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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 the Dutchman Bernhard D.H. Tellegen in 1926. 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) was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted 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.
The EL34, 6CL6, 6F6, 6G6, SY4307A and 6K6GT are some examples of true "screen pentodes" used for power amplification, using a screen for the suppressor grid, rather than the higher efficiency, lower distortion beam forming plates used in "beam pentodes". These tubes are often preferred as output tubes in electric guitar amplifiers because of their higher distortion characteristics.
The tetrode tube offered a larger amplification factor, more power and a higher frequency capability than the earlier triode. However, in the tetrode the positively charged screen grid can collect secondary electrons, electrons knocked out of the anode when the electrons from the cathode strike it, resulting in a current away from the anode. This causes the anode current Ia to decrease with increasing anode voltage Va, over part of the characteristic curve. This negative differential resistance (ΔVa/ΔIa < 0) can cause the tetrode to become unstable and results in parasitic 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 are reflected and 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.
Pentode tubes 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.
A pentode 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.
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