Among the HI FI enthusiasts for several years we are always spoken of in valves amplification as the final solution for every domestic installation and not. Never having spoken here I will try to fill this gap by explaining what are the valves, how they behave and how to design something with them.
In this series of articles I will cover the electronic tubes, they are so called because at the base of their operation there are the electrons that move in an empty space.
Indeed, electronic tubes are formed by a casing, more precisely said bulb, usually glass, in which a vacuum is practiced, extracting the air with a pump and then hermetically closing the bulb itself.
For operation using electronic thermal input, consisting in the emission of electrons by a metal brought to high temperature.
The electrons that are emitted are those that are free in the metal.
Under normal conditions, these electrons can not exit the metal because they do not have sufficient energy to overcome the obstacle consisting of the atoms that are on the same surface of the metal.
To understand what this obstacle, consider the fig.1 in which are indicated some surface atoms of a metal, that is, the atoms that separate the inside of the metal outer space.
To exit from the inside of the metal and get to the free space, the free electrons must pass through these atoms; in fig.1 you see, But, that in the area between the lines denoted by A and B, are found exclusively electrons belonging to atoms on the surface, which together they form a very thin layer which has a negative potential.
This layer constitutes a potential barrier to the free electrons, because it prevents them to move in outer space by repelling towards the interior of metal.
In order for the free electrons are able to overcome the potential barrier and leave the metal is necessary to increase their energy; this can be achieved in different ways, but what interests us here is to increase the temperature of the metal.
The more energy that the electrons acquire increasing metal temperature results in an increase in their speed.
In this way, the fastest among the free electrons can leave the metal, because the potential barrier can no longer stop their quick run to repel them back.
The Thermionic emission depends on both the temperature at which the metal is brought by both the nature of the metal itself.
And 'evident that the emission of electrons is much more abundant than the higher the temperature of the metal and therefore the energy possessed by its free electrons; one can not, however, increase the temperature to what you want, because the metal, reached its melting temperature, pass to the liquid state.
To achieve the same emission of electrons from different metals must take them at temperatures that differ significantly from one metal.
Practically, the electrons necessary to the functioning of the tubes are obtained from metals that provide a good emission at not too high temperatures and sufficiently distant from that of fusion.
Among the metals, the most suitable in this respect is the pure tungsten has proven. The use of pure tungsten is limited, But, to large power tubes used for the transmitters, while for the average power tubes, which is also used for the transmitters, We are used to thoriated tungsten, so called because it is added to the tungsten thorium oxide, that high temperature is transformed into thorium metal spreading on the surface of the tungsten.
With this arrangement is obtained by electronic emission not only much greater than that provided by pure tungsten, but at a lower temperature.
For low-powered tubes that are used in small transmitters and receivers, yes the emission obtained from the oxides of barium and strontium deposited to form a thin layer on a metal (usually nickel), which serves as a support.
The emission obtained is still more abundant of the above cases, while a more sufficing based temperature, which normally is around 700 ° C.
It should be borne in mind that all of these substances do not emit electrons indefinitely, because after a certain number of hours of operation of the tubes they undergo changes as a result of which greatly reduces the number of emitted electrons; under these conditions a tube is said exhausted and is practically unusable, now being insufficient, the number of electrons available for its regular funzionamento.Le substances from which we obtain the electron emission are brought to the required temperature using the thermal effect of the electric current.
In this respect, electronic tubes are divided into two broad categories: the direct ignition pipes and tubes in an indirect ignition.
In the direct ignition tubes the element for the issue of the electrons is fashioned in the form of wire or thin strip and is made to go from a current intensity suitable to bring it to the desired temperature; this element is said filament.
The wire or strip constituting the filament is usually folded on itself and disposed within the bulb of the tube as seen, eg, in Figure. 2-a, in which a normal tube for receivers is illustrated.
In the upper part the filament is supported by a spring which maintains the taut even when it elongates due to the increase of its temperature; at the bottom of the filament has its ends fixed to two legs that come out from the bulb and allow it to connect to the external circuit from which the ignition current flowing through the filament bringing due to temperature.
It says that the filament "turns on" because observing function in a tube through the glass bulb can be seen that its filament assumes a more or less bright red color. Therefore, in the direct ignition tubes the same filament, led to high temperature, emits electrons.
instead, to the electrons in indirect ignition tubes are issued by an element, said cathode, that is brought to the desired temperature by the filament, which shall therefore not the issue of the electrons but only to the heating of the cathode.
The cathode has the shape of a cylinder disposed around the filament, as seen in Fig. 2-b; also the cathode must be connected to the external circuit to the tube, and then is provided with a dedicated pin that protrudes from the lower part of the bulb, close to those of the filament.
In fig. 2-c is shown greatly enlarged section of a portion of the cathode: to electrically isolate the filament from the cathode, between the two elements is placed a cylinder of refractory material, ie of a material that withstands high temperatures. You can see also that the cathode is formed from a nickel cylinder, on which it is deposited the layer of oxides of barium and strontium from which the electron emission occurs.
In an indirect ignition tubes the electrons are always obtained for emission by the layer of oxide which covers the cathode, while for the direct ignition tubes are also used emitters filaments of pure tungsten or thoriated tungsten.
The direct ignition tubes enter more rapidly as a function, after their ignition, of an indirect ignition tubes, because in the latter the heat produced by the filament takes some time to reach the cathode and bring it to the temperature at which the emission takes place; precisely for this reason, after turning on the amplifier, We are a short delay before you can listen to sounds from the popular.
The indirect ignition tubes, however, have the advantage of being able to be switched with alternating current, that you can easily get from the network; the direct ignition tubes, instead, require an ignition current continues in many cases, because the alternating current would give rise to drawbacks.
CLASSIFICATION OF ELECTRONIC TUBES
After seeing how are obtained within the bulb of a tube the electrons necessary for its operation, we can begin to examine the various types of electronic tubes used in radio circuits, to see how to use the electrons emitted from the filament or from the cathode.
In the considerations that follow I will refer to an indirect ignition tubes (in which, as we know, the electron emission occurs by the cathode), because this type of tube is widely used in today's modern electronic circuits.
The operation of the tubes does not depend, however, on the type of the element that emits the electrons, whether the filament or the cathode; in some cases there may, however, be some difference in the connections to the pipe utilization circuit, but what I will talk about when we will deal with such links.
The electrons emitted from the cathode are used in a desired manner by placing within the bulb of the other tube elements, with which you can collect them or affect their movement.
All the elements that are found in the bulb of a tube, including the one that emits electrons, They are called generically ELECTRODES and various types of pipes can be distinguished according to the number of their electrodes.
In an indirect ignition tubes the filament is not included in the number of the electrodes because it does not emit electrons, but serves only to heating of the cathode; instead, in direct ignition tubes the filament is included in the number of the electrodes, as it provides directly issue electronic.
1. – DIODES ELECTRONIC
In this section we will deal with the DIODE, so named because it comprises two electrodes: in addition to the cathode, in diode there is a second electrode which has the task of collecting the electrons emitted by the cathode.
This second electrode is said ANODE or even PLAQUE, because in the first diode realized in 1904 English John Ambrose Fleming(1849-1945) this electrode was precisely constituted by a metal plate disposed adjacent to the cathode.
Currently, in the tubes used for receivers, the anode is made from a nickel cylinder arranged around the cathode and connected to a dedicated pin that serves to connect to the external circuit to the tube, as seen in Fig. 3-a. In this figure are not shown, for simplicity, the supports that hold the electrodes and maintain them in the desired positions.
To represent the diode in the circuit diagrams using the graphic sign shown in fig. 3-b, by means of which indicate the filament, the cathode and the anode, by drawing within a circle representative of the bulb that encloses them.
Very often we use, But, the graphic sign of fig. 3-c, in which it is no longer represented the filament, because of usually no need to indicate, since it knows that it is connected directly to the generator supplying the current ignition.
1.1 – Operation of the diode
It has been said earlier that the anode has the task of collecting the electrons emitted by the cathode; this occurs when the anode is at a positive potential relative to the cathode, because in that case it attracts the electrons which are negatively charged.
To bring the anode at a positive potential relative to the cathode, you can use a battery, by connecting its positive pole to the anode, and its negative pole to the cathode, as seen in Fig. 4-a.
In this case, between the anode and the cathode of the diode there is the same voltage supplied by the battery Ba; the voltage present between the anode and the cathode is called the anode voltage of the diode and is denoted by Va.
The electrons emitted from the cathode reach the anode moving inside the tube in the direction indicated by the arrow marked between the two electrodes in fig. 4-a.
When the electrons reach the anode, reject from this electrode an equal number of electrons, which are attracted to the positive pole of the battery; on the other hand, the electrons that the cathode has succumbed to the anode are replaced by as many electrons from the negative pole of the battery.
The fact that the electrons emitted from the cathode are replaced by other electrons supplied from the battery must not think that the tube can operate indefinitely without ever being exhausted: the tube is exhausted because, after a long period of operation, the surface of its cathode alters losing the property of emitting electrons.
The movement of electrons from the anode to the battery and from this to the cathode constitutes a current that is said ANODE CURRENT and indicates how it is done in fig. 4-a; the path from this current circuit is said CIRCUIT ANODIC.
In fig. 4-we considered in the movement of electrons, assigning accordingly the anode current electronic sense, while the conventional direction of the current, which it is opposite to the electronic sense, therefore the direction of the anode current will be drawn as has been done in Fig. 4-b.
In this way, we imagine that the anode current starts from the positive pole of the battery and returns to the negative pole of the same after passing through the diode from the anode to the cathode, in the sense indicated by the arrow marked between these electrodes in FIG. 4-b.
This manner of indicating the direction of current flow does not cause any inconvenience, as it is sufficient to remember that the necessary current flows actually in the opposite direction to that of conventional, also inside the diode; however, when we examine more complex electronic tubes of the diode, we will have to consider the electrons again to see how the various electrodes affect their movements.
Therefore from now on we will refer to the electrons when we have to consider their behavior inside a tube; When, instead, we will deal with the external circuit at the same tube, will indicate the conventional direction of the current that circulates.
In fig. 4, in addition to the anode circuit, It is also shown the POWER CIRCUIT diode, in which the filament is connected to the heads of Vf battery supplying the current ignition.
As shown, the ignition circuit is completely separated from the anode and no special features worthy of note.
From what has been said so far it is not clear which utility may present the diode. Suppose, But, to exchange links to the battery terminals, connecting the positive pole to the cathode and the negative pole to the anode, as seen in Fig. 4-c.
Now the anode is negative relative to the cathode, and then repels electrons emitted from this electrode instead of attracting them as previously.
Do not happening the passage of electrons between the two electrodes, the circuit in which the diode is interrupted and consequently no current is inserted can circulate more.
Thus we see that the diode is an element having the property of letting pass the electron current from cathode to anode only and not in opposto.Da sense this property of the diode derives its usefulness, eg, with regard to the transformation of alternating current into direct current.
The first operation that takes place in the course of this transformation consists in RECTIFICATION of an alternating current, to obtain a current flowing in a single direction: what provides precisely the diode.
consider, eg, the circuit of FIG. 5 in which an alternating current generator feeds a resistor having a diode in series.
If there were no diode, the current would reverse its direction of movement at every semi-period and the resistor R would be traversed by a direct current in one direction during a semi-period and in the opposite direction in the next half-period.
The presence of the modification diode instead the operation of the circuit, because it allows current to flow in one direction only.
Indeed, during the half-period in which the positive pole of the generator connected to the anode of the diode (fig. 5-a), the current can circulate in the circuit according to the conventional sense indicated by the arrows.
When, instead, seeds in the next period, the same pole of the generator becomes negative (fig. 5-b), the flow, which now would be directed in the opposite direction to the previous, It can no longer move because the circuit is interrupted between the two electrodes of the diode.
The current starts to circulate, still in the direction indicated in Fig. 3-a, when the pole of the generator connected to the anode back to being positive for another half-period.
In this way the resistor is traversed by a direct current always in the same direction, although its passage takes place only for a half-period of each period of the alternating current.
The rectified current obtained is not yet a true DC and need to make it so other circuits, to examine later, because first it is advisable to have a knowledge a little 'more in-depth of the diode operation.
1.2- Characteristic curve of the diode
By applying different voltages anodic and measuring for each of them the corresponding anode current is detected the characteristic curves; this test is carried out by the manufacturers of the tubes, by means of a circuit that, in its simplest form, It is shown in fig. 6.
It is thus seen that immediately, doubling the anode voltage by 10 V a 20 V, the anode current increases more than double, passing the 60 mA and 150 mA; tripling the anode voltage by 10 V a 30 V, the anodic current passes from 60 mA and 280 mA, increasing more than four times.
Not being able to determine with a simple relation as Ohm's law the dependence between the voltage and current, recourse is made to a Cartesian diagram to know which value assumes the current flowing through the diode for each value of the anode voltage.
Combining with a line the points A, B and C corresponding to the measures, a curve is obtained which is called the characteristic curve of the diode or, more simply, feature DIODE.
E’ well, however, have a complete view of the behavior of a diode and for this purpose we must consider how the curve would continue if the anode voltage were increased beyond the value of 30 V.
The curve does not would continue indefinitely with the same pattern, but at some point it would become horizontal, as seen in Fig. 8.
The diagram shown in this figure is intended merely to show the trend of the feature, and then on its axles have not been shown the values of voltage and current.
The fact that the characteristic present a horizontal section means that in correspondence to this same stretch the current has constantly the same value, ie no longer increases as you continue to increase the anode voltage.
The constant current flowing through the diode under these conditions is called the saturation current.
To explain the behavior of the diode we need to consider what happens, in its interior, the electrons emitted from the cathode are attracted by the anode in increasing manner as a function of accelerating voltage, until it reaches a maximum point.
When that happens, You are obtained by the saturation current, whose value, as indicated by the straight portion of the characteristic, It can no longer increase even if it continues to increase the anode voltage, because now all the electrons that the cathode is capable of emitting reach the anode.
In normal applications the diode works with currents related to the curved portion of the characteristic, which it is the only one provided by the manufacturer for an operation without damage to the diode itself.
1.3- Electrical power in the diode
To get a complete view of the diode operation it is still necessary to consider this element from the point of view of electric power.
For this purpose, we consider again the circuit of FIG. 4-and to observe that in this circuit is put into play a power given by the product of the battery voltage Va to the anode current.
Therefore ask ourselves what happens that the battery provides power to the circuit: this power is dissipated in the form of heat in the diode, as well as would happen if it were in its place a resistor.
With regard to the latter aspect, we know that the power is dissipated as heat due to the resistance that the electrons constituting the current meet nell'attraversarlo; in the case of the diode that does not happen, because the electrons that pass from cathode to anode move in a vacuum and therefore do not encounter any resistance in their movement.
We must remember, But, that a moving body possesses energy, which it is precisely said kinetic energy and which depends on the mass and the square of the velocity of the body.
Therefore also the moving electrons in the diode, though they have an extremely low mass, possess a kinetic energy, whose value increases due to the acceleration, it does increase their speed as they approach the anode.
When they reach the anode, the electrons are stopped in their fast race bump against this electrode and thus lose all the kinetic energy, having canceled their speed.
As a result of impact of electrons by the anode it heats up and this means that the kinetic energy possessed by the electrons is transferred to the anode in the form of heat: it can be concluded that in a diode in heat dissipation of the electric power takes place on the anode of the tube.
Due to this fact it is commonly said that a diode occurs in the anode dissipation of electrical power.
Because the speed of the electrons and thus their kinetic energy converted into heat at the anode depends on the anode voltage, it is evident that what is more elevated this tension, the greater the anode dissipation in the diode. Therefore two different diodes, crossed by the same current, dissipate more power that has a higher anode voltage.
To compare two diodes in this respect, It can be used to their characteristics, bringing them back on the same graph, as it was done in the example of fig. 9, for European-type diode EZ81 and the American type diode 6X4.
One sees immediately that in order to cross the two diodes from the same current, e.g. 75 mA, It should be an anodic voltage 12 V for the type EZ81 and 22,5 V for the type 6X4: it can therefore be deduced that, with such current, the second diode has an anode dissipation is almost double of the first.
The features provided by the manufacturers of the diodes are generally presented as those shown in fig. 9, ie they are formed by a first portion in solid line and by a second portion in dotted line, to take account of the anode dissipation.
It should be remembered, indeed, that the heat transferred to the anode as a result of power dissipation causes an increase in electrode temperature: to avoid so that the anode reaches an excessive temperature, such as to come corrupted, the power dissipated in the diode must not exceed a certain maximum value.
The stretch in the entire line feature precisely indicates the voltage values of the continuous and direct current for which the dissipated power does not exceed the maximum value.
When the diode is used to convert alternating current into direct current, the voltage and current can also have values greater than those indicated by the continuous line to stretch, without that exceed the maximum power dissipation, as these values are taken for a very short time.
To know the behavior of the diode in such conditions, track is therefore also the second portion of the curve, with dotted line, to remember that the values given refer to voltages and currents of short duration.
We saw that to limit the anode dissipation is necessary to reduce as far as possible to the anode voltage; when they are involved considerable powers, this is achieved by resorting to a different type of diode from the one considered so far, which we now turn.
2. – I DIODI A GAS
These diodes are practically no longer used because supplanted by those solid-state, I describe here the operation quickly. The diodes are considered previously said A high vacuum or even high vacuum, as the inside of their bulb is extracted almost all the air by means of special pumps.
however you can not pull out the air, then in a tube, there are still a few billion of the various gas molecules that constitute the air itself.
Since these molecules are extremely small, it is virtually impossible that the electrons can to encounter some obstacles that their shortest path from the cathode to the anode; under these conditions the tube then behaves as if in its interior there was the perfect vacuum.
Instead, say gas diodes in which the diodes are introduced specially certain gases to increase the number of molecules present in the bulb, and thus promote the meeting of the electrons with these molecules.
If the anode voltage is low and therefore also the velocity of the electrons is not very high, the behavior of the diode does not differ significantly from that of a vacuum diode pushed.
Increasing, But, the tension, the electrons emitted from the cathode acquire a greater speed and can impact the gas molecules they encounter in their walk with violence such as to disconnect one or more of their peripheral electrons.
This is called impact ionization, because the gas molecules that have lost electrons due to peripheral bump on the part of the electrons emitted from the cathode become positive ions.
The electrons detached from the gas molecules are directed towards the anode together with the electrons emitted from the cathode, while the positive ions are repelled from the anode, which it is also positive, and moving towards the cathode.
The positive ions, having a much greater mass of electrons, They move with much lower speed and therefore remain longer than the electrons in the space between the electrodes, making you feel more time for the effect of their positive charges.
In this way, these fillers can neutralize the negative space charge of the electron cloud which is located near the cathode, consequently nullifying the repulsion that it exerts on the electrons emitted by the cathode itself.
All the emitted electrons can thus reach the anode with an anode voltage applied at the lower tube of what would be needed to achieve the same result in a vacuum diode pushed, with the advantage of a lower anode dissipation.
For the moment I finish here this article but, I will continue the discussion of the vacuum diodes with their use circuits with the next article.