Vacuum Tube Amplifier
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A valve amplifier or tube amplifier is a type of
electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...
that uses
vacuum tubes A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...
to increase the
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
or power of a
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
. Low to medium power valve amplifiers for frequencies below the
microwaves Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency rang ...
were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as
guitar amplifier A guitar amplifier (or amp) is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which a ...
s,
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend word, blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a T ...
s such as
DirecTV DirecTV (trademarked as DIRECTV) is an American multichannel video programming distributor based in El Segundo, California. Originally launched on June 17, 1994, its primary service is a digital satellite service serving the United States. It ...
and
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
, high quality stereo amplifiers, military applications (such as
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
) and very high power
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
and UHF
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
s.


History


Origins

Until the invention of the
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
in 1947, most practical high-frequency electronic amplifiers were made using
thermionic valve A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...
s. The simplest valve (named
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
because it had two
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
s) was invented by
John Ambrose Fleming Sir John Ambrose Fleming FRS (29 November 1849 – 18 April 1945) was an English electrical engineer and physicist who invented the first thermionic valve or vacuum tube, designed the radio transmitter with which the first transatlantic rad ...
while working for the
Marconi Company The Marconi Company was a British telecommunications and engineering company that did business under that name from 1963 to 1987. Its roots were in the Wireless Telegraph & Signal Company founded by Italian inventor Guglielmo Marconi in 1897 ...
in London in 1904. The diode conducted
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
in one direction only and was used as a radio detector and a
rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
. In 1906
Lee De Forest Lee de Forest (August 26, 1873 – June 30, 1961) was an American inventor and a fundamentally important early pioneer in electronics. He invented the first electronic device for controlling current flow; the three-element "Audion" triode va ...
added a third electrode and invented the first electronic amplifying device, the
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
, which he named the ''
Audion The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906.De Forest patented a number of variations of his detector tubes starting in 1906. The patent that most clearly covers ...
''. This additional ''control grid'' modulates the current that flows between
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
and
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
. The relationship between current flow and plate and grid voltage is often represented as a series of "characteristic curves" on a diagram. Depending on the other components in the circuit this modulated current flow can be used to provide current or
voltage gain In electronics, gain is a measure of the ability of a two-port circuit (often an amplifier) to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal. It is ...
. The first application of valve amplification was in the regeneration of long distance
telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
signals. Later, valve amplification was applied to the '
wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
' market that began in the early thirties. In due course amplifiers for music and later
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
were also built using valves. The overwhelmingly dominant circuit topology during this period was the
single-ended triode A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted sig ...
gain stage, operating in class A, which gave very good sound (and reasonable measured
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
performance) despite extremely simple circuitry with very few components: important at a time when components were handmade and extremely expensive. Before
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, almost all valve amplifiers were of low gain and with linearity dependent entirely on the inherent linearity of the valve itself, typically 5% distortion at full power.
Negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...
(NFB) was invented by
Harold Stephen Black Harold Stephen Black (April 14, 1898 – December 11, 1983) was an American electrical engineer, who revolutionized the field of applied electronics by discovering the negative feedback amplifier in 1927. To some, his discovery is considered the ...
in 1927, but initially little used since at that time gain was at a premium. This technique allows amplifiers to trade gain for reduced distortion levels (and also gave other benefits such as reduced output impedance). The introduction of the
Williamson amplifier The Williamson amplifier is a four-stage, push-pull, Class A triode-output valve audio power amplifier designed by D. T. N. Williamson during World War II. The original circuit, published in 1947 and addressed to the worldwide do it yourself c ...
in 1947, which was extremely advanced in many respects including very successful use of NFB, was a turning point in audio power amplifier design, operating a push-pull output circuit in class AB1 to give performance surpassing its contemporaries.


Post-war developments

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
stimulated dramatic technical progress and industrial scale production economies. Increasing affluence after the war led to a substantial and expanding consumer market. This enabled electronics manufacturers to build and market more advanced valve (tube) designs at affordable prices, with the result that the 1960s saw the increasing spread of electronic gramophone players, and ultimately the beginnings of '' high fidelity''. Hifi was able to drive full frequency range loudspeakers (for the first time, often with multiple drivers for different frequency bands) to significant volume levels. This, combined with the spread of TV, produced a 'golden age' in valve (tube) development and also in the development of the design of valve amplifier circuits. A range of topologies with only minor variations (notably different
phase splitter A phase splitter is a device that separates a signal into multiple phases (or polarities). The term is most often applied to amplifiers that produce two "balanced" voltage outputs: of equal amplitude but opposite polarity (i.e. 180 degrees phase ...
arrangements and the " Ultra-Linear" transformer connection for tetrodes) rapidly became widespread. This family of designs remains the dominant high power amplifier topology to this day for music application. This period also saw continued growth in civilian radio, with valves being used for both transmitters and receivers.


Decline

From the 1970s the silicon
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
became increasingly pervasive. Valve production was sharply decreased, with the notable exception of
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
s (CRTs), and a reduced range of valves for amplifier applications. Popular low power tubes were
dual triodes Dual or Duals may refer to: Paired/two things * Dual (mathematics), a notion of paired concepts that mirror one another ** Dual (category theory), a formalization of mathematical duality *** see more cases in :Duality theories * Dual (grammatical ...
(ECCnn, 12Ax7 series) plus the EF86 pentode, and power valves were mostly being beam tetrode and pentodes (EL84, EL34, KT88 / 6550, 6L6), in both cases with indirect heating. This reduced set of types remains the core of valve production today. The
Soviets Soviet people ( rus, сове́тский наро́д, r=sovyétsky naród), or citizens of the USSR ( rus, гра́ждане СССР, grázhdanye SSSR), was an umbrella demonym for the population of the Soviet Union. Nationality policy in th ...
retained valves to a much greater extent than the
West West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sunset, Sun sets on the Earth. Etymology The word "west" is a Germanic languages, German ...
during the
Cold War The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...
, for the majority of their communications and military amplification requirements, in part due to valves' ability to withstand instantaneous overloads (notably due to a nuclear detonation) that would destroy a transistor. The dramatic reduction in size, power consumption, reduced
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
levels and above all cost of electronics products based on transistors has made valves
obsolete Obsolescence is the state of being which occurs when an object, service, or practice is no longer maintained or required even though it may still be in good working order. It usually happens when something that is more efficient or less risky r ...
for mainstream products since the 1970s. Valves remained in certain applications such as high power RF transmitters and the
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range. This induces polar molecules in the food to rotate and produce t ...
, and audio amplification equipment, particularly for the electric guitar, recording studios, and high-end home stereos.


Audio usage

In audio applications, valves continue to be highly desired by most professional users, particularly in recording studios' equipment and guitar amplifiers. There is a subgroup of audio enthusiasts who advocate the use of tube amplifiers for home listening. They argue that tube amplifiers produce a "warmer" or more "natural"
valve sound Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. At first, the concept of ''tube sound'' did not exist, because practically ...
. Companies in Asia and Eastern Europe continue to produce valves to cater to this market. Many professional guitar players use 'tube amps' because of their renowned 'tone'. 'Tone' in this usage is referring to timbre, or pitch color, and can be a very subjective quality to quantify. Most audio technicians and scientists theorize that the 'even harmonic distortion' produced by valve tubes sounds more pleasing to the ear than transistors, regardless of style. It is the tonal characteristics of valve tubes that have sustained them as the industry standard for guitars and studio microphone pre-amplification. Tube amplifiers respond differently from transistor amplifiers when signal levels approach and reach the point of
clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...
. In a tube amplifier, the transition from linear amplification to limiting is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping. For this reason, some guitarists prefer the sound of an all-tube amplifier; the aesthetic properties of tube versus solid state amps, though, are a topic of debate in the guitarist community.


Characteristics

Power valves typically operate at higher voltages and lower currents than transistors - although solid state operating voltages have steadily increased with modern device technologies. High power radio transmitters in use today operate in the kilovolt range, where there is still no other comparable technology available. ( ower = voltage × current so high power requires high voltage, high current, or both) Many power valves have good linearity but modest gain or
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
. Signal amplifiers using tubes are capable of very high frequency response ranges – up to
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
and many of the directly heated single-ended triode (DH-SET) audio amplifiers use radio transmitting tubes designed to operate in the megahertz range. In practice, however, tube amplifier designs typically "couple" stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at both ends.


Advantages

*Inherently suitable for high voltage circuits. *Can be constructed on a scale that can dissipate large amounts of heat (some extreme devices even being water cooled). For this reason valves remained the only viable technology for very high power applications such as radio and TV transmitters long into the age when transistors had displaced valves in most other applications. *Electrically very robust, they can tolerate overloads for minutes, which would destroy
bipolar transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...
systems in
millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be called ...
s *Withstand very high transient peak voltages without damage, suiting them to certain military and industrial applications *Generally operate at applied voltages well below their maximum capability, providing long life and reliability *Softer clipping when overloading the circuit, which many audiophiles and musicians subjectively believe gives a more pleasant and more musically satisfying sound.


Disadvantages

*Bad linearity, especially with modest feedback factors.Audio Power Amplifier Design Handbook
/ref> *Tubes require a
cathode heater In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. ...
. Heater power represents a significant heat loss and energy use. *Tubes require higher voltages for the anodes compared to solid state amplifiers of similar power rating. *Tubes are significantly larger than equivalent solid-state devices *High impedance and low current output is unsuitable for the direct drive of many real-world loads, notably various forms of
electric motor An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
s. *Valves have a shorter working life than solid state parts due to various failure mechanisms (such as heat,
cathode poisoning In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. ...
, breakage, or internal short-circuits). *Tubes are available in only a single polarity, whereas transistors are available in complementary polarities (e.g., NPN/PNP), making possible many circuit configurations that cannot be realized directly. *Valve circuits must avoid introduction of noise from AC heater supplies. *
Microphonics Microphonics, microphony, or microphonism describes the phenomenon wherein certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term comes from analogy with a microphone, which ...
– valves may sometimes be sensitive to sound or vibration, inadvertently acting like a
microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
.


Operation

All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. See
power amplifier classes In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...
. Some significantly different circuit topologies exist compared to transistor designs. *The grid (where the input signal is presented) needs to be biased substantially negative with respect to the cathode. This makes it extremely difficult to directly couple the output of one valve to the input of a following valve as is normally done in transistor designs. *Valve stages are coupled with components rated to withstand several hundred volts, typically a capacitor, occasionally a coupling transformer. The phase shifts introduced by coupling networks can become problematic in circuits that have feedback. *There is no valve analog of the complementary devices widely used in "totem pole" output stages of silicon circuits. Push-pull valve topologies therefore require a
phase splitter A phase splitter is a device that separates a signal into multiple phases (or polarities). The term is most often applied to amplifiers that produce two "balanced" voltage outputs: of equal amplitude but opposite polarity (i.e. 180 degrees phase ...
. *The very high output impedance of valves (compared with transistors) usually requires matching transformers to drive low impedance loads such as loudspeakers or cutting lathe heads. The transformer is used as the load, in place of the resistor usually used in small-signal and driver stages. The reflected impedance of the transformer primary at the frequencies in use is much higher than the DC resistance of the windings, often kilohms. High performance transformers are, however, severe engineering compromises, are expensive and, in operation, are far from ideal. Output transformers dramatically increase the cost of a valve amplifier circuit compared to a direct-coupled transistor alternative. However, in both tube and solid state amps, matching output transformers are required for public address applications where low-loss high impedance/high voltage lines are used to connect multiple distant loudspeakers. *The open loop linearity of valves, especially triodes, makes it possible to use little or no negative feedback in circuits whilst retaining acceptable or even excellent distortion performance (especially for small-signal circuits).


Topologies

*Linear small signal circuits almost invariably use a triode in the single ended gain stage topology (in class A), including the output stage. *Broadband valve amplifiers typically use class A1 or AB1. *Modern high power output stages are usually push pull, often necessitating some form of phase splitter to derive a differential/balanced drive signal from a single ended input, typically followed by a further gain stage (the "driver") prior to the output tubes. For example, a shunt regulated push-pull amplifier) *single ended power stages using very large valves exist and dominate in radio transmitter applications. A sidebar is the observation that the niche "DH-SET" topology favored by some audiophiles are extremely simple and typically constructed using valve types originally designed for use in radio transmitters *more complex topologies (notably the use of active loads) can improve linearity and frequency response (by removing Miller capacitance effects).


Output impedance

The high output impedance of tube plate circuits is not well matched to low-impedance loads such as loudspeakers or antennas. A matching network is required for efficient power transfer; this may be a transformer at audio frequencies, or various tuned networks at radio frequencies. In a ''cathode follower'' or ''common-plate'' configuration, the output is taken from the cathode resistance. Because of negative feedback (the cathode-ground voltage cancels the grid-ground voltage) the voltage gain is close to unity and the output voltage ''follows'' the grid voltage. Although the cathode resistor can be many kilohms (depending on biasing requirements), the small-signal output impedance is very low (see
operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
).


Applications


Audio frequency (AF) and broadband amplifiers

Valves remain in widespread use in guitar and
high-end audio High-end audio is a class of consumer home audio equipment marketed to audiophiles on the basis of high price or quality, and esoteric or novel sound reproduction technologies. The term can refer simply to the price, to the build quality of the ...
amplifiers due to the perceived sound quality they produce. They are largely obsolete elsewhere because of higher power consumption, distortion, costs, reliability, and weight in comparison to transistors.


Telephony

Telephony was the original, and for many years was a driving application for audio amplification. A specific issue for the telecommunication industry was the technique of multiplexing many (up to a thousand) voice lines onto a single cable, at different frequencies. The advantage of this is that a single valve "repeater" amplifier can amplify many calls at once, this being very cost effective. The problem is that the amplifiers need to be extremely linear, otherwise "
intermodulation Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency compo ...
distortion" (IMD) will result in "crosstalk" between the multiplexed channels. This stimulated development emphasis towards low distortion far beyond the nominal needs of a single voice channel.


Audio

Today, the main application for valves is audio amplifiers for high-end hi-fi and musical performance use with
electric guitar An electric guitar is a guitar that requires external amplification in order to be heard at typical performance volumes, unlike a standard acoustic guitar (however combinations of the two - a semi-acoustic guitar and an electric acoustic gui ...
s,
electric bass The bass guitar, electric bass or simply bass (), is the lowest-pitched member of the string family. It is a plucked string instrument similar in appearance and construction to an electric or an acoustic guitar, but with a longer neck The ...
es, and
Hammond organ The Hammond organ is an electric organ invented by Laurens Hammond and John M. Hanert and first manufactured in 1935. Multiple models have been produced, most of which use sliding drawbars to vary sounds. Until 1975, Hammond organs generated s ...
s, although these applications have different requirements regarding distortion which result in different design compromises, although the same basic design techniques are generic and widely applicable to all broadband amplification applications, not only audio. Post World War II, the majority of valve power amplifiers are of the Class AB-1 "push pull" ultralinear topology, or lower cost single ended i.e. 6BQ5/EL84 power tubes, but niche products using the DH-SET and even OTL topologies still exist in small numbers.


Instrumentation amplifiers

The basic moving coil
voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. Ana ...
and
ammeter An ammeter (abbreviation of ''Ampere meter'') is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit ...
itself takes a small current and thus loads the circuit to which it is attached. This can significantly alter the operating conditions in the circuit being measured. The vacuum tube voltmeter (VTVM) uses the high input impedance of a valve to buffer the circuit being measured from the load of the ammeter. Valve
oscilloscope An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
s share this very high input impedance and thus can be used to measure voltages even in very high impedance circuits. There may typically be 3 or 4 stages of amplification per display channel. In later oscilloscopes, a type of amplifier using a series of tubes connected at equal distances along
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
s, known as a
distributed amplifier Distributed amplifiers are circuit designs that incorporate transmission line theory into traditional amplifier design to obtain a larger gain-bandwidth product than is realizable by conventional circuits. History The design of the distribut ...
was employed to amplify very high frequency vertical signals before application to the display tube. Valve oscilloscopes are now obsolete. In the closing years of the valve era, valves were even used to make "
operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
s" – the building blocks of much modern linear electronics. An op-amp typically has a differential input stage and a totem pole output, the circuit usually having a minimum of five active devices. A number of "packages" were produced that integrated such circuits (typically using two or more glass envelopes) into a single module that could be plugged into a larger circuit (such as an analog computer). Such valve op-amps were very far from ideal and quickly became obsolete, being replaced with solid-state types.


Narrow band and radio frequency tuned amplifiers

Historically, pre-WWII "transmitting tubes" were among the most powerful tubes available. These usually had directly heated thoriated filament cathodes that glowed like light bulbs. Some tubes were capable of being driven so hard that the anode itself would glow cherry red; the anodes were machined from solid material (rather than fabricated from thin sheet) to withstand heat without distorting. Notable tubes of this type are the 845 and 211. Later tetrodes and pentodes such as 817 and (direct heated) 813 were also used in large numbers in (especially military) radio transmitters RF circuits are significantly different from broadband amplifier circuits. The antenna or following circuit stage typically contains one or more adjustable capacitive or inductive component allowing the resonance of the stage to be accurately matched with carrier frequency in use, to optimize power transfer from and loading on the valve, a so-called "tuned circuit". Broadband circuits require flat response over a wide range of frequencies. RF circuits by contrast are typically required to operate at high frequencies but often over a very narrow frequency range. For example, an RF device might be required to operate over the range 144 to 146 MHz (just 1.4%) Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies. However, an ever-decreasing minority of high power radio frequency amplifiers continue to have valve construction.


Notes


References

*Radio communication handbook (5th Ed),
Radio Society of Great Britain The Radio Society of Great Britain (RSGB) is the United Kingdom's recognised national society for amateur radio operators. The society was founded in 1913 as the London Wireless Club, making it one of the oldest organisations of its kind in the ...
, 1976,


See also

*
Guitar amplifier A guitar amplifier (or amp) is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which a ...
*
Vintage musical equipment Vintage musical equipment is older music gear, including instruments, amplifiers and speakers, sound recording equipment and effects pedals, sought after, maintained and used by record producers, audio engineers and musicians who are interested ...
*
Klystron A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
*
Robert von Lieben Robert von Lieben (September 5, 1878, in Vienna – February 20, 1913, in Vienna) was an Austrian entrepreneur, and self-taught physicist and inventor. Lieben and his associates Eugen Reisz and Siegmund Strauss invented and produced a gas ...
*
Traveling wave tube A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs t ...
* Valve audio amplifier – technical *
Valve RF amplifier A valve RF amplifier ( UK and Aus.) or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely rep ...
*
Valve transmitters Most high power transmitter amplifiers are of vacuum tube, valve construction because of the high power required. Anode circuits Since valves are designed to operate with much higher resistive loads than Solid state (electronics), solid state de ...


External links


The Vacuum Tube FAQ
– Henry Pasternack's FAQ from rec.audio
The Audio Circuit
– An almost complete list of manufacturers, DIY kits, materials and parts and 'how they work' sections on valve amplifiers

– distortion factor to distortion attenuation and THD
AX84.com
– Although oriented towards valve guitar amplifiers, AX84's free schematics and theory document apply well to any tube/valve project
Tube Data Archive
– Massive collection (7 GB+) of tube data sheets and information. {{Authority control