A
valve amplifier or
tube amplifier is a type of
electronic 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...
that makes use of vacuum tubes to increase the
powerIn physics, power is the rate at which work is performed or energy is converted. It is an energy per unit of time. As a rate of change of work done or the energy of a subsystem, power iswhere P is power, W is work and t is time....
and/or
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of a signal. They are typically (but not exclusively) used for
soundSound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.- Perception of sound...
amplification, either in home stereo hi-fi amplifiers or for
electric guitarAn electric guitar is a guitar that uses pickups to convert the vibration of its steel-cored strings into an electrical current, which is made louder with an instrument amplifier and a speaker. The signal that comes from the guitar is sometimes electronically altered with guitar effects such as...
amps, or for radio frequency signals.
Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by
solid stateSolid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as
guitar amplifierA 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 and modify the tone by emphasizing or de-emphasizing certain frequencies and adding electronic effects.The amplifier comprises a...
s,
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
transponderIn telecommunication, the term transponder has the following meanings:...
s such as
DirecTVDirecTV is a direct broadcast satellite service based in El Segundo, California, which transmits digital satellite television and audio to households in the United States. Its primary competitors are Dish Network and cable providers. DirecTV currently has 18 million subscribers...
and GPS systems, audiophile stereo amplifiers, military applications (such as
target acquisitionIn the military, target acquisition denotes any process that provides detailed information about enemy forces and locates them with sufficient accuracy to permit continued monitoring or target designation and engagement....
and
radarRadar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
) and very high power
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
and UHF
televisionTelevision is a widely used telecommunication medium for transmitting and receiving moving images, either monochromatic or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission...
transmitterA transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.-Transmitter types:...
s.
Origins
Until the invention of the
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's...
in 1947, all practical amplifiers were made using thermionic valves. The simplest valve was invented by
John Ambrose FlemingSir John Ambrose Fleming was an English electrical engineer and physicist. He is known for inventing the first thermionic valve or vacuum tube, the diode, then called the kenotron in 1904. He also invented the right-hand rule, used in mathematics and electronics...
while working for the
MarconiMarconi may refer to:people*Guglielmo Marconi, Italian-born radio pioneer*David Marconi, American screenwriter*Enrico Marconi, also known as Henryk Marconi, architect*Leandro Marconi, architect, son of Enrico Marconibusinesses...
Company in London in 1904 and named the
diodeIn electronics a diode is a two-terminal electronic component which conducts electric current asymmetrically or unidirectionally; that is, it conducts current more easily in one direction than in the opposite direction. The term usually refers to a semiconductor diode, the most common type today,...
, as it had two electrodes. The diode conducted
electricityElectricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge...
in one direction only and was used as a radio detector and a
rectifierA rectifier is an electrical device that converts alternating current to direct current , a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals...
.
Although he may not have at first realised the significance of his invention it was
Lee De ForestLee De Forest was an American inventor with over 180 patents to his credit. De Forest invented the Audion, a vacuum tube that takes relatively weak electrical signals and amplifies them. De Forest is one of the fathers of the "electronic age", as the Audion helped to usher in the widespread use...
who added a third electrode and invented the first electronic amplifying device, the
triodeA 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...
which he named the '
AudionAn Audion is a wireless signal detector device invented by Lee De Forest in 1906.Audion may also refer to:*Audion , a media player for Apple Macintosh created by Panic*Matthew Dear or Audion, techno music artist...
'. This additional 'control grid' modulates the current that flows between
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
and
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
for a given voltage between the cathode and anode. 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
gainGain is a measure of a system's response to feedback. If the gain in a positive feedback loop is less than 1, the feedback is not of itself sufficient to make the system become unstable. For example, water evaporating from the World's oceans causes a positive feedback, as it is a greenhouse gas...
.
The first application of valve amplification was in the regeneration of long distance telephony signals. Later, valve amplification was applied to the 'wireless' market that began in the early thirties. In due course amplifiers for music and later television were also built exclusively using valves until the 1950s. During this period power levels were usually very low (a few watts) and radios often used headphones only, with no loudspeaker at all.
The overwhelmingly dominant circuit topology during this period was the
single-ended triodeA single-ended triode vacuum tube electronic amplifier 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 signals added and the distortion components subtracted...
gain stage, operating in class A, which gave very good sound (and reasonable measured distortion performance) despite extremely simple circuitry with very few components: important at a time when components were hand made and extremely expensive. Prior to World War II, almost all valve amplifiers were of low gain and with linearity dependent entirely on the inherent linearity of the tube itself, typically 5% distortion at full power.
Post-war developments
Increasing post-war affluence and industrialised production economies, dramatic technical progress stimulated by the war, and for the first time a substantial and expanding consumer market brought more advanced valve designs to market at affordable prices, with the result that the 1960s saw the increasing spread of gramophone players, and ultimately the beginnings of "
high fidelityHigh fidelity or hi-fi reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images that are very faithful to the original performance...
" able to drive full frequency range loudspeakers (for the first time often with multiple drivers for different frequency bands) to significant volume levels, combined with the spread of TV, to produce a 'golden age' in valve development and also in the development of valve amplifier circuits.
Negative 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 :...
(NFB) was invented by
Harold Stephen BlackHarold Stephen Black was an American electrical engineer, who revolutionized the field of applied electronics by inventing the negative feedback amplifier in 1927. To some, his invention is considered the most important breakthrough of the twentieth century in the field of electronics, since it...
in 1927, but initially little used since at that time
gainGain is a measure of a system's response to feedback. If the gain in a positive feedback loop is less than 1, the feedback is not of itself sufficient to make the system become unstable. For example, water evaporating from the World's oceans causes a positive feedback, as it is a greenhouse gas...
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 amplifierA Williamson amplifier refers to a type of vacuum tube amplifier whose circuit design is similar to that originally published by D.T.N. Williamson.- Explanation :...
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.
Similar topologies with only minor variations (notably different
phase splitterA phase splitter is a device that separates a signal into multiple phases .- External links :* - Transistors Tutorial Part 2: "Bipolar Transistors"...
arrangements and the "
Ultra-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 ....
" transformer connection for tetrodes) rapidly became widespread and 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 became increasingly pervasive and valve production was sharply ramped down, with the notable exception of
cathode ray tubeThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen...
s (CRTs), and a dramatically rationalized range of valves for amplifier applications, low power tubes being mostly dual triodes (ECCnn, 12Ax7 series) plus the EF86 pentode, power tubes 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 retained valves to a much greater extent than the west during the Cold War, for the majority of their communications and military amplification requirements, in part due to tubes' ability to withstand instantaneous overloads (notably
due to a nuclear detonationThe term electromagnetic pulse has the following meanings:# A burst of electromagnetic radiation from an explosion or a suddenly fluctuating magnetic field. The resulting electric and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage...
) that would destroy a transistor.
The dramatic reduction in size, power consumption, reduced distortion levels and above all cost of electronics products based on transistors has made valves obsolete for mainstream products since the 1970s, although valves remained in niche applications for somewhat longer; applications such as high power RF transmitters. However, the difficulty in producing transistors with good gain and efficiency at very high frequencies, combined with the fragility of transistors (problems such as thermal runaway), resulted in tubes being retained for longer in high power and high frequency applications, notably large radio and TV transmitters for many more years.
Resurgence
In audio applications, valves continue to be highly desired by some users, both in the higher-end home audio market and in the guitar amplifier market. Amongst stereo enthusiasts, there is a subgroup of audio buffs who advocate the use of tube amplifiers for home listening; they argue that tube amplifiers produce a "warmer" or more "natural" sound. Companies in Russia, China and Eastern Europe continue to produce valves to cater to this market.
In the guitar amplifier market, some performers continue to use tube amps today, including in folk, blues, roots rock, and in harder genres such as metal, where tube amps are used to create heavy distortion. Audio engineers suggest that the subjectively pleasing aspects of tube amplification may be due to the non-linear overdrive that is produced with tubes. When a tube fails, it is replaceable. While solid state devices are also replaceable, it is usually a much more involved process (i.e., having the amplifier tested by a professional, removing the faulty component, and replacing it). For working musicians this is usually a huge problem in comparison to looking in the back of a tube amp at the tubes and simply replacing the faulty tube.
Tube amplifiers respond differently from transistor amplifiers when signal levels approach and reach the point of
clippingClipping is a form of waveform distortion that occurs when an amplifier is overdriven, which happens through attempts to increase the voltage or current beyond its maximum power capability...
. In a tube-powered 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
Valves are high voltage and low current devices in comparison with
transistorA transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's...
s, especially
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
transistors. The high working voltage makes valves well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a
matchingIn the mathematical discipline of graph theory, a matching or edge-independent set in a graph is a set of edges without common vertices. It may also be an entire graph consisting of edges without common vertices.- Definition :...
transformerA 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...
is required. The transformer is a critical component and heavily influences the performance (and cost) of the amplifier.
Many power valves have good linearity but modest
gainGain is a measure of a system's response to feedback. If the gain in a positive feedback loop is less than 1, the feedback is not of itself sufficient to make the system become unstable. For example, water evaporating from the World's oceans causes a positive feedback, as it is a greenhouse gas...
or
transconductanceTransconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance and transconductance is the ratio of the current at the output port and the voltage at the input ports and is written as gm:or sometimes...
. Signal amplifiers using tubes are capable of very high frequency response ranges – up to
radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
. Indeed, many of the directly heated single-ended triode (DH-SET) audio amplifiers are in fact 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 high end.
Advantages
- Very linear (especially triodes) making it viable to use them in low distortion linear circuits with little or no negative feedback.
- Extremely high input impedance (cf bipolar transistors but a characteristic shared by FETs).
- High voltage devices and thus inherently suitable for very 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, and especially high power and high voltage applications such as radio and TV transmitters long into the age when transistors had displaced valves in most other applications. However, today these also are becoming obsolete.
- Very low "drift" (of specifications) over a wide range of operating conditions, specifically high heat and high power. Semiconductors are very heat sensitive by comparison, forcing compromises in solid state amplifier designs.
- Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar
-Medicine:* Bipolar cell* Bipolar cell of the retina* Bipolar disorder* Bipolar I disorder* Bipolar II disorder* Bipolar spectrum-Astronomy:* Bipolar nebula, a two-lobed, axi ally symmetric nebula...
transistor systems in milliseconds.
- Easily removable for testing or replacement – it is much harder to replace a faulty transistor.
- Softer clipping when overloading the circuit, which many audophiles and musicians think give a more pleasant sound.
Disadvantages
- Heater
A heater is any object that emits heat or causes another body to achieve a higher temperature. In a household or domestic setting, heaters are commonly used to generate heating...
supplies are usually required for the cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
s, and dangerously high voltages are usually required for the anodes.
- They are significantly larger than equivalent solid state transistors
- High impedance and low current output is unsuitable for direct drive of many real world loads, notably various forms of electric motor
An electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
s.
- Valved audio equipment is normally heavy because of the weight of transformer
A 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...
s.
- Valves may have a shorter working life than solid state parts due to various failure mechanisms (cathode poisoning, breakages (i.e., open circuit) or shorts internally, notably of the heater or grid structures, or in the case of glass valves, physical breakage, although this should not be overstated; many valve types typically have operation lives in the tens of thousands of hours and an indefinite shelf life (many 60 year old tubes are still in regular use).
- Available in a single polarity only whereas transistors are available in complementary polarities (e.g., NPN/PNP), making possible many circuit configurations that cannot be realized directly with valves.
- In comparison to the lower impedance environment of transistors special consideration must be made to the physical layout of valve circuits in order to avoid instability and the introduction of noise from radio frequency interference and ac heater supplies.
- Microphony – valves may sometimes be sensitive to sound waves around them, inadvertently acting like a microphone
A microphone, colloquially called a mic or mike , is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1876, Emile Berliner invented the first microphone used as a telephone voice transmitter...
.
Operation
All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. See amplifier classes. However, the nature of valves results in significantly different circuit topologies and characteristics than transistor designs, and as a sweeping generalisation, valve amplifiers tend to operate in class A (or class AB1 with a heavy class A overlap), whereas transistor amplifiers tend to operate more in class B (there are however significant exceptions in both directions).
- The grid (where the input signal is presented) needs to be biased substantially negative with respect to the cathode (typically about -75 V). This makes it extremely difficult to direct couple the output of one valve (typically sitting at about + 100 V) to the input of a following valve as is normally done in modern transistor designs. (cf transistors which are usually biased just a few volts positive), at a voltage between that of the collector and emitter, facilitating direct coupling).
- Valve stages thus need to be coupled using some component able to totally block and withstand several hundred volts, typically a capacitor, occasionally a coupling transformer, adding phase shifts and possibly coloration to the signal. These introduced phase shifts 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. This is because valves work based on the flow of electrons from the cathode to the anode and it is not possible to construct a hypothetical valve in which an "electron hole" migrated the other way. Push-pull valve topologies therefore typically require a transformer.
- The very high output impedance of valves (compared with transistors) usually demands the use of matching transformers if low impedance loads (notably loudspeakers or various forms of motor, such as cutting lathe heads, etc.) are to be driven. The transformer is used as the load, in place of the resistor usually used in small-signal and driver stages. NB the impedance of the transformer primary at the frequencies in use is much higher than the DC resistance of the windings, often kOhms. High performance transformers are however severe engineering compromises, are expensive, and in operation are far from ideal. Transformers dramatically increase the cost of a valve amplifier circuit compared to a direct-coupled transistor alternative.
- The (typically) much lower open loop gain but enhanced 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 (cf silicon circuits, notably the very widely use "op-amp" configuration, which normally have a totem pole 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 SRPP)
- 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 transformerless
Julius FuttermanJulius Futterman was an American electronics engineer who designed a valve amplifier that did not have an output transformer. This was originally done to reduce costs, as output transformers can be rather expensive...
pioneered a type of amplifier known as "output transformerless" (OTL). These forego the typical output transformer by paralleling (electrically connecting and operating side-by-side) perhaps one dozen or more output tubes in an attempt to reduce effective plate resistance and more closely match it with speaker impedances (typically 8 ohms). This design and its various incarnations tend to require numerous tubes, run hot, and because they attempt to match impedances in a way fundamentally different from a transformer, they often have a unique sound quality.
Output impedance
The high voltage and low current with high output impedance (Z out) of the output (anode circuit in the overwhelming majority of valve circuits) is suitable to drive another following valve stage, and can drive an antenna system that has been arranged to resonate at the required drive frequency, but usually is not suitable to drive low impedance loads, such as (notably) loudspeakers and motors. The main techniques are used to resolve this: the use of a matching transformer, and the use of negative feedback to reduce that active output impedance (in proportion to the amount of feedback applied). In combination these techniques can reduce the Z out from hundreds of ohms to a fraction of an ohm.
Audio Frequency (AF) and Broadband amplifiers
Valves remain in widespread use in guitar and
high-end audioHigh-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...
amplifiers due to the sound quality they produce, which is subjectively preferred by some users. They are largely obsolete for most other applications, mainly due to the cost effectiveness advantages of the transistor, and due to the lower weight and heat production of transistor amps.
Telephony
(Medium voltage and low to medium power)
Telephony was the original, and for many years was a driving application for audio amplification. Although a telecoms voice connection is very undemanding compared with modern data communication (it has a very narrow frequency range, typically about 300–3000 Hz, and also has very poor signal to noise ratio), a specific issue for the telecomms 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 a 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 "
intermodulationIntermodulation or intermodulation distortion , or intermod for short, is the result of two or more signals of different frequencies being mixed together, forming additional signals at frequencies that are not, in general, at harmonic frequencies of either.Intermodulation is caused by non-linear...
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
(Medium voltage, low to medium power)
Today the main application for valves is audio amplifiers for high-end hi-fi and musical performance use with
electric guitarAn electric guitar is a guitar that uses pickups to convert the vibration of its steel-cored strings into an electrical current, which is made louder with an instrument amplifier and a speaker. The signal that comes from the guitar is sometimes electronically altered with guitar effects such as...
s,
electric bassElectric bass can mean:* Electric upright bass, the electric version of a double bass.* Electric bass guitar....
es, and
Hammond organThe Hammond organ is an electric organ which was invented by Laurens Hammond in 1934 and manufactured by the Hammond Organ Company. While the Hammond organ was originally sold to churches as a lower-cost alternative to the wind-driven pipe organ, in the 1960s and 1970s, it became a standard...
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, but niche products using the DH-SET and even OTL topologies still exist in small numbers.
Instrumentation amplifiers
(medium voltage, small signals)
The basic moving coil voltmeter and ammeter 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, clearly an undesirable feature. The vacuum tube voltmeter (VTVM) was developed by taking advantage of the near infinite input impedance of a valve to buffer the circuit being measured from the load of the ammeter. VTVMs have become obsolete since the introduction of the modern digital voltmeter (DVM) which typically also has an extremely high input impedance (FET) input.
Valve oscilloscopes 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 sets of amplification per display channel. In later oscilloscopes, a type of amplifier using a series of tubes connected at equal distances along
transmission lineA transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission....
s, known as a
distributed amplifierDistributed 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:...
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 simple "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 "integrated" (planar silicon) types.
Narrow band and radio frequency tuned amplifiers
Historically (pre WWII) "transmitting tubes" were among the most powerful tubes available, were usually direct heated by fragile thoriated filaments that glowed like light bulbs. Some tubes were capable of being driven so hard that the anode would itself glow cherry red, the anodes being machined from solid material (rather than fabricated from thin sheet) to be able to withstand this without distorting when heated. 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 (in particular transmitters, which is something more than simply an amplification gain stage) 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 often go down to near DC (10 Hz or below) and up to tens or hundreds of kilohertz to low megahertz, and are required to have essentially flat frequency response over this entire range (4 or more orders of magnitude). RF circuits by contrast are typically required to operate over higher frequencies (which makes capacitive and inductive parasitic effects much more of a design challenge) but often 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% of an octave)
Historically, distortion and out-of-band emission was less of an issue and Class C could be used. Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies (notably cellular radio base stations). However an ever decreasing minority of especially high power radio frequency amplifiers (notably for TV) continue to have valve construction. Nevertheless, since the development of radio is inseparable from valve technology, the field of valve amplification remains of historic interest, notably to radio amateurs.
External links
- Tubeopedia – Wiki of electronic tubes and related topics
- 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
- Conversion calculator – 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 (7GB+) of tube data sheets and information.