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Avalve amplifier ortube amplifier is a type ofelectronic amplifier that usesvacuum tubes to increase theamplitude or power of asignal. Low to medium power valve amplifiers for frequencies below themicrowaves were largely replaced bysolid state amplifiers in the 1960s and 1970s.Valve amplifiers can be used for applications such asguitar amplifiers,satellitetransponders such asDirecTV andGPS, high quality stereo amplifiers, military applications (such asradar) and very high powerradio andUHFtelevisiontransmitters.
Until the invention of thetransistor in 1947, most practical high-frequency electronic amplifiers were made usingthermionic valves.[1] The simplest valve (nameddiode because it had twoelectrodes) was invented byJohn Ambrose Fleming while working for theMarconi Company in London in 1904. The diode conductedelectricity in one direction only and was used as a radio detector and arectifier.
In 1906Lee De Forest added a third electrode and invented the first electronic amplifying device, thetriode, which he named theAudion. This additionalcontrol grid modulates the current that flows betweencathode andanode. 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 orvoltage gain.
The first application of valve amplification was in the regeneration of long distancetelephony signals. Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and latertelevision were also built using valves.
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The overwhelmingly dominant circuit topology during this period was thesingle-ended triode gain stage, operating in class A, which gave very good sound (and reasonable measureddistortion performance) despite extremely simple circuitry with very few components: important at a time when components were handmade and extremely expensive. BeforeWorld War II, 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 (NFB) was invented byHarold Stephen Black 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 theWilliamson amplifier 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.
World War II 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 ofhigh 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 differentphase splitter 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.
From the 1970s the silicontransistor became increasingly pervasive. Valve production was sharply decreased, with the notable exception ofcathode-ray tubes (CRTs), and a reduced range of valves for amplifier applications. Popular low power tubes weredual triodes (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.
TheSoviets retained valves to a much greater extent than theWest during theCold War, for the majority of their communications and military amplification requirements, in part due to valves' ability to withstand instantaneous overloads (notablydue to a nuclear detonation) that would destroy a transistor.[2]
The dramatic reduction in size, power consumption, reduceddistortion levels and above all cost of electronics products based on transistors has made valvesobsolete for mainstream products since the 1970s. Valves remained in certain applications such as high power RF transmitters and themicrowave oven, and audio amplification equipment, particularly for the electric guitar, recording studios, and high-end home stereos.
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. 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 ofclipping. 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.[3]
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. ([power = voltage × current], so high power requires high voltage, high current, or both)
Many power valves have good linearity but modest gain ortransconductance. Signal amplifiers using tubes are capable of very high frequency response ranges – up toradio frequency 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.
All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. Seepower amplifier classes. Some significantly different circuit topologies exist compared to transistor designs.
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 acathode follower orcommon-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 voltagefollows the grid voltage. Although the cathode resistor can be many kilohms (depending on biasing requirements), the small-signal output impedance is very low (seeoperational amplifier).
Valves remain in widespread use in guitar andhigh-end audio 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 was the original application for audio amplification, and remained as the main usage for many years. 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. A single valve "repeater" amplifier can amplify many calls at once, and was thus very cost effective.
The problem is that the amplifiers need to be extremely linear, otherwise "intermodulation 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.
Today, the main application for valves is audio amplifiers for high-end hi-fi and musical performance use withelectric guitars,electric basses, andHammond organs, 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 output transformer-less topologies still exist in small numbers.
The basic moving coilvoltmeter andammeter 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.
Valveoscilloscopes 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 alongtransmission lines, known as adistributed amplifier 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 amplifiers" – 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.
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.
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