Pentode
Encyclopedia
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s. The term most commonly applies to a three-grid vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
(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 tetrode
Beam tetrode
A beam tetrode is a type of vacuum tube specially designed to produce greater output power than a similar pentode...
s, and an improvement over conventional tetrodes
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
, which were themselves a development of triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
s.
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 distortionDistortionA distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
, and so useful in Radio frequencyRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
stages where Automatic gain controlAutomatic gain controlAutomatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
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, EF86EF86EF86 is a miniature high gain, low mu, pentode vacuum tube. It has 9 pins and connects to a B9A base.The EF86 was introduced in the late 1950s and were produced by Philips, Mullard, Telefunken, Valvo, and GEC among others...
/6267, 1N5GT, 6AU6A, 6J7GT. Often in the European valve naming schemeMullard-Philips tube designationIn Europe, the principal method of numbering vacuum tubes was the nomenclature developed and used by Mullard in the UK and applied Europe-wide thanks to their parentage by Philips who had subsidiaries in Germany and France , plus the support of Telefunken, from 1934...
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", include the Sylvania (and possibly GE) 6CA7 version of EL34, 6V6GT and the EL84EL84The EL84 is a vacuum tube of the power pentode type. It has a 9 pin Noval base and is found mainly in the final output stages of amplification circuits, most commonly now in guitar amplifiers, but originally in radios and many other devices of the pre-transistor era.It was developed to eliminate...
/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.
The EL34, 6CL6, 6F6, 6G6 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 amplifier
Guitar amplifier
A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
s because of their higher distortion characteristics.
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 tetrodeTetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
could supply sufficient power to a speaker or transmitter, and offered a larger amplification factor than the earlier triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
. However, the positively charged screen grid can collect the secondary electrons
Secondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
emitted from the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, which can cause increased current toward the screen grid, and cause the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
current to decrease with increasing anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
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
Suppressor grid
A suppressor grid is a grid used in a thermionic valve to suppress secondary emission. A suppressor grid is also called the antidynatron grid, as it helps to reduce the dynatron effect.- Placement :...
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
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
.
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.
- Pentodes tend to have a higher noise (partition noise),
- Triodes have a lower internal anode resistance, and hence higher damping factor when used in audio output circuits, compared with pentodes, when negative feedbackNegative feedbackNegative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
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. An examination of the characteristics shown here will show that the plate current hardly changes as plate voltage varies.
- 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 a square-law relationship.
Usage
High-end audio
High-end audio is a term used to describe a class of consumer home audio equipment marketed to audio enthusiasts on the basis of high price or quality, and esoteric or novel sound reproduction technologies. High-end audio can refer simply to the price, to the build quality of the components, or to...
and professional audio
Professional audio
Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...
applications, microphone preamplifier
Preamplifier
A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...
s and electric guitar amplifier
Guitar amplifier
A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
s. 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
GU-50
The GU-50, is a power pentode vacuum tube intended for 50 Watt operation as a linear RF amplifier on frequencies up to 120 MHz. It is in fact a Soviet-produced copy of the Telefunken LS-50 power pentode, possibly reverse-engineered from German military radios captured during World War II, or...
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 audiophileAudiophile
An audiophile is a person who enjoys listening to recorded music, usually in a home. Some audiophiles are more interested in collecting and listening to music, while others are more interested in collecting and listening to audio components, whose "sound quality" they consider as important as the...
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 tetrodeBeam tetrodeA beam tetrode is a type of vacuum tube specially designed to produce greater output power than a similar pentode...
- Pentode transistorPentode transistor-Early pentode transistors:One early pentode transistor was developed in the early 1950s as an improvement over the point-contact transistor.*A point-contact transistor having three emitters...
- Valve audio amplifier – technical
- List of vacuum tubes