen grid's power or voltage rating, and to prevent local oscillation. Triode-connection is a useful option for audiophiles who wish to avoid the expense of 'true' power triodes.

See also


  1. G. Holst and B.D.H. Tellegen, "Means for amplifying electrical oscillations", US Patent 1945040, January 1934.
  2. "RCA Receiving Tube Manual, 1940"; p118
  3. The Navy Electricity and Electronics Training Series, Module 06: Introduction to Electronic Emission, Tubes, and Power Supplies. United States Navy. 1998. p. 13. ISBN   132966776X.
  4. Whitaker, Jerry (2016). Power Vacuum Tubes Handbook, 3rd Edition. CRC Press. p. 87. ISBN   1439850658.
  5. Solymar, Lazlo (2012). Modern Physical Electronics. Springer Science and Business Media. p. 8. ISBN   9401165076.
  6. Stuart, B & Snow, H. A. (1930). Reduction of Distortion and Cross-Talk in Radio Receivers by Mean of Variable-Mu Tetrodes. Proc IRE, Vol 18, Issue 12, pp2102 - 2127
  7. Grayson Evans. Hollow-State Design 2nd Edition. pp. 45–. ISBN   978-1-300-96521-3.
  8. "Sylvania Receiving Tubes Technical Manual, 14th Edition" p 143
  9. "RCA Receiving Tube Manual, 1940"; p8.
  10. Tony Sale. "The Colossus Rebuild Project"
  11. Tony Sale. "The Colossus: its purpose and operation".
  12. Michael Saunby. "Small signal audio pentodes".
  13. B. Jack Copeland. "Colossus: The secrets of Bletchley Park's code-breaking computers".
  14. Sylvania. Engineering Data Service. 7AK7. July 1953.