Electron multipliers And Vacuum tubes in the 21st century

The highest possible vacuum is desired in a tube. Remaining gas atoms will ionize and conduct electricity between the elements in an undesired manner. In a defective tube residual air pressure will lead to ionization, becoming visible as a pink-purple glow discharge between the tube elements.
To prevent gases from compromising the tube's vacuum, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, barium being the most common. While the tube envelope is being evacuated, the internal parts except the getter are heated by RF induction heating to

Getter in opened tube; silvery deposit from getter

help free any remaining gases from the metal parts. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating. This causes some material from the getter to

Tube tester manufactured in 193evaporate, reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside

Voltage regulator tube in operation. Low pressure gas within tube glows due to current flow.

of the envelope of the tube. The getter continues to absorb small amounts of gas that may leak into the tube

70 watt tube audio amplifier selling for $2,680^[23] in 2011, about 10 times the price of a comparable model using transistors.[24during its working life. If a tube develops a serious leak in the envelope, this deposit turns a white color as it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getter materials, such as zirconium. Early gettered tubes used phosphorus based getters and these tubes are easily identifiable, as the phosphorus leaves a characteristic orange or rainbow deposit on the glass. The use of phosphorus was short-lived and was quickly replaced by the superior barium getters. Unlike the barium getters, the phosphorus did not absorb any further gases once it had fired.

Heat generation and transfer Power

A considerable amount of heat is produced when tubes operate, both from the filament (heater) but also from the stream of electrons bombarding the plate. The requirements for heat removal can significantly change the appearance of high-power vacuum tubes. Although the miniature tube style became predominant in consumer

The anode (plate) of this transmitting triode has been designed to dissipate up to 500W of heat

equipment, high power audio amplifiers and rectifiers would still require the larger "octal" style of enclosure. Transmitting tubes could be much larger still.
Most tubes produce heat from two sources during operation. The first source is the filament or heater. Some tubes contain a directly heated cathode. This is a filament similar to an incandescent electric lamp; some types glow brightly like a lamp, but most glow a dim orange-red. The "bright emitter" types possess a tungsten filament alloyed with 1–3 % thorium which reduces the work function of the metal, giving it the ability to emit sufficient electrons at about 2000 degrees Celsius. The "dull emitter" types also possess a tungsten filament but it is coated in a mixture of calcium, strontium and barium oxides, which emit electrons easily at much lower temperatures due to a monolayer of mixed alkali earth metals coating the tungsten; these only reach 800–1000 degrees Celsius.
The second form of cathode is the indirectly heated form which usually consists of a nickel cylinder, coated on the outside with the same strontium, calcium, barium oxide mix used in the "dull emitter" directly heated types. Inside the cylinder is a tungsten filament to heat it. This filament is usually uncoiled and coated in a layer of alumina (aluminium oxide) in order to insulate it electrically from the actual cathode. This form of construction allows for a much greater electron emitting area and allows the cathode to be held at a potential difference, typically 150 volts more positive than the heater or 50 volts more negative than the heater. For small-signal tubes such as used in radio receivers, heaters consume between 50 mW and 5 watts, (directly heated), or between 500 mW and 8 watts for indirectly heated types. Thus even a small signal amplifier might consume a watt of power just to warm its heater, compared to the milliwatts (or less) that a modern semiconductor amplifier would require for the same function. Even in power amplifiers the filament power may be responsible for an appreciable reduction in efficiency.
The second source of heat is generated at the plate (anode), as electrons accelerated by its high voltage strike it, depositing their kinetic energy there and raising its temperature. In tubes used in power amplifiers or transmitter output stages, this source of heat will far exceed the power due to the cathode heater. The plates of improperly operated or overloaded beam power tubes can sometimes become visibly red hot; this should never occur under normal operation of consumer electronics and is a precursor to tube failure.
Heat escapes the device by black body radiation from the anode (plate) as infrared radiation. Convection is not possible in most tubes since the anode is surrounded by vacuum. Considerations of heat removal can affect the overall appearance of some tubes. The anode is often treated to make its surface less shiny and darker in the infrared (see black body radiator). The screen grid may also generate considerable heat, which is radiated toward the plate which must reradiate that additional heat along with the heat it generates itself. Limits to screen grid dissipation, in addition to plate dissipation, are listed for power devices. If these are exceeded then tube failure is likely.
Tubes used as power amplifier stages for radio transmitters may have additional heat exchangers, cooling fans, radiator fins, or other measures to improve heat transfer at the anode (plate). High power transmitting tubes may have the surface of their anodes external to the tube, allowing for water cooling or evaporative cooling. Such a water cooling system must be electrically isolated to withstand the high voltage present on the anode.
Tubes which generate rather little heat, such as the 1.4 volt filament directly heated tubes designed for use in battery powered equipment, often have shiny metal anodes. 1T4, 1R5 and 1A7 are examples. Gas filled tubes such as thyratrons may also use a shiny metal anode since the gas present inside the tube allows for convection of heat from the anode to the glass enclosure.
The outer electrode in most tubes is the anode (plate). Some small signal types, such as sharp and remote cut-off R.F. and A.F. pentodes and some pentagrid converters have a shield fitted around all the electrodes enclosing the anode. This shield is sometimes a solid metal sheet, treated to make it dull and gray so that it can itself reradiate heat generated from within. Sometimes it is fabricated from expanded metal mesh, acting as a Faraday cage but allowing sufficient infrared radiation from the anode to escape. Types 6BX6/EF80 and 6BK8/EF86 are typical examples of this shielded type using expanded mesh. Types 6AU6/EF94 and 6BE6/EK90 are examples which use a gray sheet metal cylindrical shield.

Tube packages High Power

Most modern tubes have glass envelopes, but metal, fused quartz (silica), and ceramic have also been used.

High power GS-9B triode transmitting tube with heat sink at bottom.

The first version of the 6L6 used a metal envelope sealed with glass beads, while a glass disk fused to the

Metal cased tubes with "octal" bases

metal was used in later versions. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor was a modern receiving tube using a very small metal and ceramic package.
Tubes have always had their internal elements connected to external circuitry using pins at their base which plug into a socket. After all, tubes needed to be replaced rather frequently unlike modern semiconductor devices which are mostly soldered in place. Subminiature tubes were produced using wire leads rather than sockets, however these were restricted to rather specialized applications. In addition to the connections at the base of the tube, many early triodes connected the grid using a metal cap at the top of the tube; this was done in order to reduce stray capacitance between the grid and the plate leads. Tube caps were also used for the plate (anode) connection, particularly in transmitting tubes and tubes using a very high plate voltage.
High power tubes such as transmitting tubes have packages designed more to enhance heat transfer. In some tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 4CM2500KG, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt.)

Electron Tube Thermionic Valve

Vacuum tubes with two active elements ("diodes") are used for rectification. Ones with 3 or more elements ("triodes", "tetrodes", etc.) are used for amplification, functions which rely on amplification such as oscillators,

Modern vacuum tubes, mostly miniature style

In electronics, a vacuum and switching.
Standard tubes with cathode, grids, and anode are classified in various ways according to requirements. They

Early RCA triode vacuum tube, type 808

may be classified by frequency range (audio, radio, vhf, uhf, microwave), power rating (small-signal, audio power, high-power radio transmitting), design purpose (sharp versus remote cutoff; amplifying and switching, control and signal amplification, rectification, mixing, general-purpose versus very low microphonic and low noise audio amplification, and so on). Distinctions are not necessarily sharp; for example similar dual triodes can be used for audio preamplification and as flip-flops in computers, although linearity is important in the

Triodes as they evolved over 40 years of tube manufacture, from the RE16 in 1918former case and long life in the latter.
Other tubes which could be called vacuum tubes have different construction and different functions, such as cathode ray tubes which create a beam of electrons for display purposes (such as the television picture tube)

Vacuum tube with plate (anode) cut open revealing grid.

in addition to more specialized functions such as electron microscopy and electron beam lithography. X-ray tubes are also vacuum tubes. Phototubes and photomultipliers also rely on electron flow through a vacuum, though in this case the emission of electrons from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles.