Series RC-22

RADIO CORPORATION OF AMERICA

ELECTRONIC COMPONENTS AND DEVICES HARRISON, N J

Contents

Page

Electrons, Electrodes, and Electron Tubes .... 3

Electrons, Cathodes, Generic Tube Types, Diodes, Triodes, Tetrodes, Pentodes, Beam Power Tubes, Multi-Electrode and Multi-Unit Types, Receiving Tube Structure, Television Picture Tubes

Electron Tube Characteristics . 12

Electron Tube Applications . . \ 14

Amplification, Rectification, Detection, Autoni^atic Volume or Gain Control, Tuning Indication with Electron-Ray Tubes, Oscillation, De- flection Circuits, Frequency Conversion, Automatic Frequency Control

Electron Tube Installation ......... 58

Filament and Heater Power Supply, Heater-to-Cathode Connection, Plate Voltage Supply, Grid Voltage Supply, Screen-Grid Voltage Supply, Shielding, Dress of Circuit Leads, Filters, Output-Coupling Devices, High-Fidelity Systems, High- Voltage Considerations for Television Picture Tubes, Picture-Tube Safety Considerations

Interpretation of Tube Data . 69

Application Guide for RCA Receiving Tubes .... 75

Technical Data for RCA Tube Types 83

Picture-Tube Characteristics Chart 484

Electron Tube Testing 487

Resistance-Coupled Amplifiers 491

Outlines . 500

Circuits .504

Index 536

Reading List 544

Information furnished by RCA is believed to be accurate and reliable. However, no responsibility is assumed by RCA for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of RCA.

Copyright 1963 by Radio Corporation of America (All Rights Reserved)

V Trade Mark(s) Registered '@ Marca(s) Registrada(s)

7-63

Printed in U. S. A.

RCA

Receiving Tube Manual

THIS MANUAL, like its preceding editions, has been prepared to assist those who work or experi- ment with home-entertainment-type electron tubes and circuits. It will be found valuable by engineers, service technicians, educators, experimenters, radio amateurs, hobbyists, students, and many others technically interested in electron tubes.

The material in this edition has been augmented and revised to include the recent technological advances in the electronics field. Many tube types widely used in the design of new electronic equip- ment only a few years ago are now chiefly of interest for renewal purposes. Consequently, in the Tube Types Section, information on many older types is limited to basic essential data ; information on newer and more important types is given in greater detail.

RADIO CORPORATION OF AMERICA

Electronic Components and Devices Harrison, N. J.

2

Electrons, Electrodes, and Electron Tubes

The electron tube is a marvelous device. It makes possible the performing of operations, amazing in conception, with a precision and a certainty that are astounding. It is an exceedingly sensi- tive and accurate instrument— the prod- uct of coordinated efforts of engineers and craftsmen. Its construction requires materials from every corner of the earth. Its use is world-wide. Its future possi- bilities, even in the light of present-day accomplishments, are but dimly fore- seen, for each development opens new fields of design and application.

The importance of the electron tube lies in its ability to control almost in- stantly the flight of the millions of elec- trons supplied by the cathode. It accom- plishes this control with a minimum of energy. Because it is almost instantane- ous in its action, the electron tube can operate efficiently and accurately at electrical frequencies much higher than those attainable with rotating machines.

Electrons

All matter exists in the solid, liquid, or gaseous state. These three forms con- sist entirely of minute divisions known as molecules, which, in turn, are com- posed of atoms. Atoms have a nucleus which is a positive charge of electricity, around which revolve tiny charges of negative electricity known as electrons. Scientists have estimated that electrons weigh only 1/30-billion, billion, billion, billionths of an ounce, and that they may travel at speeds of thousands of miles per second.

Electron movement may be accele- rated by the addition of energy. Heat is one form of energy which can be con- veniently used to speed up the electron. For example, if the temperature of a metal is gradually raised, the electrons in the metal gain velocity. When the metal becomes hot enough, some elec- trons may acquire sufficient speed to

break away from the surface of the metal. This action, which is accelerated when the metal is heated in a vacuum, is utilized in most electron tubes to produce the necessary electron supply.

An electron tube consists of a cath- ode, which supplies electrons, and one or more additional electrodes, which con- trol and collect these electrons, mounted in an evacuated envelope. The envelope may be made of glass, metal, ceramic, or a combination of these materials.

Cathodes

A cathode is an essential part of an electron tube because it supplies the electrons necessary for tube operation. When energy in some form is applied to the cathode, electrons are released. Heat is the forifi of%nergy generally used.The method of heating the cathode may be used to distinguish between the different forms of cathodes. For example, a di- rectly heated cathode, or filament-cath- ode, is a wire heated by the passage of an electric current. An indirectly heated cathode, or heater-cathode, consists of a filament, or heater, enclosed in a metal sleeve. The sleeve carries the electron- emitting material on its outside surface and is heated by radiation and conduc- tion from the heater.

A filament, or directly heated cath- ode, such as that shown in Fig. 1 may be further classified by identifying the filament or electron-emitting material. The materials in regular use are tung- sten, thoriated tungsten, and metals which have been coated with alkaline- earth oxides. Tungsten filaments are made from the pure metal. Because they must operate at high temperatures (a dazzling white) to emit suflScient elec- trons, a relatively large amount of fila- ment power is required.

Thoriated-tungsten filaments are made from tungsten impregnated with thorium oxide. Due to the presence of

3

RCA Receiving Tube Manual

thorium, theie filaments liberate elec- trons at a more moderate temperature of about 1700°C (a bright yellow) and are, therefore, much more economical of filament power than are pure tungsten filaments.

Alkaline earths are usually applied as a coating on a nickel-alloy wire or ribbon. This coating, which is dried in a relatively thick layer on the filament, requires only a relatively low tempera- ture of about 700-750°C (a dull red) to produce a copious supply of electrons. Coated filaments operate very eflficiently and require relatively little filament power. However, each of these cathode materials has special advantages which determine the choice for a particular application.

Fig. 1 Fig. 2

Directly heated filament-cathodes require comparatively little heating power. They are used in almost all of the tube types designed for battery op- eration because it is, of course, desirable to impose as small a drain as possible on the batteries. Examples of battery-oper- ated filament types are the 1R5, 1U4, 1U5, and 3V4. AC-operated types hav- ing directly heated filament-cathodes include the 2A3 and 5Y3GT.

An indirectly heated cathode, or heater-cathode, consists of a thin metal sleeve coated with electron-emitting ma- terial such as alkaline-earth oxides. The emissive surface of the cathode is main- tained at the required temperature (ap- proximately 1050°K) by resistance-heat- ing of a tungsten or tungsten-alloy wire which is placed inside the cathode sleeve and electrically insulated from it, as shown in Fig. 2. The heater is used only for the purpose of heating the cathode sleeve and sleeve coating to an electron- emitting temperature. Useful emission does not take place from the heater wire.

A new dark heater insulating coat- ing developed by RCA has better heat transfer than earlier aluminum-oxide coatings, and makes it possible to operate heaters at lower temperatures for given power inputs. Because the tensile strength of the heater wire increases at the lower operating temperatures, tubes using dark heaters have increased re- liability, stability, and life.

The heater-cathode construction is well adapted for use in electron tubes in- tended for operation from ac power lines and from storage batteries. The use of separate parts for emitter and heater functions, the electrical insulation of the heater from the emitter, and the shield- ing effect of the sleeve may all be utilized in the design of the tube to minimize the introduction of hum from the ac heater supply and to minimize electrical inter- ference which might enter the tube cir- cuit through the heater-supply line. From the viewpoint of circuit design, the heater-cathode construction offers advantages in connection flexibility be- cause of the electrical separation of the heater from the cathode.

Another advantage of the heater- cathode construction is that it makes practical the design of a rectifier tube having close spacing between its cathode and plate, and of an amplifier tube hav- ing close spacing between its cathode and grid. In a close-spaced rectifier tube, the voltage drop in the tube is low, and, therefore, the regulation is improved. In an amplifier tube, the close spacing in- creases the gain obtainable from the tube. Because of the advantages of the heater-cathode construction, almost all present-day receiving tubes designed for ac operation have heater-cathodes.

Generic Tube Types

Electrons are of no value in an elec- tron tube unless they can be put to work. Therefore, a tube is designed with the parts necessary to utilize electrons as well as those required to produce them. These parts consist of a cathode and one or more supplementary elec- trodes. The electrodes are enclosed in an evacuated envelope having the neces- sary connections brought out through air-tight seals. The air is removed from the envelope to allow free movement of

4

Electrons, Electrodes, and Electron Tubes

the electrons and to prevent injury to the emitting surface of the cathode.

When the cathode is heated, elec- trons leave the cathode surface and form an invisible cloud in the space around it. Any positive electric potential within the evacuated envelope offers a strong attraction to the electrons (unlike elec- tric charges attract; like charges repel). Such a positive electric potential can be supplied by an anode (positive elec- trode) located within the tube in prox- imity to the cathode.

Diodes

The simplest form of electron tube contains two electrodes, a cathode and an anode (plate), and is often called a diode, the family name for a two-elec- trode tube. In a diode, the positive po- tential is suppUed by a suitable electrical source connected between the plate terminal and a cathode terminal, as shown in Fig. 3. Under the influence of

[OUTPUT

Fig. 3

the positive plate potential, electrons flow from the cathode to the plate and return through the external plate-bat- tery circuit to the cathode, thus com- pleting the circuit. This flow of electrons is known as the plate current.

If a negative potential is applied to the plate, the free electrons in the space surrounding the cathode will be forced back to the cathode and no plate cur- rent will flow. If an alternating voltage is applied to the plate, the plate is alter- nately made positive and negative. Be- cause plate current flows only during the time when the plate is positive, current flows through the tube in only one direc- tion and is said to be rectified. Fig. 4 shows the rectified output current pro- duced by an alternating input voltage.

Diode rectifiers are used in ac re- ceivers to convert the ac supply voltage to do voltage for the electrodes of the

other tubes in the receiver. Rectifier tubes having only one plate and one cathode, such as the 35W4, are called half-wave rectifiers, because current can flow only during one-half of the alternating-current cycle. When two plates and one or more cathodes are

:\ ALTERNATING 1 VOLTAGE INPUT

Fig. 4

used in the same tube, current may be obtained on both halves of the ac cycle. The 6X4, 5Y3GT, and 5U4GB are ex- amples of this type and are called full-wave rectifiers.

Not all of the electrons emitted by the cathode reach the plate. Some return to the cathode while others remain in the space between the cathode and plate for a brief period to produce an effect known as space charge. This charge has a repelling action on other electrons which leave the cathode surface and im- pedes their passage to the plate. The ex- tent of this action and the amount of space charge depend on the cathode temperature, the distance between the cathode and the plate, and the plate potential. The higher the plate potential, the less is the tendency for electrons to remain in the space-charge region and repel other electrons. This effect may be noted by applying increasingly higher plate voltages to a tube operating at a fixed heater or filament voltage. Under these conditions, the maximum number of available electrons is fixed, but in- creasingly higher plate voltages will succeed in attracting a greater propor- tion of the free electrons.

Beyond a certain plate voltage, however, additional plate voltage has little effect in increasing the plate cur- rent because all of the electrons emitted

5

RCA Receiving Tube Manual

by the cathode are already being drawn to the plate. This maximum current, illustrated in Fig. 5, is called saturation current. Because it is an indication of the total number of electrons emitted, it is also known as emission current or simply emission.

Although tubes are sometimes tested by measurement of their emission cur- rent, it is generally not advisable to measure the full value of emission be- cause this value would be sufficiently large to cause change in the tube's char- acteristics or even to damage the tube. Consequently, while the test value of emission current is somewhat larger than

Sati

rati

)n P

)int

PLATE VOLTAGE

Fig. 6

the maximum current which will be re- quired from the cathode in the use of the tube, it is ordinarily less than the full emission current. The emission test, therefore, is used to indicate whether the cathode can supply a sufficient num- ber of electrons for satisfactory opera- tion of the tube.

If space charge were not present to repel electrons coming from the cathode, the same plate current could be produced at a lower plate voltage. One way to make the effect of space charge small is to make the distance between plate and cathode small. This method is used in rectifier types having heater-cathodes, such as the 6V4GA and the 6AX5GT. In these types the radial distance be- tween cathode and plate is only about two hundredths of an inch.

Another method of reducing space- charge effect is utilized in mercury- vapor rectifier tubes. When such tubes are operated, a small amount of mercury contained in the tube is partially vapor- ized, filling the space inside the bulb with mercury atoms. These atoms are

bombarded by electrons on their way to the plate. If the electrons are moving at a sufficiently high speed, the collisions tear off electrons from the mercury atoms. The mercury atom is then said to be "ionized," i.e., it has lost one or more electrons and, therefore, has a positive charge. Ionization is evidenced by a bluish-green glow between the cathode and plate. When ionization oc- curs, the space charge is neutralized by the positive mercury atoms so that in- creased numbers of electrons are made available. Mercury-vapor tubes are used primarily for power rectifiers.

lonic-heated-cathode rectifiers depend on gas ionization for their opera- tion. These tubes are of the full-wave design and contain two anodes and a coated cathode sealed in a bulb contain- ing a reduced pressure of inert gas. The cathode in each of these types becomes hot during tube operation, but the heat- ing effect is caused by bombardment of the cathode by ions within the tube rather than by heater or filament cur- rent from an external source.

The internal structure of an ionic- heated -cathode tube is designed so that when sufficient voltage is applied to the tube, ionization of the gas occurs be- tween the anode which is instantaneously positive and the cathode. Under normal operating voltages, ionization does not take place between the anode that is negative and the cathode so that the requirements for rectification are satis- fied. The initial small flow of current through the tube is sufficient to raise the cathode temperature quickly to incan- descence whereupon the cathode emits electrons.The voltage drop in such tubes is slightly higher than that of the usual hot-cathode gas rectifiers because energy is taken from the ionization discharge to keep the cathode at operating tempera- ture. Proper operation of these rectifiers requires a minimum flow of load current at all times in order to maintain the cathode at the temperature required to supply sufficient emission.

Triodes

When a third electrode, called the grid, is placed between the cathode and plate, the tube is known as a triode, the family name for a three-electrode tube.

6

Electrons, Electrodes, and Electron Tubes

The grid usually consists of relatively fine wire wound on two support rods (siderods) and extending the length of the cathode. The spacing between turns of wire is large compared with the size of the wire so that the passage of electrons from cathode to plate is practically un- obstructed by the grid. In some types, a frame grid is used. The frame consists of two siderods supported by four metal straps. Extremely fine lateral wire (di- ameter of 0.5 mil or less) is wound under tension around the frame. This type of grid permits the use of closer spacings between grid wires and between tube electrodes, and thus improves tube per- formance.

The purpose of the grid is to control the flow of plate current. When a tube is used as an amplifier, a negative dc volt- age is usually applied to the grid. Under this condition the grid does not draw ap- preciable current.

The number of electrons attracted to the plate depends on the combined effect of the grid and plate polarities, as shown in Fig. 6. When the plate is posi- tive, as is normal, and the dc grid volt- age is made more and more negative, the plate is less able to attract electrons to it and plate current decreases. When the

ELECTRON FLOW

OUTPUT

Fig. 6

grid is made less and less negative (more and more positive), the plate more read- ily attracts electrons to it and plate cur- rent increases. Hence, when the voltage on the grid is varied in accordance with a signal, the plate current varies with the signal. Because a small voltage ap- plied to the grid can control a compara- tively large amount of plate current, the signal is amplified by the tube. Typical three-electrode tube types are the 6C4 and 6AF4A.

The grid, plate, and cathode of a triode form an electrostatic system, each

electrode acting as one plate of a small capacitor. The capacitances are those existing between grid and plate, plate and cathode, and grid and cathode. These capacitances are known as inter- electrode capacitances. Generally, the capacitance between grid and plate is of the most importance. In high-gain radio- frequency amplifier circuits, this capaci- tance may act to produce undesired coupling between the input circuit, the circuit between grid and cathode, and the output circuit, the circuit between plate and cathode. This coupling is un- desirable in an amplifier because it may cause instability and unsatisfactory per- formance.

Tetrodes

The capacitance between grid and plate can be made small by mounting an additional electrode, called the screen grid (grid No. 2), in the tube. With the addition of the grid No.2, the tube has four electrodes and is, accordingly, called a tetrode. The screen grid or grid No.2 is mounted between the grid No.l (con- trol grid) and the plate, as shown in Fig. 7, and acts as an electrostatic shield be- tween them, thus reducing the grid-to- plate capacitance. The effectiveness of

Fig. 7

this shielding action is increased by a bypass capacitor connected between screen grid and cathode. By means of the screen grid and this bypass capacitor, the grid-plate capacitance of a tetrode is made very small. In practice, the grid- plate capacitance is reduced from sev- eral picofarads (pf) for a triode to 0.01 pf or less for a screen-grid tube.

The screen grid has another desir- able effect in that it makes plate current practically independent of plate voltage over a certain range. The screen grid is operated at a positive voltage and.

7

RCA Receiving Tube Manual

therefore, attracts electrons from the cathode. However, because of the com- paratively large space between wires of the screen grid, most of the electrons drawn to the screen grid pass through it to the plate. Hence the screen grid sup- plies an electrostatic force pulling elec- trons from the cathode to the plate. At the same time the screen grid shields the electrons between cathode and screen grid from the plate so that the plate ex- erts very little electrostatic force on electrons near the cathode.

So long as the plate voltage is higher than the screen-grid voltage, plate cur- rent in a screen-grid tube depends to a great degree on the screen-grid voltage and very little on the plate voltage. The fact that plate current in a screen-grid tube is largely independent of plate volt- age makes it possible to obtain much higher amplification with a tetrode than with a triode. The low grid-plate capaci- tance makes it possible to obtain this high amplification without plate-to-grid feedback and resultant instability. In receiving-tube applications, the tetrode has been replaced to a considerable de- gree by the pentode.

Pentodes

In all electron tubes, electrons strik- ing the plate may, if moving at sufficient speed, dislodge other electrons. In two- and three-electrode types, these dis- lodged electrons usually do not cause trouble because no positive electrode other than the plate itself is present to attract them. These electrons, therefore, are drawn back to the plate. Emission caused by bombardment of an electrode by electrons from the cathode is called secondary emission because the effect is secondary to the original cathode emis- sion.

In the case of screen-grid tubes, the proximity of the positive screen grid to the plate offers a strong attraction to these secondary electrons and particu- larly so if the plate voltage swings lower than the screen-grid voltage. This effect lowers the plate current and limits the useful plate-voltage swing for tetrodes.

The effects of secondary emission are minimized when a fifth electrode is placed within the tube between the screen grid and plate.This fifth electrode is known as the suppressor grid (grid

8

No.3) and is usually connected to the cathode, as shown in Fig. 8. Because of its negative potential with respect to the plate, the suppressor grid retards the flight of secondary electrons and diverts them back to the plate.

SUPPRESSOR GRID

Fig. 8

The family name for a five-electrode tube is "pentode". In power-output pentodes, the suppressor grid makes pos- sible higher power output with lower grid-driving voltage; in radio-frequency amplifier pentodes the suppressor grid makes possible high voltage amplifica- tion at moderate values of plate voltage. These desirable features result from the fact that the plate-voltage swing can be made very large. In fact, the plate volt- age may be as low as, or lower than, the screen-grid voltage without serious loss in signal-gain capability. Representative pentodes used for power amplification are the 3V4 and 6K6GT; representative pentodes used for voltage amplification are the 1U4, 6AU6A, 6BA6, and 5879.

Beam Power Tubes

A beam power tube is a tetrode or pentode in which directed electron beams are used to increase substantially the power-handling capability of the tube. Such a tube contains a cathode, a con- trol grid (grid No.l), a screen grid (grid No.2), a plate, and, optionally, a sup- pressor grid (grid No.3). When a beam power tube is designed without an ac- tual suppressor grid, the electrodes are so spaced that secondary emission from the plate is suppressed by space-charge effects between screen grid and plate. The space charge is produced by the slowing up of electrons traveling from a high-potential screen grid to a lower- potential plate. In this low-velocity re- gion, the space charge produced is suffi-

Electrons, Electrodes, and Electron Tubes

cient to repel secondary electrons emit- ted from the plate and to cause them to return to the plate.

Beam power tubes of this design employ beam-confining electrodes at cathode potential to assist in producing the desired beam effects and to prevent stray electrons from the plate from re- turning to the screen grid outside of the beam. A feature of a beam power tube is its low screen-grid current. The screen grid and the control grid are spiral wires wound so that each turn of the screen grid is shaded from the cathode by a grid turn. This alignment of the screen grid and control grid causes the electrons to travel in sheets between the turns of the screen grid so that very few of them strike the screen grid. Because of the effective suppressor action provided by space charge and because of the low cur- rent drawn by the screen grid, the beam power tube has the advantages of high power output, high power sensitivity, and high efficiency.

Fig. 9 shows the structure of a beam power tube employing space-charge sup- pression and illustrates how the electrons

are confined to beams. The beam condi- tion illustrated is that for a plate po- tential less than the screen-grid poten- tial. The high-density space-charge re- gion is indicated by the heavily dashed lines in the beam. Note that the edges of the beam-confining electrodes coincide with the dashed portion of the beam. In this way the space-charge potential re- gion is extended beyond the beam boundaries and stray secondary electrons are prevented from returning to the

screen grid outside of the beam. The^ space-charge effect may also be obtained by use of an actual suppressor grid. Ex- amples of beam power tubes are 6AQ5A, 6L6GB, 6V6GT, and 50C5.

Multi-Electrode and Multi-Unit Tubes

Early in the history of tube develop- ment and application, tubes were de- signed for general service; that is, a single tube type— a triode— was used as a radio-frequency amplifier, an inter- mediate-frequency amplifier, an audio- frequency amplifier, an oscillator, or a detector. Obviously, with this diversity of application, one tube did not meet all requirements to the best advantage.

Later and present trends of tube de- sign are the development of "specialty" types. These types are intended either to give optimum performance in a particu- lar application or to combine in one bulb functions which formerly required two or more tubes. The first class of tubes in- cludes such examples of specialty types as the 6CB6 and 6BY6. Types of this class generally require more than three electrodes to obtain the desired special characteristics and may be broadly classed as multi-electrode types. The 6BY6 is an especially interesting type in this class. This tube has an unusually large number of electrodes, namely seven, exclusive of the heater. Plate cur- rent in the tube is varied at two different frequencies at the same time. The tube is designed primarily for use as a com- bined sync separator and sync clipper in television receivers.

The second class includes multi- unit tubes such as the twin-diode triodes 6BF6 and 6AV6, as well as triode-pen- todes such as the 6U8A and 6X8. This class also includes class A twin triodes such as the 6CG7 and 12AX7, and types such as the 6CM7 containing dissimilar triode units used primarily as combined vertical oscillators and vertical deflec- tion amplifiers in television receivers. Full-wave rectifiers are also multi-unit types.

A third class of tubes combines fea-^ tures of each of the other two classes. Typical of this third class are the penta- grid-converter types 1R5, 6BE6, and 6SA7. These tubes are similar to the

9

RCA Receiving Tube Manual

multi-electrode types in that they have seven electrodes, all of which affect the electron stream; and they are similar to the multi-unit tubes in that they per- form simultaneously the double function of oscillator and mixer in superhetero- dyne receivers.

Receiving Tube Structure

Receiving tubes generally utilize a glass or metal envelope and abase. Orig- inally, the base was made of metal or molded phenolic material. Types hav- ing a glass envelope and a molded phen- olic base include the "octal" types such as the 5U4GB and the 6SN7GTB. Types having a metal envelope and molded phenolic octal base include the 6AC7 and the 6AG7. Many modern types utilize integral glass bases. Present-day conventional tube designs utilizing glass envelopes and integral glass bases in- clude the seven-pin and nine-pin mini- ature types, the nine-pin novar and neo- noval types, and the twelve-pin duodecar types. Examples of the seven-pin mini- ature types are the 6AU6A and 6BN6. Examples of the nine-pin miniature types are the 12 AU7A and 6E A8. Examples of the novar types are the 6BH3 and 7868. The nine-pin base for the novar types has a relatively large pin-circle diameter and long pins to insure firm retention of the tube in its socket.

The nuvistor concept provided a new approach to electron tube design.

Nuvistor tubes utilize a light-weight can- tilever-supported cyUndrical electrode structure housed in a ceramic-metal en- velope (see page 2 for cutaway view). These tubes combine new materials, proc- esses, and fabrication techniques. Ex- amples of the nuvistor are the 2CW4 and the 6CW4.

Television Picture Tubes

The picture tube, or kinescope, is a multi-electrode tube used principally in television receivers for picture display. It consists essentially of an electron gun, a glass or metal-and-glass envelope and face-plate combination, and a fluores- cent screen.

The electron gun includes a cathode for the production of free electrons, one or more control electrodes for acceler- ating the electrons in the beam, and, optionally, a device for "trapping" un- wanted ions out of the electron beam.

Focusing of the beam is accom- plished either electromagnetically by means of a focusing coil placed on the neck of the tube, or electrostatically, as shown in Fig. 10, by means of a focusing electrode (grid No. 4) within the enve- lope of the tube. The screen is a white- fluorescing phosphor P4 of either the silicate or the sulfide type.

Deflection of the beam is accom- plished either electrostatically by means of deflecting electrodes within the enve- lope of the tube, or electromagnetically

DEFLECTING YOKE

GLASS NECK SECTION

-CENTERlislG MAGNET!

t

REFERENCE LINE— »J Fig. 10

10

Electrons, Electrodes, and Electron Tubes

by means of a deflecting yoke placed on the neck of the tube. Fig. 10 shows the structure of the gun section of a pic- ture tube and illustrates how the elec- tron beam is formed and how the beam is deflected by means of an electromag- netic deflecting yoke. In this type of tube, ions in the beam are prevented from damaging the fluorescent screen by an aluminum film on the gun side of the screen. This film not only "traps" un- wanted ions, but also improves picture contrast. In many types of non-alumi- nized tubes, ions are separated from the electron beam by means of a tilted-gun and ion-trap-magnet arrangement.

Color television picture tubes are similar to black-and-white picture tubes, but differ in three major ways. (1) The light-emitting screen is made up of trios of phosphor dots deposited in an inter- laced pattern. Each dot of a trio is capa- ble of emitting light in one of the three primary colors (red, green, or blue). (2) A shadow mask mounted near the screen of the tube contains over 300,000 aper- tures, one for each of the phosphor dot trios. This mask provides color separa- tion by shadowing two of the three phosphor dots of each trio. (3) Three closely spaced electron guns, built as a unit, provide separate beams for excita- tion of the three different color-phos- phor-dot arrays. Thus it is possible to control the brightness of each of the three colors independently of the other two.

The three electron guns are mounted with their axes tilted toward the central axis of the envelope, and are spaced 120 degrees with respect to each other. The focusing electrodes of the three guns are interconnected internally, and their po- tential is adjusted to cause the separate beams to focus at the phosphor-dot screen. All three beams must be made to converge at the screen while they are simultaneously being deflected, Con- vergence is accomplished by the action of static and dynamic magnetic fields set up by the radial-converging magnet assembly mounted on the neck of the tube. These fields are coupled into the radial-converging pole pieces within the tube. Another pair of pole pieces in the tube is activated by the lateral-converg- ing magnet also mounted on the neck of the tube. These pole pieces permit lateral shift in position of the blue beam in opposition to the lateral shift of the green and red beams.

A purifying magnet is used with color picture tubes to provide a mag- netic field, adjustable in magnitude and direction, to effect register over the en- tire area of the screen. A magnetic shield is used to minimize the effects of the earth's magnetic field.

Deflection of the three beams is ac- complished simultaneously by a deflect- ing yoke consisting of four electromag- netic coils similar to the deflecting yoke used for black-and-white picture tubes.

11

Electron Tube Characteristics

The term "characteristics" is used to identify the distinguishing electrical features and values of an electron tube. These values may be shown in curve form or they may be tabulated. When the characteristics values are given in curve form, the curves may be used for the determination of tube performance and the calculation of additional tube factors.

Tube characteristics are obtained from electrical measurements of a tube in various circuits under certain definite conditions of voltages. Characteristics may be further described by denoting the conditions of measurements. For ex- ample Static Characteristics are the val- ues obtained with different dc potentials applied to the tube electrodes, while Dy- namic Characteristics are the values ob- tained with an ac voltage on a control grid under various conditions of dc po- tentials on the electrodes. The dynamic characteristics, therefore, are indicative of the performance capabilities of a tube under actual working conditions.

Static characteristics may be shown by plate characteristics curves and trans- fer (mutual) characteristics curves.These curves present the same information, but in two different forms to increase its usefulness. The plate characteristic curve is obtained by varying plate volt- age and measuring plate current for dif- ferent grid bias voltages, while the trans- fer-characteristic curve is obtained by varying grid bias voltage and measuring plate current for different plate voltages. A plate-characteristic family of curves is illustrated by Fig. 11. Fig. 12 gives the transfer-characteristic family of curves for the same tube.

Dynamic characteristics include amplification factor, plate resistance, control-grid— plate transconductance, and certain detector characteristics, and may be shown in curve form for varia- tions in tube operating conditions.

The amplification factor, or is the ratio of the change in plate voltage

to a change in control-electrode voltage in the opposite direction, under the con- dition that the plate current remains un- changed and that all other electrode voltages are maintained constant. For example, if, when the plate voltage is

/

'V

/ 1

Q

/ 1

) 5

0 1

}0 \i

>0 2(

)0 2S

PLATE VOLTS

Fig. 11

made 1 volt more positive, the control- electrode (grid-No. 1) voltage must be made 0.1 volt more negative to hold plate current unchanged, the amplifica- tion factor is 1 divided by 0.1, or 10. In other words, a small voltage variation in the grid circuit of a tube has the same effect on the plate current as a large

-20 GRID VOLTS

Fig. 12

plate- voltage change— the latter equal to the product of the grid-voltage change and amplification factor. The il of a tube is often useful for calculating stage gain. This use is discussed in the ELECTRON TUBE APPLICATIONS SECTION.

Plate resistance (rp) of an electron tube is the resistance of the path between cathode and plate to the flow of alter- nating current. It is the quotient of a

12

Electron Tube Characteristics

small change in plate voltage divided by the corresponding change in plate cur- rent and is expressed in ohms, the unit of resistance. Thus, if a change of 0.1 milliampere (0.0001 ampere) is produced by a plate voltage variation of 1 volt, the plate resistance is 1 divided by 0.0001, or 10000 ohms.

Control-grid plate transconduct- ance, or simply transconductance (gm), is a factor which combines in one term the amplification factor and the plate resistance, and is the quotient of the first divided by the second. This term has also been known as mutual conduct- ance. Transconductance may be more strictly defined as the quotient of a small change in plate current (amperes) di- vided by the small change in the control- grid voltage producing it, under the con- dition that all other voltages remain un- changed. Thus, if a grid-voltage change of 0.5 volt causes a plate-current change of 1 milliampere (0.001 ampere), with all other voltages constant, the trans- conductance is 0.001 divided by 0.5, or 0.002 mho. A "mho" is the unit of con- ductance and was named by spelling ohm backwards. For convenience, a millionth of a mho, or a micromho (/tmho),is used to express transconduct- ance. Thus, in the example, 0.002 mho

is 2000 micromhos.

Conversion transconductance (ge) is a characteristic associated with the mixer (first detector) function of tubes and may be defined as the quotient of the intermediate-frequency (if) current in the primary of the if transformer di- vided by the applied radio-frequency (rf) voltage producing it; or more pre- cisely, it is the limiting value of this quotient as the rf voltage and if current approach zero. When the performance of a frequency converter is determined, conversion transconductance is used in the same way as control-grid— plate transconductance is used in single-fre- quency amplifier computations.

The plate efficiency of a power am- plifier tube is the ratio of the ac power output (Po) to the product of the aver- age dc plate voltage (Eb) and dc plate current (lb) at full signal, or

Plate eflSciency Po watts

(%) ""Eb volts X lb amperes^

The power sensitivity of a tube is the ratio of the power output to the square of the input signal voltage (Em) and is expressed in mhos as follows:

/ . X Po watts Power sensitivity (mhog) =

13

Electron Tube Applications

The diversified applications of an electron receiving tube have, within the scope of this section, been treated under seven headings. These are: Amplifica- tion, Rectification, Detection, Auto- matic Volume or Gain Control, Oscilla- tion, Frequency Conversion, and Au- tomatic Frequency Control. Although these operations may take place at either radio or audio frequencies and may in- volve the use of different circuits and different supplemental parts, the gen- eral considerations of each kind of oper- ation are basic.

Amplification

The amplifying action of an electron tube was mentioned under Triodes in the section on ELECTRONS, ELEC- TRODES, and ELECTRON TUBES. This action can be utilized in electronic circuits in a number of ways, depending upon the results desired. Four classes of amplifier service recognized by engineers are covered by definitions standardized by the Institute of Radio Engineers. This classification depends primarily on the fraction of input cycle during which plate current is expected to flow under rated full-load conditions. The classes are class A, class AB, class B, and class C. The term "cutoff bias" used in these definitions is the value of grid bias at which plate current is very small.

Classes of Service

A class A amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube fiows at all times.

A class AB amplifier is an ampli- fier in which the grid bias and alter- nating grid voltages are such that plate current in a specific tube flows for ap- preciably more than half but less than the entire electrical cycle.

A class B amplifier is an amplifier in which the grid bias is approximately equal to the cutoff value, so that the plate current is approximately zero when no exciting grid voltage is applied,

and so that plate current in a specific tube flows for approximately one-half of each cycle when an alternating grid voltage is applied.

A class C amplifier is an amplifier in which the grid bias is appreciably greater than the cutoff value, so that the plate current in each tube is zero when no alternating grid voltage is applied, and so that plate current flows in a specific tube for appreciably less than one-half of each cycle when an alter- nating grid voltage is applied.

The suffix 1 may be added to the letter or letters of the class identification to denote that grid current does not flow during any part of the input cycle. The suffix 2 may be used to denote that grid current flows during part of the cycle.

For radio-frequency (rf) amplifiers which operate into a selective tuned cir- cuit, as in radio transmitter applications, or under requirements where distortion is not an important factor, any of the above classes of