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plate resistance

Valve amplifier

A valve amplifier or tube amplifier is a type of electronic amplifier that make use of vacuum tubes instead of solid-state semiconductor devices (such as transistors). As any other electronic amplifier they serve to increase the power and/or amplitude of a signal —typically (but not exclusively) sound or radio frequency signals.

Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS systems, audiophile stereo amplifiers, military applications (such as target acquisition and radar) and very high power radio and UHF television transmitters.

History

Origins

Until the invention of the transistor in 1947, all practical amplifiers were made of thermionic valves.

The simplest valve was invented by John Ambrose Fleming while working for the Marconi Company in London in 1904 and named the Diode, as it had two electrodes. The diode conducted electricity in one direction only and was used as a radio detector and a rectifier.

Although he may not have at first realised the significance of his invention it was Lee De Forest who added a third electrode and invented the first electronic amplifying device, the Triode which he named the 'Audion'. This additional 'control grid' modulates the current that flows between cathode and anode 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 gain

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 triode 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 1960's saw the increasing spread of Gramophone players, and ultimately the beginnings of "HiFi" able to drive full frequency range loudspeakers (for the first time often with multiple drivers for different frequencies) to significant volume levels, combined with the spread of B&W TV, to produce a 'golden age' in Valve development and also in the development of Valve amplifier circuits.

Negative feedback (NFB) was invented by Harold 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 the Williamson 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 far ahead of its time.

Similar topologies with only minor variations (notably different phase splitter arrangements and the "ultra-Linear" 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 70s the silicon transistor became increasingly pervasive and valve production was sharply ramped down, with the notable exception of Cathode Ray Tubes, 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/Pentodes (EL84, EL34, KT88 / 6550, 6L6), in both cases with indirect heating and this rationalized set of types remains the core of all subsequent production and remain in production (in the east) today.

It has subsequently come to light that 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 detonation) 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 (mainly high power RF transmitters) applications for somewhat longer. 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 and guitar amplifier for many more years. However, developments in semiconductor production have now mostly closed that market also.

Resurgence of Valves

In audio applications valves continue to be highly desired by some users. Companies in Russia, China and Eastern Europe continue to produce valves to cater to this market.

Characteristics of linear valve amplifiers

Valves are high voltage/low current devices in comparison with transistors (and especially MOSFETs). The high working voltage makes them 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 matching transformer 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 gain or transconductance. Signal amplifiers using tubes are capable of very high frequency response ranges - up to radio frequency. 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.

Circuit advantages of valves

  • 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).
  • valves are high voltage devices and thus inherently suitable for very high voltage circuits.
  • valves 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/high voltage applications such as Radio & TV transmitters long into the age when transistors had displaced valves in most other applications. However, today these also are becoming obsolete.
  • electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds. In the worst case a failed tube can simply be unplugged and replaced by the user.

Disadvantages of valves

  • heater supplies are usually required for the cathodes.
  • dangerously high voltages are usually required for the anodes.
  • significantly larger than equivalent solid-state transistors
  • high impedance / low current output unsuitable for direct drive of many real world loads, notably various forms of electric motor.
  • valved audio equipment is normally heavy because of the weight of transformers.
  • 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 physical the layout of valve circuits in order to avoid instability and the introduction of noise from radio frequency interference and ac heater supplies.

Operation

All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. See Amplifier. 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 ~ -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.
  • 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" (directly heated single-ended triode) 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).

Futterman OTL

Julius Futterman 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 / low current / 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.

Applications

Audio Frequency (AF) / Broadband amplifiers

Valves remain in widespread use in guitar and high-end audio amplifiers due to the sound quality they produce, but are largely obsolete for most other applications, mainly due to the cost effectiveness advantages of the transistor.

Telephony

(Medium voltage / 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 ~ 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 "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.

Audio

(Medium voltage, low to medium power)

Today the main application for valves is audio amplifiers for High-end HiFi and performance (notably Guitar) use, 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 WWII the majority of valve power amplifiers are of the Class AB1 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 volt/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 Volt Meter (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 Volt Meter (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 lines, known as a distributed 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 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 / Radio Frequency (RF) / tuned amplifiers

(High voltage / High power)

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 tetrode/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 / 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 sound be used

Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies (notably consider cellular radio base stations). However an ever decreasing minority of especially high power radio frequency amplifiers (notably for TV) continue to have valve construction

The development of radio is inseparable from valve technology and the field remains of historic interest, notably to radio amateurs

Notes

References

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

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