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The Klystron 81

transit time no longer may be ignored; an appreciable fraction of a cycle of input signal may be required for an electron to leave the cathode space charge, pass through the grid wires, and travel through the space between grid and plate.

Effects of The effect of lead induct-

Leod Inductance ance is two-fold. First, as shown in figure 21, the combination of grid-lead inductance, grid-cathode capacitance, and cathode lead inductance tends to reduce the effective grid-cathode signal voltage for a constant voltage at the tube terminals as the frequency is increased. Second, cathode lead inductance tends to introduce undesired coupling between the various elements within the tube.

Tubes especially designed for v-h-f and u-h-f use have had their lead inductances minimized. The usual procedures for reducing lead inductance are: (1) using heavy lead conductors or several leads in parallel (examples are the 6SH7 and 6AK5), (2) scaling down the tube in all dimensions to reduce both lead inductances and interelectrode capacitances (examples are the 6AK5, 6F4, and other acorn and miniature tubes), and (3) the use of very low inductance extensions of the elements themselves as external connections (examples are lighthouse tubes such as the 2C40, oilcan tubes such as the 2C29, and many types of v-h-f transmitting tubes).

Effect of When an electron tube is op-

Transit Time erated at a frequency high enough that electron transit time between cathode and plate is an appreciable fraction of a cycle at the input frequency, several undesirable effects take place. First, the grid takes power from the input signal even though the grid is negative at all times. This comes about since the grid will have changed its potential during the time required for an electron to pass from cathode to plate. Due to interaction, and a resulting phase difference between the field associated with the grid and that associated with a moving electron, the grid presents a resistance to an input signal in addition to its normal cold capacitance. Further, as a result of this action, plate current no longer is in phase with grid voltage.

An amplifier stage operating at a frequency high enough that transit time is appreciable:

(a) Is difficult to excite as a result of grid loss from the equivalent input grid resistance,

(b) Is capable of less output since trans-conductance is reduced and plate current is not in phase with grid voltage.

The effects of transit time increase with the square of the operating frequency, and they

increase rapidly as frequency is increased above the value where they become just appreciable. These effects may be reduced by scaling down tube dimensions; a procedure which also reduces lead inductance. Further, transit-time effects may be reduced by the obvious procedure of increasing electrode potentials so that electron velocity will be increased. However, due to the law of electron-motion in an electric field, transit time is increased only as the square root of the ratio of operating potential increase; therefore this expedient is of limited value due to other limitations upon operating voltages of small electron tubes.

Special Microwave Electron Tubes

Due primarily to the limitation imposed by transit time, conventional negative-grid electron tubes are capable of affording worthwhile amplification and power output only up to a definite upper frequency. This upper frequency limit varies from perhaps 100 Mc. for conventional tube types to about 4000 Mc. for specialized types such as the lighthouse tube. Above the limiting frequency, the conventional negative-grid tube no longer is practicable and recourse must be taken to totally different types of electron tubes in which electron transit time is not a limitation to operation. Three of the most important of such microwave tube types are the klystron, the magnetron, and the travelling wave tube.

The Power Klystron The klystron is a type of electron tube in which electron transit time is used to advantage. Such tubes comprise, as shown in figure 22, a cathode, a focussing electrode, a resonator connected to a pair of grids which afford velocity modulation of the electron beam (called the buncher ), a drift space, and another resonator connected to a pair of grids (called the catcher ). A collector for the expended electrons may be included at the end of the tube, or the catcher may also perform the function of electron collection.

The tube operates in the following manner: The cathode emits a stream of electrons which is focussed into a beam by the focussing electrode. The stream passes through the buncher where it is acted upon by any field existing between the two grids of the buncher cavity. When the potential between the two grids is zero, the stream passes through without change in velocity. But when the potential between the two grids of the buncher is increasingly positive in the direction of electron

collector plate catcher

Figure 22


A conventional two-cavlty klystron Is shown with о feedback loop connected between the two eovllles so that the tube may be used as an oscillator.

motion, the velocity of the electrons in the beam is increased. Conversely, when the field becomes increasingly negative in the direction of the beam (corresponding to the other half cycle of the exciting voltage from that which produced electron acceleration) the velocity of the electrons in the beam is decreased.

When the velocity-modulated electron beam reaches the drift space, where there is no field, those electrons which have been sped up on one half-cycle overtake those immediately ahead which were slowed down on the other half-cycle. In this way, the beam electrons become bunched together. As the bunched groups pass through the two grids of the catcher cavity, they impart pulses of energy to these grids. The catcher grid-space is charged to different voltage levels by the passing electron bunches, and a corresponding oscillating field is set up in the catcher cavity. The catcher is designed to resonate at the frequency of the velocity-modulated beam, or at a harmonic of this frequency.

In the klystron amplifier, energy delivered by the buncher to the catcher grids is greater than that applied to the buncher cavity by the input signal. In the klystron oscillator a feedback loop connects the two cavities. Coupling to either buncher or catcher is provided by small loops which enter the cavities by way of concentric lines.

The klystron is an electron-coupled device. Wheti used as an oscillator, its output voltage is rich in harmonics. Klystron oscillators of various types afford power outputs ranging from less than 1 watt to many thousand watts. Operating efficiency varies between 5 and 30 per cent. Frequency may be shifted to some extent by varying the beam voltage. Tuning is

Figure 23 REFLEX KLYSTRON OSCILLATOR A conventionai reflex klystron oscillator of the type commonly used as a local oscillator in superheterodyne receivers operating above about 2000 Mc is shown above. Frequency modulation of the output frequency of the oscillator, or a-f-c operation in a receiver, may be obtained by varying the negative voltage on the repeller electrode.

carried on mechanically in some klystrons by altering (by means of knob settings) the shape of the resonant cavity.

The Reflex Klystron The two-cavity klystron as described in the preceding paragraphs is primarily used as a transmitting device since quite reasonable amounts of power are made available in its output circuit. However, for applications where a much smaller amount of power is required-power levels in the milliwatt range - for low-power transmitters, receiver local oscillators, etc., another type of klystron having only a single cavity is more frequently used.

The theory of operation of the single-cavity klystron is essentially the same as the multi-cavity type with the exception that the velocity-modulated electron beam, after having left the buncher cavity is reflected back into the area of the buncher again by a repeller electrode as illustrated in figure 23. The potentials on the various electrodes are adjusted to the value such that proper bunching of the electron beam will take place just as a particular portion of the velocity-modulated beam reenters the area of the resonant cavity. Since this type of klystron has only one circuit it can be used only as an oscillator and not as an amplifier. Effective modulation of the frequency of a single-cavity klystron for FM work can be obtained by modulating the repeller electrode voltage.

i- plate 1


grid anode anode

terminal terminal glass


GRID \ \

Figure 24 CUTAWAY VIEW OF WESTERN ELECTRIC 416-B/6280 VHF PLANAR TRIODE TUBE The 416-B, designed by the Bell Telephone Laboratories is intended for amplifier or frequency multiplier service in the 4000 mc region. Employing grid wires having a diameter equal to fifteen wavelengths of light, the 416-B has a transconductance of 50,000. Spacing between grid ond cathode is ,0005 , to reduce transit time effects. Entire tube is gold plated.

The Magnetron The magnetron is an s-h-f oscillator tube normally employed where very high values of peak power or moderate amounts of average power are required in the range from perhaps 700 Mc. to 30,000 Mc. Special magnetrons were developed fot wartime use in radar equipments which had peak power capabilities of several million watts (megawatts) output at frequencies in the vicinity of 3000 Mc. The normal duty cycle of operation of these radar equipments was approximately 1/10 of one per cent (the tube operated about 1/1000 of the time and rested for the balance of the operating period) so that the average power output of these magnetrons was in the vicinity of 1000 watts.








Figure 25


An external tank circuit Is used with tills type of magnetron oscillator for operation in the lower u-h.f range.

In its simplest form the magnetron tube is a filament-type diode with two half-cylindrical plates or anodes situated coaxially with respect to the filament. The construction is illustrated in figure 25A. The anodes of the magnetron are connected to a resonant circuit as illustrated on figure 25B. The tube is surrounded by an electromagnet coil \diich, in turn, is connected to a low-voltage d-c energizing source through a rheostat R for controlling the strength of the magnetic field. The field coil is oriented so that the lines of magnetic force it sets up are parallel to the axis of the electrodes.

Under the influence of the strong magnetic field, electrons leaving the filament are deflected from their normal paths and move in circular orbits within the anode cylinder. This effect results in a negative resistance which sustains oscillations. The oscillation frequency is very nearly the value determined by L and C. In other magnetron circuits, the frequency may be governed by the electron rotation, no external tuned circuits being employed. Wavelengths of less than 1 centimeter have been produced with such circuits.

More complex magnetron tubes employ no external tuned circuit, but utilize instead one

or raore resonant cavities which are integral

with the anode structure. Figure 26 shows a magnetron of this type having a multi-cellular

Cathode anode straps anode bldck

cathode leads

.maonetron permanent jL . magnet

waveguide INPUT




Figure 26


llluslrated is an external-anode strapped magnetron of the type commonly used In radar equipment for the TO-cm, range. A permanent magnet of the general type used with such a magnetron is shown In the right-hand portion of the cfraw/ng, with the magnetron fn place between the pole pieces of the magnet.

anode of eight cavities. It will be noted, also, that alternate cavities (which would operate at the same polarity when the tube is oscillating) are strapped together. Strapping was found to improve the efficiency and stability of high-power radar magnetrons. In most radar applications of magnetron oscillators a powerful permanent magnet of controlled characteristics is employed to supply the magnetic field rather than the use of an electromagnet.

The Travelling The Travelling Wave Tube Wove Tube (figure 27) consists of a helix

located within an evacuated envelope. Input and output terminations are affixed to each end of the helix. An electron beam passes through the helix and interacts with a wave travelling along the helix to produce broad band amplification at microwave frequencies.

When the input signal is applied to the gun end of the helix, it travels along the helix wire at approximately the speed of light. However, the signal velocity measured along the axis of the helix is considerably lower. The electrons emitted by the cathode gun pass axially through the helix to the collector, located at the output end of the helix. The average velocity of the electrons depends upon the potential of the collector with respect to the cathode. When the average velocity of the electrons is greater than the velocity of the helix wave, the electrons become crowded together in the various regions of retarded field, where they impart energy to the helix wave. A power gain of 100 or more may be produced by this tube.

The Cathode-Roy Tube

The Cothode-Ray Tube The cathode-ray tube is a special type of

Figure 27 THE TRAVELLING WAVE TUBE Operation of this tube is the result of interaction between the electron beam and wave travelling along the helix.

electron tube which permits the visual observation of electrical signals. It may be incorporated into an oscilloscope for use as a test instrument or it may be tbe display device for radar equipment or a television receiver.

Operotion of A cathode-ray tube always in-the CRT eludes an electron gun for pro-

ducing a stream of electrons, a grid for controlling the intensity of the electron beam, and a luminescent screen for converting the impinging electron beam into visible light. Such a tube always operates in conjunction with either a built-in or an external means for focussing the electron stream into a narrow beam, and a means for deflecting the electron beam in accordance with an electrical signal.

The main electrical difference between types of cathode-ray tubes lies in the means employed for focussing and deflecting the electron beam. The beam may be focussed and/or deflected either electrostatically or magnetically, since a stream of electrons can be acted upon either by an electrostatic or a magnetic field. In an electrostatic field the electron beam tends to be deflected toward the positive termination of the field (figure 28). In a magnetic field the stream tends to be deflected at right angles to the field. Further, an electron beam tends to be deflected so that it is normal (perpendicular) to the equipotential lines of an electrostatic field-and it tends to be deflected so that it is parallel to the lines of force in a magnetic field.

Large cathode-ray tubes used as kinescopes in television receivers usually are both focused and deflected magnetically. On the other hand, the medium-size CR tubes used in oscilloscopes and small television receivers usually are both focused and deflected electrostatically. But CR tubes for special applications may be focused magnetically and deflected electrostatically or vice versa.

There are advantages and disadvantages to


The Cathode Ray Tube 85


V \\ electrode (F)



cathode (k)




Figure 28


both types of focussing and deflection. However, it may be stated that electrostatic deflection is much better than magnetic deflection when high-frequency waves are to be displayed on the screen; hence the almost universal use of this type of deflection for oscillographic work. But when a tube is operated at a high value of accelerating potential so as to obtain a bright display on the face of die tube as for television or radar work, the use of magnetic deflection becomes desirable since it is relatively easier to deflect a high-velocity electron beam magnetically than electrostatically. However, an ion trap is required with magnetic deflection since the heavy negative ions emitted by the cathode are not materially deflected by the magnetic field and hence would burn an ion spot in the center of the luminescent screen. With electrostatic deflection the heavy ions are deflected equally as well as the electrons in the beam so that an ion spot is not formed.

Construction of The construction of atypical Electrostatic CRT electtostatic-focus, electrostatic-deflection cathode-ray tube is illustrated in the pictorial diagram of figure 28. The indirectly heated cathode К releases free electrons when heated by the enclosed filament. The cathode is surrounded by a cylinder G, which has a small hole in its front for the passage of the electron stream. Although this element is not a wire mesh as is the usual grid, it is known by the same name because its action is similar: it controls the electron stream when its negative potential is varied.

Next in order, is found the first accelerating anode, H, which resembles another disk or cylinder with a small hole in its center. This electrode is run at a high or moderately high positive voltage, to accelerate the electrons towards the far end of the tube.

The focussing electrode, F, is a sleeve which usually contains two small disks, each with a small hole.

After leaving the focussing electrode, the electrons pass through another accelerating

anode. A, which is operated at a high positive potential. In some tubes this electrode is operated at a higher potential than the first accelerating electrode, H, wile in other tubes both accelerating electrodes are operated at the same potential.

The electrodes which have been described up to this point constitute the electron gun, which produces the free electrons andfocusses them into a slender, concentrated, rapidly-traveling stream for projecting onto the viewing screen.

Electrostatic To make the tube useful, means Deflection must be provided for deflecting the electron beam along two axes at right angles to each other. The more common tubes employ electrostatic deflection plates, one pair to exert a force on the beam in the vertical plane and one pair to exert a force in the horizontal plane. These plates are designated as В and С in figure 28.

Standard oscilloscope practice with small cathode-ray tubes calls for connecting one of the В plates and one of the С plates together and to the high voltage accelerating anode. With the newer three-inch tubes and with five-inch tubes and larger, all four deflecting plates are commonly used for deflection. The positive high voltage is grounded, instead of the negative as is common practice in amplifiers, etc., in order to permit operation of the deflecting plates at a d-c potential at or near ground.

An Aquadag coating is applied to the inside of the envelope to attract any secondary electrons emitted by the flourescent screen.

In the average electrostatic-deflection CR tube the spot will be fairly well centered if all four deflection plates are returned to the potential of the second anode (ground). However, for accurate centering and to permit moving the entire trace either horizontally or vertically to permit display of a particular waveform, horizontal and vertical centering controls usually are provided on the front of the oscilloscope.

After the spot is once centered, it is necessary only to apply a positive or negative voltage (with respect to ground) to one of the ungrounded or free deflector plates in order to move the spot. If the voltage is positive with respect to ground, the beam will be attracted toward that deflector plate, while if negative the beam and spot will be repulsed. The amount of deflection is directly proportional to the voltage (with respect to ground) that is applied to the free electrode.

With the larger-screen higher-voltage tubes it becomes necessary to place deflecting voltage on both horizontal and both vertical plates. This is done for two reasons: First, the amount of deflection voltage required by the high-

-focus COIL



/ /control

/ grid(g) cathode(k)


voltage tubes is so great that a transmitting tube operating from a high voltage supply would be required to attain this voltage without distortion. By using push-pull deflection with two tubes feeding the deflection plates, the necessary plate supply voltage for the deflection amplifier is halved. Second, a certain amount of de-focussing of the electron stream is always present on the extreme excursions in deflection voltage when this voltage is applied only to one deflecting plate. When the deflecting voltage is fed in push-pull to both deflecting plates in each plane, there is no de-focussing because the average voltage acting on the electron stream is zero, even though the net voltage (which causes the deflection) acting on the stream is twice that on either plate.

The fact that the beam is deflected by a magnetic field is important even in an oscilloscope which employs a tube using electrostatic deflection, because it means that precautions must be taken to protect the tube from the transformer fields and sometimes even the earths magnetic field. This normally is done by incorporating a magnetic shield around the tube and by placing any transformers as far from the tube as possible, oriented to the position which produces minimum effect upon the electron stream.

Construction of Electro- The electromagnetic mognetic CRT cathode-ray tube allows

greater definition than does the electrostatic tube. Also, electromagnetic definition has a number of advantages when a rotating radial sweep is required to give polar indications.

The production of the electron beam in an electromagnetic tube is essentially the same as in the electrostatic tube. The grid structure is similar, and controls the electron beam in an identical manner. The elements of a typical electromagnetic tube are shown in figure 29. The focus coil is wound on an iron core which may be moved along the neck of the tube to focus the electron beam. For final adjustment.

Figure 30

Two poirs of coils arranged for electromagnetic deflection in two c/irections.

the current flowing in the coil may be varied. A second pair of coils, the deflection coils are mounted at right angles to each other around the neck of the tube. In some cases, these coils can rotate around the axis of the tube.

Two anodes are used for accelerating the electrons from the cathode to the screen. The second anode is a graphite coating (Aquadag) on the inside of the glass envelope. The function of this coating is to attract any secondary electrons emitted by the flourescent screen, and also to shield the electron beam.

In some types of electromagnetic tubes, a first, or accelerating anode is also used in addition to the Aquadag.

Electromagnetic A magnetic field will deflect Deflection an electron beam in a direc-

tion which is at right angles to both the direction of the field and the direction of motion of the beam.

In the general case, two pairs of deflection coils are used (figure 30). One pair is for horizontal deflection, and the other pair is for vertical deflection. The two coils in a pair are connected in series and are wound in such directions that the magnetic field flows from one coil, through the electron beam to the other coil. The force exerted on the beam by the field moves it to any point on the screen by application of the proper currents to these coils.

The Troce The human eye retains an image for about one-sixteenth second after viewing. In a CRT, the spot can be moved so quickly that a series of adjacent spots can be made to appear as a line, if the beam is swept over the path fast enough. As


Gas Tubes

long as the electron beam strikes in a given place at least sixteen times a second, the spot will appear to the human eye as a source of continuous light with very little flicker.

Screen Materials- At least five types of lumi- Phosphors nescent screen materials

are commonly available on the various types of CR tubes commercially available. These screen materials are called phosphors; each of the five phosphors is best suited to a particular type of application. The P-1 phosphor, which has a green flourescence with medium persistence, is almost invariably used for oscilloscope tubes for visual observation. The P-4 phosphor, with white fluorescence and medium persistence, is used on television viewing tubes ( Kinescopes ). The P-5 and Р-П phosphors, with blue fluorescence and very short persistence, are used primarily in oscilloscopes where photographic recording of the trace is to be obtained. The P-7 phosphor, which has a blue flash and a long-persistence greenish-yellow persistence, is used primarily for radar displays where retention of the image for several seconds after the initial signal display is required.

Gas Tubes

The space charge of electrons in the vicinity of the cathode in a diode causes the plate-to-cathode voltage drop to be a function of the current being carried between the cathode and the plate. This voltage drop can be rather high when large currents are being passed, causing a considerable amount of energy loss which shows up as plate dissipation.

Action of The negative space charge can

Positive Ions be neutralized by the presence of the proper density of positive ions in the space between the cathode and anode. The positive ions may be obtained by the introduction of the proper amount of gas or a small amount of mercury into the envelope of the tube. When the voltage drop across the tube reaches the ionization potential of the gas or mercury vapor, the gas molecules will become ionized to form positive ions. The positive ions then tend to neutralize the space charge in the vicinity of the cathode. The voltage drop across the tube then remains constant at the ionization potential of the gas up to a current drain equal to the maximum emission capability of the cathode. The voltage drop varies between 10 and 20 volts, depending upon the particular gas employed, up to the maximum current rating of the tube.

Mercury Vapor Mercury-vapor tubes, although Tubes very widely used, have the

disadvantage that they must be operated within a specific temperature range (25° to 70° C.) in order that the mercury vapor pressure within the tube shall be within the proper range. If the temperature is too low, the drop across the tube becomes too high causing immediate overheating and possible damage to the elements. If the temperature is too high, the vapor pressure is too high, and the voltage at which the tube will flash back is lowered to the point where destruction of the tube may take place. Since the ambient temperature range specified above is within the normal room temperature range, no trouble will be encountered under normal operating conditions. However, by the substitution of xenon gas for mercury it is possible to produce a rectifier with characteristics comparable to those of the mercury-vapor tube except that the tube is capable of operating over the range from approximately -70° to 90° C. The 3B25 rectifier is an example of this type of tube.

Thyrotron If a grid is inserted between the ca-Tubes thode and plate of a mercury-vapor gaseous-conduction rectifier, a negative potential placed upon the added element will increase the plate-to-cathode voltage drop required before the tube will ionize or fire. The potential upon the control grid will have no effect on the plate-to-cathode drop after the tube has ionized. However, the grid voltage may be adjusted to such a value that conduction will take place only over the desired portion of the cycle of the a-c voltage being impressed upon the plate of the rectifier.

Voltage Regulotor In a glow-discharge gas tube Tubes the voltage drop across the

electrodes remains constant over a wide range of current passing through the tube. This property exists because the degree of ionization of the gas in the tube varies with the amount of current passing through the tube. When a large current is passed, the gas is highly ionized and the internal impedance of the tube is low. When a small current is passed, the gas is lightly ionized and the internal impedance of the tube is high. Over the operating range of the tube, the product (IR) of the current through the tube and the internal impedance of the tube is very nearly constant. Examples of this type of tube are VR-150, VR-105 and the old 874.

Vacuum Tube Vacuum tubes are grouped into Clossificatian three major classifications:

commercial, ruggedized, and

premium (or reliable). Any one of these three groups may also be further classified for

military duty (JAN classification). To qualify for JAN classification, sample lots of the particular tube must have passed special qualification tests at the factory. It should not be construed that a JANtype tube is better than a commercial tube, since some commercial tests and specifications are more rigid than the corresponding JAN specifications. The JAN-stamped tube has merely been accepted under a certain set of conditions for military service.

Ruggedixed or Radio tubes are being used in Premium Tubes increasing numbers for industrial applications, such as computing and control machinery, and in aviation and marine equipment. When a tube fails in a home radio receiver, it is merely inconvenient, but a tube failure in industrial applications may bring about stoppage of some vital process, resulting in financial loss, or even danger to life.

To meet the demands of these industrial applications, a series of tubes was evolved incorporating many special features designed to ensure a long and pre-determined operating life, and uniform characteristics among similar tubes. Such tubes are known as ruggedized or premium tubes. Early attempts to select re-

triode plate

triode Grid

ate -] r-f


(-fluorescent anode

-ray control electrode


liable specimens of tubes trom ordinary stock tubes proved that in the long run the selected tubes were no better than tubes picked at random. Long life and ruggedness had to be built into the tubes by means of proper choice and 100% inspection of all materials used in the tube, by critical processing inspection and assembling, and by conservative ratings of the tube.

Pure tungsten wire is used for heaters in preference to alloys of lower tensile strength. Nickel tubing is employed around the heater wires at the junction to the stem wires to reduce breakage at this point. Element structures are given extra supports and bracing. Finally, all tubes are given a 50 hour test run under full operating conditions to eliminate early failures. When operated within their ratings, ruggedized or premium tubes should provide a life well in excess of 10,000 hours.

Ruggedized tubes will withstand severe impact shocks for short periods, and will

jj 40

I p= 2.5 ma.

0 10 za 30 40 50 60

Ep (volts)

Figure 32


operate under conditions of vibration for many hours. The tubes may be identified in many cases by the fact that their nomenclature includes a W in the type number, as in 807W, 5U4W, etc. Some ruggedized tubes are included in the 5000 series nomenclature. The 5654 is a ruggedized version of the 6AK5, the 5692 is a ruggedized version of the 6SN7, etc.

4-10 Miscellaneous Tube Types

Electron Xhe electron-ray tube or magic eye Ray Tubes contains two sets of elements, one of which is a triode amplifier and the other a cathode-ray indicator. The plate of the triode section is internally connected to the ray-control electrode (figure 31), so that as the plate voltage varies in accordance with the applied signal the voltage on the ray-control electrode also varies. The ray-control electrode is a metal cylinder so placed relative to the cathode that it deflects some of the electrons emitted from the cathode. The electrons which strike the anode cause it to fluoresce, or give off light, so that the deflection caused by the ray-control electrode, which prevents electrons from striking part of the anode, produces a wedge-shaped electrical shadow on the fluorescent anode. The size of this shadow is determined by the voltage on the ray-electrode. When this electrode is at the same potential as the fluorescent anode, the shadow disappears; if the ray-electrode is less positive than the anode, a shadow appears the width of which is proportional to the voltage on the ray-electrode. Magic eye tubes may be used as tuning indicators, and as balance indicators in electrical bridge circuits. If the angle of shadow is calibrated, the eye tube may be used as a voltmeter where rough naeasurements suffice.

(ma.) 20


Eg 1 = 12.6 V.

в 12 16 20

Eg 1 = 12.6 V

-10 -8 -6 -4 -2 0


Figure 33


Controlled Series heater strings are employed Warm-up in ac-dc radio receivers and tele-Tubes vision sets to reduce the cost, size, and weight of the equipment. Voltage surges of great magnitude occur in series operated filaments because of variations in the rate of warm-up of the various tubes. As the tubes warm up, the heater resistance changes. This change is not the same between tubes of various types, or even between tubes of the same type made by different manufacturers. Some 6-volt tubes show an initial surge as high as 9-volts during warm-up, while slow-heating tubes such as the 25BQ6 are underheated during the voltage surge on the 6-volt tubes.

Standardization of heater characteristics in a new group of tubes designed for series heater strings has eliminated this trouble. The new tubes have either бОО ma. or 400 ma. heaters, with a controlled warm-up time of approximately 11 seconds. The 5U8, 6CG7, and 12BH7-A are examples of controlled warm-up tubes.

Low Introduction of the 12-volt ignition

Plate system in American automobiles

Potential has brought about the design of a Tubes series of tubes capable of operation

with a plate potential of 12-14 volts. Standard tubes perform poorly at low plate potentials, as the amplification factor of the tube drops rapidly as the plate voltage is decreased (figure 32). Contact potential effects, and change of characteristics with variations of filament voltage combine to make operation at low plate potentials even more erratic.

By employing special processing techniques

and by altering the electrode geometry a series of low voltage tubes has been developed by Tung-Sol that effectively perform with all electrodes energized by a 12-volt system. With a suitable power output transistor, this makes possible an automobile radio without a vibrator power supply. A special space-charge tube (12K5) has been developed that delivers 40 milliwatts of audio power with a 12 volt plate supply (figure 33).

Foreign The increased number of imported Tubes radios and high-fidelity equipment

have brought many foreign vacuum tubes into the United States. Many of these tubes are comparable to, or interchangeable

with standard American tubes. A complete listing of the electrical characteristics and base connection diagrams of all general-purpose tubes made in all tube-producing countries outside the Iron Curtain is contained in the Radio Tube Vade Mecum (Worlds Radio Tubes) available at most larger radio parts jobbers for $5.00, or by mail from the publishers of this Handbook at $5-50 posфaid. The Equivalent Tubes Vade Mecum (Worlds Equivalent Tubes) available at the same prices gives all replacement tubes for a given type, both exact and near-equivalents (with points of difference detailed). (Data on TV and special-рифозе tubes if needed is contained in a companion volume Television lubes Vade Mecum).


Transistors and Semi-Conductors

One of the earliest detection devices used in radio was the galena crystal, a crude example of a semiconductor. More modern examples of semiconductors are the copper-oxide rectifier, the selenium rectifier and the germanium diode. All of these devices offer the interesting property of greater resistance to the flow of electrical current in one direction than in the opposite direction. Typical conduction curves for these semiconductors are shown in Figure 1. The copper oxide rectifier action results from the function of a thin film of cuprous oxide formed upon a pure copper disc. This film offers low resistance for positive voltages, and high resistance for negative voltages. The same action is observed in selenium rectifiers, where a film of selenium is deposited on an iron surface.

in ы x kj a 2 < -I


-о.г -0


1N34 STAL Dl(







-20 -10



5-1 Atomic Structure of Germanium and Silicon

It has been previously stated that the electrons in an element having a large atomic number are grouped into rings, each ring having a definite number of electrons. Atoms in which these rings are completely filled are called inert gases, of which helium and argon are examples. All other elements have one or more incomplete rings of electrons. If the incomplete ring is loosely bound, the electrons may be easily removed, the element is called metallic, and is a conductor of electric current. If the incomplete ring is tightly bound, with only a few missing electrons, the element is called non-metallic and is an insulator of electric current. Germanium and silicon fall between these two sharply defined groups, and exhibit both metallic and non-metallic characteristics. Pure germanium or silicon may be considered to be a good insulator. The addition of certain impurities in carefully controlled amounts to the pure germanium will alter the conduaivity of the material. In addition, the choice of the impurity can change the direction of conductivity through the crystal, some impurities increasing conductivity to positive voltages, and others increasing conductivity to negative voltages.

5-2 Mechanism of Conduction

As indicated by their name, semiconductors are substances which have a conductivity intermediate between the high values observed for metals and the low values observed for insulating materials. The mechanism of conduction in semiconductors is different from that

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