A Shall I help you?
B. Will you give me some information about tetrodes?
A.Certainly.
B.First of all, what is a tetrode?
A.A tetrode is a four element tube.
B.And what is the difference between a tetrode and a triode?
A.You see, a tetrode has a screen grid which is inserted between the control grid and the plate.
B.Oh, now I understand why sometimes a tetrode is called a screengrid tube!
A.Quite right! B. For what purpose is the screen grid inserted between the control grid and the plate?
A.In order to remove the influence of their parasitic capacitance and to avoid the instability of the amplifier. By the way, shall I explain to you the term "parasitic"?
B.No, thank you! I know that there exist parasitic oscillations and currents which are very undesirable in radio engineering.
14 Grammar exercises
14.1 Переведите предложения, обращая внимание на видовременные формы
1. We have already learned that when we double the current, it will reduce four times as much heat. 2. The use of this two-circuit system protects the turbine and the workers from the radioactive radiations. 3. If the number of negative charges is unlike the number of positive charges, the matter will produce electrical effects. 4. The electrons are moving from the filament to the plate and through the circuit to the battery. 5. In the sixties of the last century electrical engineering was just making its first, rather timid, steps. 6. In a normal atom there is an equal number of protons and electrons; this produces an electrical balance. 7. The use of various substances as insulators enabled the constructor to change the colour of the arc itself. 8. The precise frequency control of the transmitter enables the receiver to select its station. 9. When the ball was touching the surface of the rod, some of the static electricity passed to it. 10. As long as the current is steady, the magnetic effect does not represent any continuous expenditure of energy as does the heating effect. 11. Nowadays the incandescent lamp has become the most common source of electric light. 12. A friend of mine will have been working as a radio man for three years before he enters the Institute. 13. By the beginning of the nineteenth century physicists had already been able to produce powerful "artificial" magnets. 14. Man has been using no more than a portion of one per cent of the various forms of energy, which until recently he owed only to the sun. 15. Plate current in a screen grid tube depends to a great degree on the screen voltage and very little on the plate voltage. 16. Early transistors used point contacts. Later, the junction types appeared and soon replaced the older types. 17. A direct current generally remains at constant voltage, that is, the voltage does not rise and fall. 18. When we turn on the light and our electric lamp is burning, the tungsten filament
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of the lamp has about 30 times the resistance what it had, when cold. 19. The stronger the light, the more electrons per second will leave the cathode and so the larger will be the current. 20. If the current does not change, there is no induced e.m.f
14.2 Переведите предложения, обращая внимание на пассивную конструкцию
l.What will happen if the screen grid is inserted into a triode? 2. Arc is formed when plasma is produced between the plate and the cathode. 3. What happens when the arc has been extinguished? 4. Once ionization takes place the plasma is produced. 5. An ammeter should be connected to the circuit in series. 6. Little packets of energy are emitted in space in the form of radio waves 24 centimetres long. 7. A new telephone for noisy places is developed in Japan. The transmitter section of the new telephone is equipped with a special microphone and an amplifier to enlarge the voice of the speaker. 8. The semiconductor resistance thermometers are called thermistors. 9. Thermistors are used for automatic gradual switching on of electrical machines. 10. Thermistors may be used as time relays when an interval is required between the switching on of a given electric device and that of another. 11. This experiment has just been carried out. 12. Electric signals are amplified with the help of a semiconductor device. 13. In tuning radio frequency receiver the incoming voltage is amplified at the carrier frequency.
14.3 Переведите предложения, обращая внимание на причастные, герундиальные обороты и отглагольные существительные
1. The electron was discovered more than fifty years ago, but has never been seen. 2. In this type, of receiver a crystal detector is followed by an audio amplifier. 3. Some circuits are affected by changes in line voltage. 4. The distribution of the flux is influence by the frequency of the flux and the resistance of the coil. 5. Most electronic devices are enclosed in glass tubes from which the air has been pumped out. 6. Semiconductor devices which are being used to help electron valves reduce the size of instruments considerably. 7. The receiver has been developed to illustrate the principle of molecular electronics. 8. The turbines are driven by the kinetic energy of the water. 9. A luminescent screen is placed on the inner side of the front face of the tube. 10. When two or more cells are joined, they form a primary battery. 11. A new steam boiler will be installed on a 131-foot tower and will have a heating surface of 1,300 square feet. 12. Pumps which are driven by sunshine energy have been developed in Italy. 13 One may be sure that by the end of our century some new discoveries in the field of X-ray and atomic energy application will have been made by scientists. 14. The electric energy is obtained due to some definite mechanical source. 15. The lamp does not flash when the switch is being turned off. 16. The electric current reaches its maximum value when the magnetic lines are being cut most rapidly by the conductors
14.4 Переведите предложения, обращая внимание на согласование
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времен
1 .It was found that there were three types of coils used in core type transformers. 2. We knew that a dynamo consisted of two essential parts: the field magnet which produced the magnetic field and the armature winch carried the conductors. 3. We saw at once that the inductance of a coil, and therefore also its inductive reactance, were increased when an iron core was put into it. 4. She was asked i. It new Ohm's law. 5. It was proved that radio had been invented by A.S Popov. 6. As a result he established the nuclear theory of atomic line and proved that the atom consisted of electrons revolving around a very small but very heavy nucleus. 7. It was reported that practically all the vacuum tubes used in radio work would depend upon the thermionic conduction for their operation. 8. It was apparent that there was no sharp line of demarcation between conductors and insulators. 9. The physicists assumed that the proton would have the same mass as the electron. 10. Marie Curie found that the ore from which uranium is obtained, gave off rays four times as strong as those of pure uranium
14.5 Переведите предложения, обращая внимание на инфинитивные обороты
1. A vacuum tube is a device consisting of a number of electrodes contained within an evacuated bulb. 2. Increasing the voltage across a resistor, we increase the current which flows through the resistor. 3. The heat produced per second depends both upon the resistance of the conductor and upon the amount of current flowing through it.4. Perhaps the most widely used source of power in large power plants is the steam turbine. 5. Capacity is one of the important properties greatly affecting an electric circuit. 6. The voltage being increased, the field becomes strong enough to cause the electrons to produce additional ions by collision. 7. Using transformer you can increase or decrease the voltage of the alternating current. 8. The energy supplied by the light is sufficient to cause emission from some substances. 9. The fact that a small voltage acting on the grid is equivalent to a large voltage acting on the plate indicates the possibility of amplification with the triode tube. 10. The polarity of the poles does not change, those on one side being permanently of north polarity, those on the other side of south polarity. 11. Part of the energy being changed into heat, not all the chemical energy of the cell or battery is transformed into electric energy. 12. The crowded electrons on the zinc plate of the cell tend to move towards the copper one because of the electrical pressure created. 13. The process by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called detection, or sometimes demodulation. 14. While experimenting in their laboratory Pierre and Marie Curie discovered a new ray-shooting element— radium. 15. Two or more cells connected together constitute a battery. 16. A current passing through the winding of a solenoid acquires magnetic properties. 17. In this way it is possible to make strong magnets that are called electromagnets, the strength of the electromagnet depending on the kind of iron placed within the coil. 18. Being subjected to radiation, various substances undergo radical changes of their properties. 19. Sometimes a transformer is used merely to insulate two electrical circuits while
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still permitting an interchange of energy between them. 20. When heated or subjected to strong electric charges, the cathode emits large quantities of electrons. 21. Being a good insulator, rubber is often used in cables. 22. Having conducted many experiments scientists proved that electricity, like matter, had an atomic character. 23. When focused, the beam from the gun produces only a small spot on the screen. 25. The fluorescent screen materials used have varying characteristics according to the type of work for which the tube is intended. 24. A magnetic substance is any material which acquires this property when placed near a magnet, or near a conductor carrying an electric current. 25. The space surrounding a charged body is called an electric field offer. 26. No matter how small the pieces are into which we cut the magnet, each piece is a magnet in itself having two poles. 27. Having already made remarkable progress, radio and television continue to develop and to find wider and wider application in science, industry and agriculture
15 Texts for supplementary reading
15.1 The power amplifier
The final tube in an audio amplifier which is feeding audio frequencies into a loudspeaker must be essentially a power amplifier. Its task is to deliver undistorted power to the loudspeaker, and not to develop any great amount of voltage amplification. The task of the previous amplifier stages is to build up the small output voltages of the detector, 11 that the large voltages necessary to swing the grid of the rower tube may be obtained. The A-C power in the plate circuits of tubes previous to the power stage is small; what is required is that each previous stage shall give a maximum voltage amplification without distortion, and the fact that maximum power may not be developed in these plate circuits is not important. These tubes work at very high impedances in which it is not possible to generate much power although 11 is possible to build up considerable voltages across them.
The a-c plate current of the last tube then must be rather large and this means that the grid a-c voltages must be large, which in turn means that the Eg— Ip curve of this tube must have a long and straight line part. The 171 tube, for example, which can deliver about 700 milliwatts without much distortion must have an r.m.s. (root mean square) grid voltage applied to it of about 27 volts; there must be a portion of the Eg — Ip curve which is straight over at least twice this characteristic must be straight from minus 78,5 to minus 2,5 volts. The next preceding tube has much smaller voltage swings to handle and so its characteristic need not be straight over such a long part.
Because of the high cost of power apparatus when power tubes requiring high voltages are used the final tube in the average radio receiver is a low R, low resistance tube which will deliver considerable power to the loudspeaker without demanding excessive plate voltages. Such tubes require large input a-c voltages and therefore require greater amplification before them than tubes of higher values of.
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15.2 The task of the radio frequency amplifier
The r.-f amplifier employed in a broadcast frequency receiver may differ decidedly from that used in a receiver serving other purposes or tuned to other frequencies. A broadcast receiver, for example, must be capable of amplifying at any frequency between 500 and 1500 kc. It must be easily changed from one frequency to another, and its amplification at all frequencies within this band should be uniform. It is selects and amplifies too, its task is much more difficult to perform, as we shall see.
The energy thrust upon the other from a given broadcasting station is a complex bit of wave motion. If the microphone is idle, what comes, from the antenna may be considered as a very narrow band, at say 600 kc, called the carrier wave. If a single tone, say 1000 cycles, is put into the microphone, the antenna current has frequencies of not only 600 kc in it but 599 and 601 as well, and when music is broadcast the frequencies in the antenna may be varying between zero and 5000 cycles above and below the carrier from instant to instant. These frequencies on either side of the carrier are called the side bands. The characteristics of the transmitter must be such that each of these audio frequencies is given equal power compared to the others. The resonance curve of the antenna system of the transmitter, then, must not be sharp but must be rather flat or dull. It must have a rather flat top from 5 kc below to 5 kc above its carrier frequency. If audio frequencies up to 5000 cycles are transmitted, each station requires a channel 10 fee wide for its transmission, and if there are 1000 kc available there are 1000 channels or places for 100 simultaneous transmissions.
At the listening station, the receiver must be able to pick out any of these stations, and to receive it without being bothered by others on other channels. This means that a receiver with a good degree of selectivity is one which will receive and amplify signals on the band from 595 to 605 kc but not recognize a signal in the adjacent channels, that is on the channel extending from 605 to 615 kc. In other words, to cope with conditions in the broadcasting band a receiver should have "ten kilocycle selectivity".
15.3 Variable-MU or remote-cutoff pentodes
In certain pentodes, such as the 6K7 and the 6SK7, the grid helix has a variable pitch, so that some of the turns are closer together than others. The closely spaced turns have a greater control over electron flow from the cathode than those with greater separation, and as a result the mutuachar-acteristics of the tube are considerably modified. As the control grid is made more negative, cutoff of the electron stream is approached much more gradually than if the grid spacing were uniformly close. Tubes of this type are known as variable-mu, or remote-cutoff, or super-control pentodes. A typical example is the 6K7.
In radio receivers, which are required to respond to signals covering a wide range of intensity, the grid bias is made to depend upon the amplifier output, and as a result the sensitivity is automatically reduced when a strong signal is being received. This type of control, known as automatic volume control or a.v.c. is incorporated in practically all modern receivers.
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15.4 Dual-rurpose tubes
For reasons of economy or convenience several functions that would otherwise be accomplished by two or more tubes may be handled by a single multipurpose tube. Such a tube consists of the elements of two or more tubes all mounted within a single envelope, each unit acting independently of the others, or it may be a combination that depends for its operation of some sort between the several elements.
An example of the first class in the twin triode, such as the 608-G, which contains all the elements of two entirely distinct triodes, except that a single heater is used for both cathodes. It may be used in any circuit applications calling for two similar triodes. Another example is the 3 A8-GT, which contains a diode, a triode and a pentode but in this case the cathode, which is of the filamentary type, is common to all three. An added feature making for flexibility is that a filament tap is brought out, so that the tube may be operated at either 1,4 or 2,8 Vfor filament heating, by using the two halves in parallel or series respectively. The second class of multipurpose tubes can be illustrated by the 6A8, called a pentagrid converter. It contains five grids, which are referred to by number counting from cathode towards plate. In a typical application. No. 1 grid and No. 2 grid serve as grid and anode respectively of a triode oscillator. Grids No. 3 and No. 5 are tied together and serve as screen, shielding No. 2 and No. 4 from each other and from the plate. Grid No. 4 receives the incoming signal, and its A.C. voltage modulates the electron stream passing from the oscillator section to the plate. The plate current is then a combination of the effects of oscillator voltage and the signal voltage, but the only type of coupling between the two sources is by the electron flow, electrically the two circuits are isolated by the shielding action of the two screen grids. Among other tubes depending on electron coupling are the 6L7, the 6J8, and the 6K8, which should be studied in the tube manuals
15.5 Filter circuits
The output wave forms are entirely satisfactory for many applications, such as the operation of relays, battery charging, and so forth, but they are not smooth and continuous enough to be useful for B-voltage supply of amplifiers and radio receivers. Service of this sort requires that the supply voltage be practically pure D.C., with very little ripple superimposed upon it. Smoothing of the rectified A.C. voltage is accomplished by the use of filter circuits composed of inductance and capacitance or resistance and capacitance.
Another form of filter circuit is known as the condenser-input filter, since the condenser Cl is supplied directly by the rectifier. In operation, the condenser Cl is charged to the peak voltage available from the rectifier and this charge is, withdrawn gradually by the load current. Fluctuations in current and voltage are smoothed out by L and С 1 as in the choke-input filter. No further current is supplied by the rectifier until its voltage is again higher than that remaining on Cl.
In comparing the two types of filters, it is seen that rectified current flows con-
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tinuously in the choke-input filter, whereas it flows for only a brief part of each cycle in the condenser-input circuit. For the same D.C. load current, the peak anode current in the rectifier will therefore be much larger with condenser input than with choke input to the Hiter. Rectifiers for supplying large amounts of load current are commonly provided with filters using choke input, for this reason.
Another comparison between the two circuits concerns the voltage regulation, or the variation of output D.C. voltage with load current.
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