Electron Tube

A electron tube ( vacuum tubes American or thermionic valve in English), also called vacuum tube or even lamp , is an active electronics component, generally used like amplifying of Signal. The rectifying or amplifying vacuum tube was replaced in many applications by different Semi-conducteur S, but remains irreplaceable to date in certain fields like the linear amplification of strong power or the ultra high frequencies.

Principle

General structure

The electron tubes indicate the components which use electrode S, placed in the Vide or a Gaz, isolated between them by this medium, and including/understanding at least a source of electron S. an envelope resistant to the temperature isolates the unit from outside. Although the envelope is generally in Verre, the tubes of power often use the Céramique and the Métal. The electrodes are connected to the terminations which pass through the envelope; of course, the passage is tight. On the majority of the tubes, the terminations are pins designed to be installed in a electronic Tube socket for an easy replacement.

Thermionic effect

The vacuum tubes use the effect Thermoïonique to create electrons then to direct them and modulate them. At the origin, the vacuum tube resembles a lamp incandescent, because it has a heating filament inside an envelope of glass emptied of air. When it is hot, the filament slackens electron S in the vacuum: this process is called thermionic emission . It results a cloud from it from electrons, whose load negative, is called “Space charge”. The tubes with direct heating are not used any more except out of tube of strong power, replaced by the indirect heating. In a tube with indirect heating, the filament heats an electrode which emits the electrons, the Cathode.

The diode with vacuum

It is the simplest tube, used like Redresseur. In a Diode with vacuum, the electrons emitted by the Cathode will be attracted by a metal plate ( punt in English) called Anode and located inside the tube. This plate is positively charged. It results a flow from them from electrons, called running, which go from the Filament towards the plate. The power cannot be on in the other direction because the plate is not heated and thus does not emit electrons. One thus obtains a diode with vacuum with direct heating. This component leads the current only in one and single direction.

The triode

The addition of an intermediate electrode of order by Lee De Forest, allowed the development of the first amplifying electronics component: the Three-electrode . The triode is composed of a transmitting Cathode of electron S, of a receiving Anode, and of a grid placed between the two.

While modulating the tension applied to the grid compared to cathode, a number less less large of electrons emitted by cathode arrive until the anode, creating a variable current between anode and cathode. A load in series in the anode converts the current fluctuation into variation of tension and power: amplification is carried out.

Evolution of the triode

See also: Historical of the electron tubes

The triode presented defects, in particular a tendency to oscillate because of the capacity which form the couple cathode-anode. It was quickly improved by the addition of an intermediate grid to a potential close to the anode, reducing this harmful capacity, the Tétrode.

Finally the Pentode made it possible to remove the effect of the secondary emissions of electrons on the grid screen of the tetrode thanks to a third grid with the potential of cathode. Other combinations like the hexode, provided with two grids of order, make it possible to carry out the mixture of frequencies necessary to the receivers.

The evolution continued towards the miniaturization, the tubes multifunction, the improvement of the lifespan and reliability, the increase in the power and the frequency progressively of the needs for the radio and electronics.

Applications

In the majority of the uses, the vacuum tube was replaced by a smaller component and less expensive: the Transistor and its derivatives. This last is a Semi-conducteur and it allows the realization of integrated circuits. The transistor allows a density and a reliability quite higher than the tube for amplification and signal processing. The tubes are however always used for specific applications like the audio amplifiers, and for the applications of “very” strong powers or with High frequency (HF) as for the microwawe ovens, the industrial heating by radio frequency, and the power gain for the television and radio transmitters.

The transistor is however limited in power and frequency by two phenomena: the voluminal dissipation and the time of transit, which make difficult the simultaneous increase in the power and the frequency of operation. To mitigate this HF limitation, of the combinations of amplifiers in parallel are possible, but ask multiple couplers in entry and exit, and higher costs with a single tube amplifier of the same power. The vacuum tubes on the contrary do not dissipate any energy in the transit of the electrons, but only with the impact on the anode, which can be cooled.

In sound diffusion, certain people find that the electron tubes allow a quality of its higher than the systems transistors. It is thus not very probable that they disappear, considering the main interest that the Audiophile S carry to them, in particular for the amplifiers of Electric guitar.

Triodes and pentodes of power

In spite of the technological advance of the semiconductors of powers, the vacuum tubes thus kept the advantage for reliability and the cost price during their use out of amplifiers of strong power in certain fields. Three-electrode S out of common grid, or the Pentode S in common cathode are the most used diagrams, until approximately 100 MHz. The powers vary from 500 W for an amplifier of Radioamateur with a triode 3-500Z cooled by radiation, to 1 MW for a transmitter of broadcasting.

Design of the amplifiers has tubes

Classes of operation

Amplifying diagrams

The tube amplifiers use the three usual diagrams also applied for the transistors:

the assembly “roasts common” where the signal to be amplified is applied to cathode, with the advantage of a weak coupling input-output allowing to avoid the “Neutrodynage”. The disadvantage is a limited power gain (from ten to twenty times), these amplifiers thus require a preamplifier able to deliver 100 W for a power of final exit of 1 kw for example. This diagram is well adapted to the amplifiers functioning on all the tape HF (high frequency), for example for the Marine, the Radioamateur S or telecommunications professional HF;
the assembly in “common cathode” used with the pentodes and tetrodes, where the signal to be amplified is applied to the grid with high impedance, the power gain is very high, the preamplifier is thus economic, but the coupling input-output in general requires a neutrodynage except in BF (low frequency): (amplifiers of stages, vibrating pots, etc). Out of HF the neutrodynage on a frequency is easy to realize, it is thus the usual diagram of the transmitters of broadcasting out of PO (small waves) and OC (short waves);
the assembly in “common anode” also called “following” or “ cathode follower ”, where the signal enters on the grid and leaves on cathode. It is an aerial matching transformer without profit in tension, little used except for the amplifiers Hi-Fi.

Dissipation and cooling

In an electron tube of small signals, the principal source of heat is the filament, whose dissipation is carried out by radiation. In a tube of power, the cooling of the anode is the principal limitation in power. It is solved by radiation for the powers lower than kw, by letting the anode go up at the maximum temperature compatible with material used (Métal to 100-200 W, Graphite from 200 to 2000 W), the envelope of special glass ensuring the radiation. Air Force cooling is used until a few tens of kw, the “external” anode and being provided with wings of convection. Finally the water circulation, even the water boiling on the Vapotron S makes it possible to dissipate hundreds of kw.

Tubes ultra high frequency

The traditional tubes (three-electrode and pentodes) make it possible to design amplifiers up to approximately 1 GHz. Beyond, of the tubes were especially developed, like the Magnétron, the Travelling wave tube ( dolly-wave tubes ), and the Klystron by combining the magnetic and electrostatic effects. Their names are multiple, but sometimes with few differences except a specific improvement. Among most known:

the magnetron is used in the microwawe ovens and the Radar S.
the Klystron allows higher powers and is mainly used in the military applications of radars and electronic countermeasures, in and the transmitter particle accelerators of television;
the Travelling wave tube (SIGNAL or TWT for Travelling Wave Tubes ) is used in the applications ultra high frequency of average power and weak noise: transmitters of radio-relay system, repeaters of satellites.

The industrial market of these tubes goes to the two extremes:

of the travelling wave tubes from 10 to 50 W able to function during 10 to 15 years in orbit for direct Television. Such a tube costs approximately 50000 €;
with the magnetrons of the domestic furnaces produced per million at a price of production lower than 10 €.

Magnetron

The Magnétron is a vacuum tube without grid, with a central cathode, heated by a filament, and a mass and concentric anode in which several cavity resonators are dug. An axial magnetic field, generally created by two permanent magnets at each end of the tube. The course in spiral (because of the magnetic field) of the electrons is done at a frequency granted to the cavities résonnantes.
The magnetron being car-oscillating, it allows simple assemblies, as in the microwawe ovens.

The powers available are about a few kw uninterrupted (MW peak) with 3 GHz and hundreds of Watts (of the hundreds of kw peak) to 10 GHz. Magnetrons are available up to 35 GHz (Bande Ka).

To obtain these powers a tension of several thousands of volts is necessary.

Nowadays, the magnetron has two principal uses:

the Radar where it is competed with by the Klystron, the SIGNAL (Travelling wave tube) and from now on semiconductors;
the Microwawe oven.

Klystron

The Klystron is a vacuum tube which makes it possible to produce amplifiers of average and strong power with narrow band in ultra high frequencies. Their powers can reach more than 60 [[kw]].

The klystrons are used in particular in the Radar S, the linear particle accelerators, the stations of televisual emission UHF, and the satellite stations of diffusion.

Travelling wave tube

The Travelling wave tube ( dolly-wave tubes ) is used in ultra high frequencies to carry out Amplificateur S of weak, average or strong power. It makes it possible to produce amplifiers with wide strip and very weak Bruit basic. It is appropriate particularly well for the amplifiers of the communications satellite .

The travelling wave tube (SIGNAL, Travelling Wave Tubes ) is composed of four principal parts:

the electron gun (Filament, Cathode, Wehnelt and Anode);
the propeller;
collecting ;
the vacuum chamber.

Other vacuum tubes

A great component count were built between 1920 and 1960 by using the technique of the vacuum tube:

the Photomultiplicateur S which remain currently the most sensitive detectors of light, and which are used in Astronomie and in Nuclear physics;
the tubes meters, replaced by the integrated circuits;
bill-posters with gases, replaced by the bill-posters with LED or LCD;
camera tubes replaced by CCC;
the Cathode tube S still used for the stations of Television, the Oscilloscope S and screens of computers, although the flat panel displays (with plasma or Liquid crystals) spread.

Recent evolutions

At the beginning of the 21e century, the interest which one carries to the vacuum tubes began again, this time with the vacuum tube transmitting field ( Field-emitter microtube ).

This type of tube involved an renewed interest for the tubes; it is appeared as a Integrated circuit. The most current design uses a Cold cathode, where the electrons are emitted by ends of angles, of scale nanometric and generated on the surface of metal cathode.

Let us quote like advantages a great robustness combined with the capacity to provide great powers of exit with a good output. Functioning on the same principle that the traditional tubes, these prototypes were built with a transmitter of electrons made of small points using of the Nanotube S, and by engraving the electrodes as small folding plates (by a technique similar to that which is used to create the microscopic mirrors used in the DIGITAL Light Processing ) which are maintained upright by a Magnetic field.

These microtubes integrated should find applications in the apparatuses using of the Micro-onde S such as the Cellphone S, for the receiving transmitters/Bluetooth and Wi-Fi, the Radar S and the satellite S. Actuellement they are studied for a possible application in the manufacture of flat panel displays.

Simulation of the electron tubes

Technologies of data-processing simulation are also used with the tubes, on SPICE , for example. Many manufacturers directly provide the models of their components, models which will be used by the software of simulation. These models of the manufacturers make it possible to have reliable data leading to correct results. They are however in the majority of the cases a simplification of the real behavior of the modelled tube. In a general way, plus the number of electrodes of the tube increases, plus the model deviates from the real component. Their principal defect is to model correctly only the current of anode, and in the case of the tubes multigrilles, that for a fixed G2 tension.

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