Beam tetrode
Encyclopedia
A beam tetrode is a type of vacuum tube
specially designed to produce greater output power than a similar pentode
. It has found extensive application in power amplification
.
in the tetrode
tube
(valve) was solved by Philips
/Mullard
with the introduction of a suppressor grid
to produce the pentode
construction. Since Philips held a patent on this design, other manufacturers were keen to produce pentode type tubes without infringing the patent. In the UK, two EMI
engineers, Cabot Bull and Sidney Rodda, produced and patented an alternate design. Their design had the following features (compared to the normal pentode).
The design is today known as the beam tetrode but historically was also known as a kinkless tetrode, since it is a four-electrode device without the negative resistance
kink in the anode
current vs anode
voltage characteristic curves of a true tetrode. Some authorities, notably outside the United Kingdom, argue that the beam plates constitute a fifth electrode.
The EMI design had the following advantages compared to the pentode:
The beam tetrode was not without its disadvantages:
The MOV (Marconi Osram Valve) company, under the joint ownership of EMI
and GEC, considered the design too difficult to manufacture (due to the need for good alignment of the grid wires). As MOV had a design-share agreement with RCA
of America, the design was passed to that company. RCA had the resources to produce a workable design - the result was the famous 6L6
. Not long after, the beam tetrode appeared in a variety of offerings, but for power audio purposes, the best examples were produced by the MOV company - the KT66
in 1937 and the KT88
in 1956. This latter tube was never bettered, and both are still manufactured today for discerning audiophile
s (but no longer by MOV which ceased production in 1988).
Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34
(6CA7), although manufactured by Mullard as a pentode, was also produced by many manufacturers around the world as a beam tetrode instead. Even Philips/Mullard themselves were not immune; several examples of Mullard-marked ECL82s (a signal triode plus low-power pentode intended for single-ended operation) have turned out to contain beam tetrodes.
The most common beam tetrodes of all time were probably the 25L6
, 35L6, and 50L6, and their miniature versions the 50B5 and 50C5, which were to be found in millions of All American Five
AM radio receivers.
In military equipment the 807, with a rated anode dissipations of 25 watts and operating at a supply voltage of up to 750, was in widespread use as the final amplifier in transmitters of up to 50 watts output power and in push-pull applications for audio. Large numbers entered the market after World War II
and were used widely by radio amateurs in the USA and Europe through the 1950s and 1960s.
The beam tetrode produces the lowest distortion of this class of tube by producing significantly less third-harmonic distortion, and lower intermodulation distortion when used in ultralinear mode. Even-harmonic distortion is automatically cancelled in a push-pull design. The beam tetrode also lends itself to being operated as a triode
(by connecting its screen grid to its anode
), and in this mode functions more efficiently than a pentode operated in the same manner. An alternative was to connect the screen-grid to the control grid and use the device in a push-pull amplifier, a configuration known as 'Class-B zero-bias'.
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...
specially designed to produce greater output power than a similar pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
. It has found extensive application in power amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
.
History
The problem of secondary emissionSecondary 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...
in the tetrode
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:...
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...
(valve) was solved by Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
/Mullard
Mullard
Mullard Limited was a British manufacturer of electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed valves for the Admiralty before becoming managing director of the Z Electric Lamp Co. The...
with the introduction of a 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 :...
to produce the pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
construction. Since Philips held a patent on this design, other manufacturers were keen to produce pentode type tubes without infringing the patent. In the UK, two EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
engineers, Cabot Bull and Sidney Rodda, produced and patented an alternate design. Their design had the following features (compared to the normal pentode).
- The controlControl gridThe control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
and screen grids were wound so that the pitches were the same and the wires were in alignment (the pentode used different pitches).
- A pair of beam-forming plates was added at the two ends of the oval grid structure to focus the electron stream into a pair of beams 180 degrees apart (the pentode added a third grid). These plates are normally connected to the cathode.
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
kink in the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
current vs anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
voltage characteristic curves of a true tetrode. Some authorities, notably outside the United Kingdom, argue that the beam plates constitute a fifth electrode.
The EMI design had the following advantages compared to the pentode:
- The screen grid current was about 5-10% of the anodeAnodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
(anode) current compared with about 20% for the pentode, thus the beam tetrode was more efficient.
- The design introduced significantly less third-harmonic distortion into the signal than did the pentode.
- The design produced a greater output power compared with a similar pentode.
The beam tetrode was not without its disadvantages:
- It had higher intermodulation distortion than the pentode, though this could be eliminated with the application of 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 :...
, or (more usually) by adopting an ultra-linearUltra-LinearUltra-linear is a term used to describe a type of electronic circuit that is used to couple a tetrode or pentode vacuum-tube to a load ....
design in a push-pullElectronic amplifierAn electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
circuit. This connection links the screen grids to taps on the output transformerTransformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
.
- The beam tetrode required a higher control gridControl gridThe control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
voltage than the pentode and thus required a higher-gain driver stage preceding it.
- The beam tetrode had a tendency to oscillate if the circuit was not designed properly.
The MOV (Marconi Osram Valve) company, under the joint ownership of EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
and GEC, considered the design too difficult to manufacture (due to the need for good alignment of the grid wires). As MOV had a design-share agreement with RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
of America, the design was passed to that company. RCA had the resources to produce a workable design - the result was the famous 6L6
6L6
6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were fast replacing power triodes due to their greater efficiency...
. Not long after, the beam tetrode appeared in a variety of offerings, but for power audio purposes, the best examples were produced by the MOV company - the KT66
KT66
The KT66 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.KT66 is the designator for a vacuum tube introduced by Marconi-Osram Valve Co. Ltd. of Britain in 1937....
in 1937 and the KT88
KT88
The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.- Features :The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550...
in 1956. This latter tube was never bettered, and both are still manufactured today for discerning audiophile
Audiophile
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...
s (but no longer by MOV which ceased production in 1988).
Interestingly, many tubes that are described as pentodes actually turn out to be beam tetrodes. The ubiquitous Mullard EL34
EL34
The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater...
(6CA7), although manufactured by Mullard as a pentode, was also produced by many manufacturers around the world as a beam tetrode instead. Even Philips/Mullard themselves were not immune; several examples of Mullard-marked ECL82s (a signal triode plus low-power pentode intended for single-ended operation) have turned out to contain beam tetrodes.
The most common beam tetrodes of all time were probably the 25L6
25L6
The 25L6 is an octal-based vacuum tube of the beam-power tetrode type. It found common application in AC/DC radio receivers - such as those of the All American Five type - and was also found in large numbers in early computers, such as the UNIVAC I....
, 35L6, and 50L6, and their miniature versions the 50B5 and 50C5, which were to be found in millions of All American Five
All American Five
The term All American Five is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation broadcasts in the medium wave band, and were manufactured in the United States from the mid...
AM radio receivers.
In military equipment the 807, with a rated anode dissipations of 25 watts and operating at a supply voltage of up to 750, was in widespread use as the final amplifier in transmitters of up to 50 watts output power and in push-pull applications for audio. Large numbers entered the market after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
and were used widely by radio amateurs in the USA and Europe through the 1950s and 1960s.
The beam tetrode produces the lowest distortion of this class of tube by producing significantly less third-harmonic distortion, and lower intermodulation distortion when used in ultralinear mode. Even-harmonic distortion is automatically cancelled in a push-pull design. The beam tetrode also lends itself to being operated as a 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...
(by connecting its screen grid to its anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
), and in this mode functions more efficiently than a pentode operated in the same manner. An alternative was to connect the screen-grid to the control grid and use the device in a push-pull amplifier, a configuration known as 'Class-B zero-bias'.