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. Pentodes (termed "triple-grid amplifiers" in some early literature) are closely related to beam tetrodes, and an improvement over conventional tetrodes, which were themselves a development of triodes.
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).
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 .
Comparisons with the triode
Pentodes (and tetrodes) tend to have a much lower feedback capacitance, due to the screening effect of the second grid.
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.
^ G. Holst and B.D.H. Tellegen, "Means for amplifying electrical oscillations", US Patent 1945040, January 1934.