Graphic symbol representing a pentode of the indirectly heated cathode class Electrodes, listed from top to bottom: anode, suppressor grid, screen grid, control grid, cathode
Apentode is an electronic device having fiveelectrodes. The term most commonly applies to a three-grid amplifyingvacuum tube or thermionic valve that was invented byGilles Holst andBernhard D.H. Tellegen in 1926.[1] The pentode (called atriple-grid amplifier in some literature[2]) was developed from thescreen-grid tube orshield-grid tube (a type oftetrode tube) by the addition of a grid between the screen grid and the plate. The screen-grid tube was limited in performance as an amplifier due tosecondary emission of electrons from the plate.[3] The additional grid is called thesuppressor grid. The suppressor grid is usually operated at or near the potential of the cathode and prevents secondary emission electrons from the plate from reaching the screen grid.[4][5] The addition of the suppressor grid permits much greater output signal amplitude to be obtained from the plate of the pentode in amplifier operation than from the plate of the screen-grid tube at the same plate supply voltage. Pentodes were widely manufactured and used in electronic equipment until the 1960s to 1970s, during which time transistors replaced tubes in new designs. During the first quarter of the 21st century, a few pentode tubes have been in production for high power radio frequency applications, musical instrument amplifiers (especially guitars), home audio and niche markets.
Ordinary pentodes are referred to assharp-cutoff orhigh-slope pentodes and have uniform aperture size in the control grid. The uniform construction of the control grid results in the amplification factor (mu or μ) and transconductance changing very little with increasingly negative grid voltage, resulting in fairly abrupt cutoff of plate current.[6] These pentodes are suitable for application in amplifier designs that operate over limited ranges of signal and bias on the control grid. Examples include: EF37A,EF86/6267, 1N5GT, 6AU6A, 6J7GT. Often, but not always, in theEuropean 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).
Remote-cutoff,variable-mu,super-control orvariable slope pentodes handle much greater signal and bias voltages on the control grid than ordinary pentodes, without cutting off the anode current. The control grid of the variable-mu pentode is constructed so as to result in a given incremental change of control grid voltage having less effect on change of anode current as the control grid voltage increases negatively relative to the cathode.[7] The control grid often has the form of a helix of varying pitch.[8] As the control grid voltage becomes more negative, the amplification factor of the tube becomes smaller.[7][9] Variable-mu pentodes reduce distortion and cross-modulation (intermodulation) and permit much larger amplifier dynamic range than ordinary pentodes.[10] Variable-mu pentodes were first applied in radio frequency amplifier stages of radio receivers, typically withautomatic volume control, and are applied in other applications requiring the ability to operate over large variations of signal and control voltages. The first commercially available variable-mu pentodes were the RCA 239 in 1932 and the Mullard VP4 in 1933.[11][12]
Power pentodes,output pentodes orpower-amplifier pentodes. Power pentodes are designed to operate at higher currents, higher temperatures and higher voltages than ordinary pentodes. The cathode of the power pentode is designed to be capable of sufficient electron emission to give the required current through the tube to produce the desired power in the load impedance.[13] The plate or anode of a power pentode is designed to be capable of dissipating more power than that of an ordinary pentode.[14] TheEL34,EL84, 6CL6, 6F6, 6G6,SY4307A and 6K6GT are some examples of pentodes designed for power amplification. Some power pentodes for specific television requirements were:
video output pentodes, e.g. 15A6/PL83, PL802
frame output orvertical deflection pentodes, such as thePL84 and the pentode sections of the 18GV8/PCL85.
line output orhorizontal deflection pentodes, such as the PL36, 27GB5/PL500, PL505 etc.
A "triode-pentode" is a single envelope containing both a triode and a pentode, such as an ECF80 or ECL86.
Image of a type GU-81 power pentode, a Russian electron tube used in military radio stations in the 70s and 80s
The simpletetrode orscreen-grid tube offered a larger amplification factor, more power and a higher frequency capability than the earliertriode. However, in the tetrodesecondary electrons knocked out of the anode (plate) by the electrons from the cathode striking it (a process calledsecondary emission) can flow to the screen grid due to its relatively high potential. This current of electrons leaving the anode reduces the net anode currentIa. As the anode voltageVa 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 currentIa is found todecrease with increasing anode voltageVa, over part of thecharacteristic curve. This property (ΔVa/ΔIa < 0) is callednegative resistance. It can cause the tetrode to become unstable, leading toparasitic oscillations in the output, calleddynatron oscillations in some circumstances.
The pentode, as introduced byTellegen, has an additional electrode, or third grid, called thesuppressor 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.[15]
Pentodes (and tetrodes) tend to have a much lower feedback capacitance, due to the screening effect of the second grid.
Pentodes tend to have higher noise (partition noise) because of the random splitting of the cathode current between the screen grid and the anode,
Triodes have a lower internal anode resistance, and hence higher damping factor when used in audio output circuits, compared with pentodes, whennegative 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 stabilized supplies than triodes.
Pentodes and triodes (and tetrodes) do have essentially similar relationships between grid (one) input voltage and anode output current when the anode voltage is held constant, i.e. close to asquare-law relationship.
Pentode tubes were first used in consumer-type radio receivers. A well-known pentode type, theEF50, was designed before the start of World War II, and was extensively used inradar 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-knownvalve sound) inhigh-end andprofessional audio applications, microphonepreamplifiers and electricguitar 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 theGU-50 transmitter tube.
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 inaudiophile 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.
