VACUUM TUBES

McGraw-Hill

ELECTRICAL AND ELECTRONIC ENGINEERING SERIES

FREDERICK EMMONS TERMAN, Consulting Editor

VACUUM TUBES

1IcGRA'V-IllIjI~ ETlECTRICAL AND EI.JECTRONIC

:F~~G-lNEERING SI~RIES

~"REDERICK l~~IMONS TERMAN, Consulting Editor W. \V. IIARMAN andJ. G.TRUXAL, Associate Consulting Editors

AHREXDT AND SAVANT' Servomechanism Practice AXGf:LO . Electronic Circuits

ASELTINE . Transform Method in Linear System Analysis

BAILI'~YAND GAULT' Alternating-current Machinery BERANEK · Acoustics

BRENNER AND JAVID . Analysis of Electric Circuits

BRU~SAND SAUNDERS· Analysis of Feedback Control Systems CAGE . Theory and Application of Industrial Electronics CAUER . Synthesis of Linear Communication Networks CHIRLIAN AND ZKMAXL\~ . Electronics

CL}i~MENTAND JOHNSON . Electrical Engineering Science COTE AND OAKES · Linear Vacuum-Tube and Transistor Circuits

CUCCIA · Harmonics, Sidebands~and Transients in Communication Engineering

CU~NINGHA~I· Introduction to Nonlinear Analysis

EAST~IAN. Fundamentals of Vacuum Tubes EVANS · Control-system Dynamics

FEINSTEIN . Foundations of Information Theory

FITZGERALDAl'i~ HIGGIXBOTIL\~I . Basic Electrical Engineering FITZGERALD AND KINGSLEY . Electric Machinery

FRANK . Electrical Measurement Analysis GEPPERT· Basic Electron Tubes

GI~ASFORD· Fundamentals of Television Engineering GREINER . Semiconductor Devices and Applications

HA~I:MOND· Electrical Engineering

lIANCOCK • An Introduction to the Principles of Communication Theory HAPPELL AND HESSELBERTH · Engineering Electronics

HAR~fAN· Fundamentals of Electronic Motion

HARRINGTON' Introduction to Electromagnetic Engineering HARRINGTON · Time-harmonic Electromagnetic Fields HAYT . Engineering Electromagnetics

HILL · Electronics in Engineering

JOHNSON • Transmission Lines and Networks

KOENIG AND BLACK'VELL . Electromechanical System Theory KRAUS . Antennas

KRAUS' Electromagnetics

KUH AND PEDERSON · Principles of Circuit Synthesis LEDLEY . Digital Computer and Control Engineering LEPAGE' Analysis of Alternating-current Circuits

LEPAGE . Complex Variables and the Laplace Transform for Engineers LEPAGE AND SEELY' General Network Analysis

LEY, LrTZI AND REHBERG· LinearCircuit Analysis

LY~CH A~OTRUXAL • Introductory System Analysis

MILI.~lAN. Vacuum-tube and Semiconductor Electronics

MII~I~:\{ANAND SEELY , Electronics

MILL~IANANDTAUB • Pulse and Digital Circuits

MISHKIN AND BRAUN • Adaptive Control Systems

MOORE' Traveling-wave Engineering

PETTIT' Electronic Switching, Timing, and Pulse Circuits PETTITAND MCWHORTER • Electronic Amplifier Circuits

P~'EIFFER• Linear Systems Analysis

PLO:SSEY ANDCOLI~lN· Principles and Applications of Electromagnetic :F'ields

R[~ZA • An Introduction to Information Theory

REZA AND SI':ELY • Modern Network Analysis

ROGERS • Introduction to Electric Fields

11uDEXBERG •Trausient Performance of Electric Po"rer Systems

RYDER • Engineering Electronics

SCII\VARTZ . Information Transmission, Modulation, and Noise SEEI.Y . Electron-tube Circuits

SEELY • Electrical Engineering

SEELY' Introduction to Electromagnetic E'ields

S~:ELY· H.adio Electronics

SEIFI':RTANDSTEEG' Control Systems Ejngineering

SISKIND' Direct-current Machinery

SKILLING • Electronic Transmission Lines SKILLIXG . Transient Electric'Currents

SPAXGEXBERG • Fundamentals of Electron Devices

SPAXGENBEBG • Vacuum Tubes

STr-:vENsoN • ElementsofPower System Analysis

STE\VART . Fundamentals of Signal Theory STORER . Passive Network Synthesis

STRAUSS ^# Wave Generation and Shaping TERllAN • Electronic and Radio Engineering

TER~L\NAXDPETTIT · Electronic l\1:easurements

TIL\Lr":R • ElementsofServomechanismTheory

THALERAND BRO\VN • Analysisand Design of Feedback Control Systems

THOMPSON • Alternating-current and Transient Circuit Analysis Torr . Digital and Sampled-data ControlSystems

TRUXAL . Automatic Feedback Control System Synthesis VALDES · ThePhysical Theory of Transistors

WILLIA~IS AND YOUNG' Electrical Engineering Problems

VACUUM TUBES

KARL R. SPANGENBERG

PROFESSOR OF ELECTRICAL ENGINEERING

STANFORD UNIVERSITY

NEW YORK TORONTO LONDON

McGRAW-HILL BOOK COMPANY, INC.

1948

VACUUM TUBES

COPYRIGHT, 1948, BY THE MCGRAW-HILL BOOK COMPANY, INC.

PRINTED IN THE UNITED STATES OF AMERICA

All rights reserved. This book, or paris thereof, may not be reproduced in any form without permission of the prtblishers.

x 59880

TO MY COLLEAGUES AND STTJDENTS

PREFACE

This book is the outgro,vth of a course in vacuum-tube design given for many yearsatStanford University to senior and graduate students in electrical engineering and physics. It is concerned \vith the determina- tion of vacuum-tube characteristics in terms of the electron action within the tube. The book attempts to bridge the gap between the physical laws that lie behind the electron behavior and the external characteristics of the tubes themselves.

It is hoped that the point of vie'v taken ,vill be acceptable to both physicists and engineers. The development of the physicalla\vs involved is indicated, after which emphasis is placed upon their description and utilization. Although this book cannot pretend to give much design information, the attempt has been to include enough of the basic relations, physical data, and significant references to make it a useful reference source to vacuum experimenters and tube designers.

Vacuum tubes may seem a rather special subject to which to restrict the material in a book. Actually this is not so. In preparing the book so much material was collected that the contents had to be restricted to first-order effects. It is felt that although engineers and physicists work- ing \vith vacuum tubes are primarily concerned with the utilization of already developed tubes, the successful application of these tubes is greatly enhanced by a kno\vledge of their limitations and an understand- ing of the origin of their characteristics. This is particularly true since there are many occasions when it is desired to use tubes under conditions different from those specified by the manufacturer. Under these condi- tions it is imperative to know ho\v far one may depart from recommended operating conditions \vithout exceeding some design limitation of the tube. This, in turn, requires a kno\vledge of how the tube operates.

Circuits and tube applications are so completely covered in the text- book and periodical literature that no effort has been made to include information on these subjects. Only in the case of ultra-high-frequency tubes where the tube cannot be completely separated from the circuit have circuit considerations been included.

The author is indebted to many people for assistance rendered in the preparation of this book. lIe is particularly indebted to Dr. F. E. Ter-

viii PREFACE

man, dean of the Stanford School of Engineering, who was a constant source of inspiration and encouragement, and who made many valuable suggestions and gave much direct assistance in checking the work. The author is also indebted to Prof. Paul Kirkpatrick, head of the Physics Department at Stanford, for suggestions on the material of Chaps. 3 to 6 and 9; to Prof. L. Marton for suggestions on the material of Chaps. 13 to 15 and 20; and to C. V. Litton for much information and suggestions relative to Chap. 21. He is indebted to Evelyn G. Sarson, who typed a large part of the manuscript in its final form. O. O. Pardee and Will Harman assisted in the correction of the entire work. Lastly, the author is more than a little indebted to his ,vife, ,vho personally typed much of the manuscript and ,vas a source of constant assistance.

K~RL R. SPANGENBERG

PALO ALTO, CALIF.

Januarv, 1948

PREFACE . . . . • • • • •

CONTENTS

PAGE

vii

CHAPTER I-INTRODUCTION . . . . . . . . . . . . . . . . . 1 1.1 Devices Using Electron Tubes . . . . 1

Il-adio Receivers-Radio Transmitters--Long-distance 'Yire Tele- phones-rrelevision Systems-l\feasurement I)evices-Industrial Con- trol

1.2 Functionsof Vacuum Tubes. . . . . 2 ltectiflcation- Amplification- Oscillation- Frequency Conversion- Modulation - Detection - Light-image Production - Photoelectric Action

CHAPTER 2-BASIC TUBE1."YPES

2.1 Vacuum Diode. . 2.2 Vacuum Triode . . 2.3 Screen-grid Tube.

2.4 Pentocle. . . . . 2.5 Beam-power Tube . 2.6 Cathode-ray Tubes.

2.7 Klystron . 2.8 Magnetron 2.9 Phototubes

5 5 6 7 8 9 12 13 15 17

CHAPTER 3-ELECTRONS AND IONS. 19

3.1 The Electron. . . . 19

3.2 The Proton. , . . . . 19

3.3 Other Fundamental Particles. . 20

3.4 Atoms and Molecules. 20

3.5 Ions . . . . 22

CHAPTER 4-ELECTRONIC F~MISSION . . . . 23 4.1 Theory of Thermionic E~mi8sion . . . . 24

Work Function-TheF~mission F~quation-Typesofl~:mitter

4.2 Emission of Pure Metals . . . . 35 Tungsten-Tantalum

4.3 Atomic-film Emitters. . . . 39 4.4 Oxide Emitters. . . . 42

Theory of Oxide F~mi8sion-Activationof ()xide F:mitters-Specific Emission Characteristics-Transient Emission

4.5 SchottkyE~fIeet . . . . 46

4.6 Contact Difference of Potential. . . . 48

x CONTENTS

PAGIJ

4.7 Secondary Emission. . . . 48 Variation of Secondary Emission with Primary-electron Potential- Velocity Distribution of Secondary Electrons-Variation of Secondary Emission of Composite I.Jayers-Secondary F~missionof Insulators

CHAPTER 5--DETERMINATION OF POTENTIAL FIJi1LDS . 58

5.1 Units and IJimensions. . . . 58

5.2 Fundamental Quantities and Definitions. . . . 59 Forces between Charges

5.3 Solution of Potential Fields by Summation of Intensities . 61

5.4 SummationofPotentials . . . . 62

5.5 Gauss's Law. . . . 64 5.6 Poisson's and Laplace's Equations. . . . . 67

Interpretations of Laplace's Equation-Solutions of Laplace's Equa- tioninTwo Dimensions-Difference Form of Laplace's Equation

5.7 Elastic-membrane Models of Potential. . . . . . 75

5.8 Current-flow Models of Potential. . . 76

5.9 Sketching of Flux and Potential Fields. 80

Properties Useful in Sketching Fields

5.10 Method of Conformal Transformations. 82

Complex Functions Satisfy Laplace's Equation-Definition of Analytic Functions-The Logarithmic Transformation-The FunctionW = ^Zl/n

CHAPTER 6--LAWS OF ELECTRON MOTION • • • • • • • 97

6.1 Electron in a Uniform Electric Field . 97

6.2 Initial Velocity not Parallelto Field . . . . 99 6.3 Electrostatic Deflection of Cathode-ray Beams. . 101 6.4 Relativity Correction for Velocity . . . . 103 6.5 Two-dimensional Electric Fields. . . . . 107 6.6 Electron in a Uniform Magnetic Field. . . . . 111 6.7 Behavior of Electrons in Nonuniform Magnetic Fields. . . . 114 6.8 Combined Electric and Magnetic Fields. . . . . 116 6.9 Approximate Numerical and Graphical Methods for I)etermining Elec-

tron Paths. . . 121 Method of Joined Circular Segments-Useof Elastic-membrane Model of Potential to Determine Electron Paths-Application of the Principle of Least Action

. 125 . 125 125

135

142 Method of Solution. . . .

Electrostatic:Field of a Plane-electrode Low-mu Triode . .

Conwur Representation of Potential Field-Profile Representationof Potential Field

ElectrostaticField of aLow-rou Cylindrical-electrode Triode. . . . . Potential Contours of a Cylindrical Triode-Potential Profiles of a Cylindrical Triode

Analysis of the High-rou Triode . . . . Potential Contours and Profiles-Amplification Factor of a High-mu Plane-electrode Triode-Amplification Factor of a High-mu Cylin- dricalTriode

7.3

7.4

CHAPTER 7-THE ELECTROSTATIC FIELD OF A TRIODE . . . • • .

7.1 7.2

CONTENTS Xl

· 165 7.5

7.6

7.7 7.8

7.9

PAGE

The Equivalent Electrostatic Circuit of a Triode. . . • • . . . 152 Equivalent-diode Spacing of a Triode. . . . 153 IJiode Equivalent to a Plane-electrode Triode-Diode Equivalent to aCylindrical-electrode Triode

Application of Amplification-factor Formulas to Actual l'riodes . . . 156 More Accurate Amplification-factor :Formulas . . . 158 Formula for Small Grid-plate Spacings-Formulas for Small Screening Fraction-Formula for Small Cathode-grid Spacings

Amplification Factor of Unconventional Tubes.

CHAPTER 8-SPACE-CHARGE EFFECTS • . • •

8.1 Effects of Current Flow. . . . 8.2 Plane-electrode Space-charge Flow . 8.3 Cylindrical-electrode Space-charge :Flow.

8.4 Space-charge Flow for Other Geometries. . . . . Spherical Electrodes-The General Case

8.5 Current I.Jaw for Plane Triodes. . . • • • • • • CurrentLaw in Terms of :Electrode I)imensions

8.6 Mutual Conductance of a Plane Triode . . . .

8.7 Mutual Conductance of a Cylindrical Triode . . • • • • • • • • • 8.8 Effect of Filamentary Emitters. . . .

8.9 Effect of Initial:F~lectronVelocity. . . . . • • • . 8.10 Effect of Space Charge upon Transit Timein Diodes . . .

8.11 Summary . . . .

· 168

· 168

· 170

· 173

· 181

· 183

· 188

· 188

· 189

· 191

· 19E

· 198

· 201

· 201 202

CHAPTER 9-TRIODE CHARACTERISTICS . •

9.1 Control Action of the Grid. . .

9.2 Current-voltage Characteristics of the Triode . . . .

Plate-current- Grid-voltage Characteristics - Plate-current--- Plate- voltage Characteristics-Contours of Constant Plate Current-The Plate-current Suriace

9.3 Definition of Triode Constants. . . . Amplification Factor-l\Iutual Conductance-Plate Resistance-Re- lation between Tube Constants-Variation of Tube "Constants"

9.4 Effective Tube Constants of Combinations of Tubes

9.5 Electron Paths. . . . 9.6 Grid Current " . . " " . . " " " . . . " . . . . . Grid-current-(} rid-voltag e Characteristics-Grid-current-Plate-vo1- tage Characteristics-Constant-grid-current Contours-The Grid-cur- rent Surface-Effect of Secondary Electrons

9.7 Primary-grid-currentI~w . . . . Current-division Factor-Approximate Primary-grid-current Law- Current-division-factor Formula-Current-division I.Jaw in the Pres- ence of SecondaryF~mission

205

212 213 218

224

CHAPTER lo-TETRODES . . . . . . . . . • • • • • • • • • • . 238 10.1 Types of Tetrode. . . 238 10.2 Current-voltage Characteristics of the Screen-grid Tube. . . . 238

Plate-current-Plate-voltage Characteristics of Screen-grid Tube- Screen-current-Plate-voltage Characteristics of the Screen-grid Tube-- General Characteristics of Screen-grid Tubes

~ CONTENTS

PAGg

10.3 Beam-power Tubes. . . 245 10.4. The Electrostatic Field of a Beam-power ".rube. . . 245 10.5 Space-charge E~ffectsinthe Screen-grid-Anode Region of Beam-power

Tubes 248

Type A Distribution-Type B Distribution-Type C })istribution- Type DDistributions

10.6 I)ynamic CharacteristicsofBeam-power Tubes. . . . 259 Injected Current Varied, Potentials Constant-Plate Potential Varied, Screen Potential and Injected Current Constant

# 266

266 267

288 288 . 289 Electrode Arrangementina Pentode . . . .

Current-voltage Characteristics of the Pentocle.

Plate-current-Control-grid-voltage Characteristics - Plate-current- Plate-voltage Characteristics of a Pentode-Space-current-Plate-vol- tage Characteristics of the Pentode-Screen-grid-current-Plate-voltage Characteristics of a Pentode-Suppressor-grid :Effects

Current DivisioninPentodes . . . . 272 Amplification Factor of a Pentode . . . , . . . . 278 Electrostatic Field of a Pentode-F~lectrostatic Amplification Factor of a Pentode-True Amplification Factorofa Pentode

Transconductance of a Pentocle. " . . . . . Plate Resistance ofa Pentode .

Design Considerations. . . . 11.3

11.4

11.5 11.6 11.7

CHAPTER 11-PENTODES • • • • • • • • • • • . • .

11.1 11.2

CHAPTER 12-NOISE IN VACUUM TUBES . 298

12.1 Noise as a Limiting Factor in the lJltimate SensitivityofElectronic

Devices. . . . 298

12.2 NoiseinResistors . . . . 299 12.3 SourcesofNoiseinTubes. . . . 305 12.4 Shot Noise in I)iodes with Temperature-limitedEmission. 306 12.5 Tteduced Shot Effect in Diodes with Space-charge-limited F.illlission . . 308 12.6 Tteduced Shot EffectinTriodes with Space-charge-limited Current . . 310

12.7 Noise DuetoGasinTubes ' 312

12.8 Reduced Shot Effect in Multielectrode Tubes with Space-charge- limited Currents . . . . . 313 12.9 NoiseinMixer Tubes. . . 314 12.10 Noise Induced at Ultra-high Frequenciesby }landom F~mission. . . 316 12.11 NoiseinVelocity-modulation Tubes. . . . . . 317 12.12 NoiseinPhototubes . . . . ^{# " •} 318 12.13 NoiseinSecondary-emission Multipliers. . . 319 12.14 Definition of Noise Figure. . . • . . 321

Noise :Figure for Two Networks in Cascade

12.15 Measurement of Noise and Noise Il'igure . • • • • • • • . . 325 Typical Tube-noise Values

CHAPTER 13-ELECTROSTATIC ELECTRON OPTICS . • • • • • • • • • • • • . 328 13.1 Introduction. . . . . . . 328

Snell's Law-The l>rinciple of Least Action-Simple Lenses-Lens Formulas

CONT}l~NTS xiii

365 . . 349 . 350

369 . 375 . 377

PAGE

13.2 Electrostatic-lens Fields. . . 337 General Form of :Fields with Rotational Symmetry-The Equal-diam- eter Two-cylinder Lens-Equal-diameter Spaced Cylinders-Two- diameter Cylinder Lenses-Aperture I.JCnses

13.3 Electron Paths. . . . . . . . 13.4 General Lens Properties. . . • . .

Thin Lenses-Thick Lenses

13.5 Calculation of Lens Characteristics . . . . 360 Method of Linear Axial-potential Segments-Method of Equivalent Thin Lenses

13.6 Measurement of Lens Characteristics. . . . I)ouble-grid Method of Measuring Lens Characteristics 13.7 ()ptical Characteristics of I.lenses . . . .

13.8 Calculation of Lens Characteristics . . . .

13.9 P-Q Curves of Lenses. . . .

Comparison ofI£nses-The E'£nzel Lens

13.10 Aberrations . . . . . . 387 Chromatic Aberration-Coma-Astigmatism-Curvature of Field- Distortion of It-"ield-Spherical Aberration-Other LensDerects

CHAPTER 14-l\fAGNETIC I.JENSEs. • . . • • . • • • • • 394

14.1 Focusing Action of Axial Magnetic :Fields . . . . 394 14.2 l\t[agnetic Fields with lrotational Symmetry . . . 396 14.3 Flectron Motionina Magnetic Field Fixpressed in Cylindrical Coordi-

nates. . . . . . 397 14.4 })ifferential Equations of Motion of the Paraxial Electron. . . . 398 14.5 }"ocusing Properties of~lagneticI..Ienses. . . 399

General-Magnetic Lensofa Circular Turn of Wire-The Glazer Lens 14.6 Practical Magnetic Lenses. . . 404 14.7 Magnetic-lens I)efects. . . 405 14.8 The General Equations of_~lotionin Combined_F~lectricand l\lagnetic

Fields 406

412 . 412 414

CHAPTER 15--CATHODE-RAYTUBES. • • • . . • • •

15.1 The General Form of Cathode-ray Tubes . . 15.2 }l~lectron-gunI)esign . . . .

CutoffRelationsinthe Electron Gun-Electron Paths in the Electron Gun-Focusing System-Alternative Electrode Structures

15.3 Deflection Devices . . . . 425 Electrostatic Deflecting Plates-Magnetic I)eflection-Relative Merits of Electrostatic and Magnetic Deflection-Visual versus Deflection Sensitivity-Postdeflection Acceleration

15.4 Fluorescent Materials. . . . 429 Definitions-General Make-up of Phosphors-Luminous Properties of Fluorescent Materials-F~lectricalCharacteristics of Phosphors

15.5 Limitations of Spot Size. . . 437 Effect of Thermal Velocity of Emission-Spaee-charge Limitation of Spot Size-Effect of Secondary Emission-Halation

DV CONTENTS

PAGE

15.6 High-efficiency Cathodes . . . 449 Parallel Flow of a Rectangular Beam-Parallel-flow Cylindrical 13eam-Convergent Radial Flow of a Conical Beam

15.7 Ultra-high-frequency Deflection Effects . . . . 465 15.8 Photography of Cathode-ray Traces. . . 470

Beam Power-Screen Types-Writing Speed-Time, Stop, and Mag-

nification-~llmSensitivity-Developers and Development

CHAPTER 16--ULTRA-IIIGH-FREQUENCY EFFECTS IN CONVENTIONAL TUBES . 475 16.1 Introduction. . . . . 475 16.2 Causes of I)ecreased Output at Ultra-high Frequencies . . . . . 475

16.3 Onset of Tube-reactance Limitations 477

16.4 The Nature of Currents Induced by Electron Motion at lJltra-high Frequencies . . . . 482 The General Case-The Diode without Space Charge-The l)iode with Space(~harge-CurrentsInducedinthe Electrodesof a Triode

16.5 Onsetof Transit-time Effects in Triodes. . . 490 16.6 Transit-time Effects in the Space-charge-limited Diode . . . 495 16.7 Small~signalTransit-time Effects in the Space-charge-limited Triode. 501 16.8 Similitude and Scalingin Lltra-high-frequency Triodes. . . 504 16.9 High-frequency I.limitofTriode Oscillation . . . . 507 16.10 Large-signal Effects. . . 516

Transit-time Effects in Diodes-Transit-time F~ffects in Triodes-- Transit-time EffectsinTetrodes-The Resnatron

16.11 Disk-seal Tubes . . . . . 524

CHAPTER I7-VELOCITY-MODULATED TUBES OR KLYSTRONS. . • 527 17.1 The Bunching Principle. . . 527 17.2 Cavity Resonators . . . . . . 529 17.3 Mechanism of Energy Interchange between Electrons and Cavity

Resonators. . . . . . 537

17.4 First-order Bunching Theory . . . 541

17.5 The Klystron Amplifier 556

Structure of the Klystron Amplifier-Output Power of the Klystron Amplifier-Efficiencyof the Klystron Amplifier-Mutual Conductance of the Klystron Amplifier-Power Itequired to Bunch the Beam

17.6 The Cascade Amplifier. . . . . 564

17.7 Frequency-multiplier Klystrons . . . . . 566

17.8 Second-order 13unching Effects. . 567

17.9 The Reflex-klystron Oscillator . . . . 571

Behavior of Electrons in the Reflector Space-Distance-time Diagram of a Reflex-klystron Oscillator-Bunching Theory of the Reflex-klystron Oscillator-Self-admittance of the Beam-Mechanism by Which Oscillations Start-Variation of Beam Conductance with Amplitude of Oscillation-The ~~lectronic-admittanceSpiral-Reflex-klystron 0s- cillation with a Simple H,esonant Circuit-}:)ower I~elationsin the Reflex-klystron Oscillator-Voltage Stability of Iteflex-klystron Oscillators

17.10 Broad-band Operation of l~eflex-klystronOscillators. . . 591 Equivalent Circuit of Concentric-line Itesonator-Possible Modes of

CONTENTS xv

· 606

· 616

· 616

631 636

. . 621

· 621 · 622 . . 625

1)A,GE Oscillation-l\Iethod of Calculating Oscillation Mode Plot-Mode Interference 17.11 The T,vo-resonator Klystron Oscillator . . 17.12 The Heil Tube . 17.13 Bunching Effects in Negative-grid Tubes CHAPTER I8-MAGNETRON OSCILLATORS 18.1 Introduction. . . . 18.2 Structural Form of Magnetrons 18.3 Resonant Properties of Multicavity Magnetrons . . 18.4 Flectron 13ehavior in Crossed Static l\lagnetic and Static Jl~lectric Fields: Plane Case . . . . 18.5 Jl~lectronBehavior in Crossed~1agneticand AlternatingI~~lectricFields: Plane Case. . . . . . Alternating Transverse F~lectric-field F~ffectof a Traveling Electric Field 18.6 Electron Behavior in Crossed ~1agneticand Radial :Electric Fields . . 642

18.7 TheF~ffectof Space Charge. . . . 648

18.8 Electron Behavior in Crossed Magnetic and Alternating ltadial Electric Fields. . . . . 651

18.9 Basic Design Relations for l\fulticavity l\.fagnetrons. . . . . 660

18.10 J)imensional Relations in Magnetrons. . . . . . 665

18.11 Output Characteristics of Magnetrons. . . . . 667

· 693 · 694 · 675 · 675 · 675 · 676 · 677 · 681 · 683 . . . 684

CHAPTER 19-PHOTOELECTRIC TUBli~S. • . • • • • 19.1 The General :Form of Photoelectric Tubes. 19.2 Fundamental Photoelectric I{,elations . . 19.3 History of the Photoelectric Effect . . 19.4 Specific Photoemission Characteristics. . 19.5 Fundamental Theory of Photoemission 19.6 Spectral Itesponse Curves of Photoemissive Surfaces . 19.7 Vacuum-phototube Characteristics . . . . Current-voltage Characteristics-SpectralCharacteristics 19.8 Gas-phototube Characteristics . . . . Factors in the Design of Gas Phototubes-Frequency l)istortion in Gas Phototubes-Summary of Gas-phototube Characteristics 19.9 ·Utilization of Phototube Characteristics. . . . 19.10 Photomultiplier Tubes . . . . 688 CHAPTER 2Q-SPECIAL TlJ""BES • • 701 20.1 Introduction. . . . . • . . . . . 701

20.2 The Hexode . . . . . 702

20.3 The IIeptode. . . . 710

20.4 The Pentagrid Converter . . 714

20.5 The Octode . . . . . 716

20.6 Space-charge Grid Tubes . . 717

20.7 Negative-resistance Tubes. . . . . . . 718 Glow-discharge Tubes-The J)ynatron-Direct-coupled Negative- resistance I)evices-Negative Screen-grid Resistance of a Pentode-

xvi CONTENTS

· 807

· 808

· 809

PAG~

Push-pull Negative-resistance Circuit-l"eedback Circuits-Special N egative-resistance Tubes

2U8 Negative-transconductance Tubes . . . . . 723

20.9 Electron-ray Indicator Tubes . . . . . . . 723

20.10 Directed-rayF~lectronTubes. . . . 724

20.11 Deflection Tubes. . . . . 727

20.12 Television Camera Tubes . . . . . . 728

The Image-dissectorTube-The Iconoscope-The Image lconoscope- The Orthicon-The ImageOrthicon-The Monoscope 20.13 TheElectron Microscope. . . 738

Structure of the :ElectronMicroscope-F~quivalentWaveI..ength of Elec- trons-Theoretical Resolving Power of the Electron Microscope- Operating Principle of the Electron Microscope-Limits of Resolving Power-Electrostatic Electron Microscopes CHAPTER 21-HIGH-VACUUM PRACTICE . . . . 747

21.1 Introduction. . . . . 747

21.2 Fundamental Gas Laws. . . . . 749

Boyle's Law-Charles's Law or Gay-Lussac's Law-Avogadro'sLaw- General Gas-expansion Law-Distribution of Velocities in a Gas- Mean Free Pathofa Gas Molecule-Mean Free Path of an Electron amongGas Molecules 21.3 Measurement of Vacuum 757 Manometers-The McLeod Gauge-The Spark-discharge Tube-The Pirani Gauge--The Thermocouple Gauge-Triode Ionization Gauge 21.4 Pumping Speed 775 Speed of an Aperture-Definition of Pump Speed-Speed of Tubing- Efiect of Tubing Upon Pumping Speed 21.5 Production of Low Vacuum . . . 780

21.6 Production of High Vacuum. . . 781

The Mercury-diffusion Pump-Oil Pumps-Fractionating Pumps 21.7 Glassand Its Properties. . . , 791

Composition of Glass-Physical Properties of Glass-Working of Glass 21.8 Sealing ofGlass to Other l\{aterials. . . . . 796

Sealing of Small I£ads Into Glass-Copper-to-glass Seals-Kovar and Femico-Glass-to-porcelain Seals-Glass-to-mica Seals 21.9 Metals Useful in Tube Construction . . . SOl Nickel-Copper-Aluminum-Molybdenum-Tantalum-Tungsten- Relative Properties of the Metals-Spot Welding 21.10 Insulators. . . . 21.11 Degassing of Glass and Metals. 21.12 Getters . . . . ApPENDIXES T. Properties of the Elements . . . . II. Differential Operators and Vector Notation. . . . III. A Note on Mks Units . . . . IV. Characteristics of Fluorescent Screens . . . . . . 811

. . 813

. 817 . . . 821

CONTENTS xvii

PAGE

V. Surface Resistance and Depth of Penetration of Current Resulting from Skin Effects . . . . . 822 VI. Principal Properties of the Bessel Functions. . . 823 VII. Values of a²as a Function of rc/rfor use in Eq. (15.63). . . 825 VIII. Nomographic Chart Relating Object and Image Distance to the Focal

Lengthof aThin Lens . . . . 826 IX. Nomographic Chart Relating Tube Constants. 827 X. Designation ofFrequencyBands. . . . . 828

PROBLEMS • •

NAME INDEX . .

SUBJECT INDEX.

829 849 853

CIIAPTER 1 INTRODUCTION

V

^ACUUM tubes are found as basic or auxiliary elements in numerous technical devices no\v in use. rI'hey are indispensable in communication systems and industrial control. Their development has facilitated advances in the fields of po\ver and transportation. Without the vacuum tube \ve should be back in the days of the gravity-cell telegraph and the ringer telephone.

In the United States the number of vacuum tubes in use is several times the number of human beings and household pets. The 50,000,000 radio sets manufactured in the United States in the year 1947J alone contained more vacuum tubes than the adult population of the country.

Associated with the 25,000,000 telephones and 120,000,000 miles of tele- phone and telegraph wire in the United States are many more vacuum tubes. Various industrial devices include almost as many more. The United States uses nearly half the world's total of vacuum tubes.

One may conclude that there are many vacuum tubes in use. They must be of some importance. Theyare.

1.1. Devices Using Vacuum Tubes. This book is more concerned with the properties and functions of vacuum tubes than with the systems utilizing these properties. Ho\vever, it is well to be reminded of the extent of vacuum-tube applications and the degree to \vhich \ve are dependent upon them. The follo\ving devices are totally dependent upon vaccum tubes.

Radio Receivers. These are too well kno,vn to require much descrip- tion. They range from portable receivers the size of a brick and capable of receiving local broadcast stations to large-size all-\vave receivers capable of picking up a signal stronger than the noise level from any point on the globe. ~~venthe smallest receivers use 4 or 5 vacuum tubes.

The average home receiver has about 7 tubes. An all-wave receiver may have 20 or more tubes.

Radio Transmitters. Transmitters range from portable ,valkie-talkie sets to large po,ver-broadcast and short-wave stations. In output power they vary from 0.1 \vatt to hundreds of kilo\vatts. In frequency they mayrange from 100 kc to 6O,(X){) mc. The short-,vave transmitters are capable of producing an audible signal at any point on the earth's surface.

1

2 VACUUM TUBES

Transmitters may use voice or code. 'fhey may incorporate static- elimination or secrecy features in their operation. A small transmitter may use only a few vacuum tubes. The largest transmitters may use 50 or more tubes.

Long-distance Wire Telephones. The connections between telephone stations on the same continent are effected by wire transmission lines rather thanby radio. When the distance between telephone stations is large, it is necessary to amplify the speech energy about every 16 miles for cablesand every50milesfor open-wire lines. Each speech amplifier contains several vacuum tubes and amplifies the speech power from about 10microwatts to about 1milliwatt, a power amplification of 100. Thus atelephone call from San Francisco to New York passes through30 or morespeech amplifiers.

Television Systems. Televisionsystemsachievethe modern miracle of reproducing a visual scene at a point remote from the original. This is done entirely with vacuum tubes and electrical-circuit elements. No me- chanical devices are needed. In its present stage of development the reproduced picture as vie,ved from 6 it on an 8-in. cathode-ray-tube screen is as good as a motionpicture seenfrom the firstrowofthebalcony.

Each television transmitter contains hundreds of vacuum tubes, including a special camera tube. Every television receiver contains 20 or more tubes, including a special viewing tube.

Measurement Devices. Electronic measurement devices are too numerous to mention. Quantities that can be measured, besides all the electrical quantities, are color, weight, light intensity, odor, time interval, and many others. In fact, it can be said that any quantity which can bemeasured at all can probably bemeasured by electronicmeans.

Industrial Control. The number of electronic industrial-control devicesis legion. They include counting circuits, sorting systems, illu- mination-control systems, welding-control devices, and liquid- and gaseous-flow regulators. Typical devices are those which automatically regulate temperature or humidity. Allthese devices have their primary dependence upon the vacuum tubes in them.

In addition to the above devices, \vhich are totally dependent upon vacuum tubes, there are many others that have acquired a strong depend- ence upon electronic devices. Thus all commercial flying makes constant use of radio communications to keep posted on the weather and on terminal traffic and to keep ground stations posted on plane positions as well as to guide the planes directly. The invasion of other fields by electronics has already been considerable and is bound to be greater in time to come.

1.2. Functions of Vacuum Tubes. Although the applications of vacuum tubes are almost infinite, the specific functions that vacuum tubes

INTRODU·~ . ION 3 can perform by virtue of their o\vn properties are relatively few. It is these few fundamental functions and their combinations that give rise to the numerous applications.

A list of the functions of vacuum tubes is bound to be an arbitrary one since the tube cannot function by itself without an associatedcircuit.

Ho,vever, some of the jobs that vacuum tubes can perform are so funda- mental that they may be considered properties of the tube itself, inde- pendent of the associated circuits.

The principal functions that may be performed by vacuum tubes are listed below.

Rectification. Vacuum tubes are able to convert alternating currents to direct currents. This is known as "rectification." Rectification is an inherent property of vacuum tubes because current can flow inonly one direction from a source of electrons.

If a sinusoidal wave of voltage is applied to a vacuum tube of the right type, current will flow in only one direction, giving rise toasucces- sion of half-wave pulses all of the same polarity. It is possible to connect another like tube to insert half-wave pulses of the same polarity between the pulses of the first tube. The average of these pulses constitutes a direct current; the other frequency components are rejected by a filter circuit.

Rectification is important because electronic devices operate best on direct current, while power is usually generated and transmitted in alter- nating form. Itis thus necessary to convert, or rectify, the a-c power to d-c power.

Amplification. The amplification of voltage or power is the Qutstp·nd- ingfunction that vacuum tubes are able to perform. With the exception of the mechanical torque amplifier, no other device can do anything like it. Strictly speaking, the vacuum tube does not amplify power but rather controls the flow of a relatively large amount of power from one source with a small amount of power from another source. The British use the expression "electric valve" for certain types of electron tubes.

This term is really better than ours, for it indicates the nature of the amplifying action.

OsciUation. The generation of high-frequency alternating currents, or oscillation, is another remarkable function that vacuum tubes can perform. Oscillation is obtained by causing part of the output of an amplifier to excite the amplifier and thus make the device self-excited and self-sustaining. Tubes can be built that ,vill produce oscillations at frequencies as low as 1 cycle per sec, while other tubes can be built that will oscillate at frequencies as high as 60,000 me per sec.

Frequency Conversion. Vacuum tubes are able to shift the frequency of a wave. This they are able to do by an electrical "beat" action ..

4 VACUU~1 TIJBES

Thus a wave of a given frequency can be mixed with a \vave of another frequency in a vacuum tube, and among the products of the interaction is found the difference of the two frequencies. If one of the original waves had certain effects associated "rith it, these same effects are associated with the difference frequency. The beat action results from the nonlinear characteristics of the vacuum tube.

Modulation. The transmission of intelligence by radio waves or by certain types of wire telephony requires the use of frequencies higher than those audible. It is necessary to superimpose the audible frequencies upon the higher transmitted frequency. This superimposition is known as "modulation." Modulation is best performed by vacuum tubes.

Basically, modulation takes the form of varying some property ofthe r-fwave at the audible rate. The commonest form of modulation varies the amplitude of the r-f wave in accordance with the intelligence to be transmitted. This is known a~ "amplitude modulation." Frequency modulationis also common.

Detection. Detection is the inverse of modulation and issometimes known as " demodulation." It is the process of extracting the intelligence from the modulated 1vave. In the case of amplitude modulation the detectionmay he effected by rectifying the r-f wave and then utilizing the average value of the rectified ,vave, sinceit follo,vs the amplitude varia- tions in magnitude. Detection of modulated radio signals is best per- formed by vacuum tubes over most of the range of radio frequencies.

Light-image Production. It is possible for vacuum tubes to convert part of their output energy into visible light. rrhis is done in cathode- ray tubes in which a stream of electrons is caused to hit a fluorescent screen, causing light to be emitted. l'he cathode-ray tube can be used for viewing wave forms and for doing many other wonderful things, including the reproduction of visual scenes. The fundamental property involved here is the conversion of electrical energy into visual energy.

Photoelectric Action. Vacuum tubes can be made that will convert light energy into electrical energy. l'his is possible by virtue of the photoelectric effect, which is the emission of electrons from certain sur- faces when illuminated with visual energy. The liberated electrons con- stitutean electric current whose measure is related to the frequency and intensity of the exciting light. Tubes making use of this principle are known as i tphotoelectric tubes." The photoelectric tube is one of the tubes most extensively used in industrial-control systems.

The above paragraphs have givenabird's-eye vie,v of the functions of vacuum tubes. The reader is probably familiar \vith all the above functions, which are now commonly encountered in everyday life. The rest of the book is devoted to the description and explanation of the characteristics of the vacuum tubes themselves.

CH-Lt\PTER 2

BASIC TU.BE TYPES

T

^HJ4] electronic engineer has about a dozen types of vacuum tube he can call upon for his high-frequency and industrial-control circuits_

This is a surprisingly small number of distinct tube types. The small number of types is balanced, however, by the large number of forms in which each type may appear, as determined by the required power capacity and frequency range.

The purpose of this chapter is to list the basic types and their funda- mental characteristics as a prelude to a detailed study of their charac- teristics and the physical laws from ,vhich these are derived.

2.1. Vacuum Diode. The vacuum diode is a t\vo-electrode vacuum tube. One electrode acts as an

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\vith plate voltage.

The most useful property of the diode is that it passes current only in one direction. This property makes the diode useful as a detector

~ndas a rectifier for d-c power supplies.

5

6 VACIJUM TUBES

2.2. Vacuum Triode. A vacuum triode is a three-electrode tube con- taining an emitting electrode called the "cathode," a control electrode called the "grid," and a current-collecting electrode called the "Rllode"

or "plate."

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FIG.2.2.-Plate-current-plate-voltage characteristics of a triode.

est form of triode. Byvirtue of its proximity to the cathode the grid is able toinfluence the electrostatic field at the cathode to a greater extent than can the plate, and thus it is able to control the flowofcurrent from the cathode. The grid is usually operated on a slight negative potential so that the electrons will pass between the grid wires without hitting the wires themselves.

Some typical characteristics of a triode illustrating the variation of plate current with plate voltage for various fixed values of grid voltage are shown in Fig. 2.2. The plate current increases if either grid or plate voltage is increased. The increase in plate current for a given increase in grid voltage is always much larger than the increase in plate current for the same increase in plate voltage.

BA.SIC TUBE TYPES 7 The relative effectiveness of the plate and grid potentials in controlling the plate current is known as the amplification factor of the tube (mu;

symbol p,). The amplification factor is the maximumamplification that can be obtained by using the tube as an amplifier. With triodes the useful amplification is about two-thirds of the amplification factor.

Study of the family of curves of Fig. 2.2 shows that all the curves are alikeinshape and further are somewhat similar to the characteristic of a diode. This is true in that the plate current of a triode is found to vary nearly as the three-halves po,ver of an equivalent voltage which is

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the sum of the plate voltage divided by mu and the grid voltage.

Triodes have their greatest use as power amplifiers. They are also used extensively in control applications wherever a small voltage is wanted to control an appreciable amount of current.

2.3. Screen-grid Tube.. The screen-grid tube is a four-element vacuum tube. ThefOUf elements are cathode, control grid, screen grid, and plate. The electrode construction is similar to that of the triode except thatanextra grid of meshalittle coarserthanthatofthe control gridis inserted between the control grid and the plate.

The screen-grid tube is the historical predecessor of the pentode.

Its inventionwasthe result of an effort to overcome a limitation of the triode. Triodes do not work well as amplifiers of high frequencies, for the high interelectrode capacity between plate and grid causes the tube to regenerate and oscillate. In the screen-grid tube the capacity between

8 VACUUM TUBES

the control grid and plate is reduced by inserting the extra grid, known as the "screen grid," bet,veen these elements. The insertion of the screen grid and its operation at a constant potential succeeded in produc- ing the low control-grid-plate capacity desired but caused distortions in the plate-current-plate-voltage characteristics, for the neW electrode arrangement permitted secondary electron.flowbetweenplate andscreen grid. Thisdetrimental effect was overcome in the pentode by the addi- tionof acoarse-mesll suppressor grid between screen grid and plate.

The screen-grid tube is usually operated with cathode near ground potential, control grid at a small negative potential, and screen grid and plate at a medium and high positive potential, respectively. Some typical screen-grid-tube plate-current-plate-voltage characteristics are shown in Fig. 2.3. The dips in the low-voltage portion of the curves are the result of secondary electron current flo,ving from plate to screen.

The low slope of the high-voltage portion of the curves results from the fact that the cathode is screened from the plate by the screen grid as wellasby the control grid, and hence the magnitude ofthe plate current is increased only a little by an increase in plate voltage. Screen-grid tubes have been rendered virtually obsolete by the development of the pentode and some special tetrodes not subject to the tremendous dis- tortions of current characteristics by secondary emission. Screen-grid tubes may be used as a-fand r-f amplifiers. They are also occasionally used in laboratory apparatus in which it is desirable to utilize the negative resistance characteristic ,vhich is available at the points on the current characteristics where the slope is negative.

2.4. Pentode. The pentode is a five-element high-vacuum tube.

The five electrodes, in the order in which they occur in the tube, are cathode, control grid, screen grid, suppressor grid, and plate. In normal operation the cathode is operated near ground potential,the control grid ata small negative potential, the screen grid at a relatively large positive potential, the suppressor grid at cathode potential, and the plate at the screen potential or a more positive potential.

Some typical plate-current-plate-voltage curves of a pentode are shown in Fig. 2.4. In theseitis seen that the insertion of the suppressor grid at cathode potential bet\veen screen grid and plate has eliminated the distortions in the characteristic observed in the case of the screen-grid tube. This it does by causinga negative potential gradient at both the screen grid and plate, which suppresses the secondary electrons from these electrodes. The slope of the plate-current characteristic for high plate voltages is even less than in the screen-grid tube, for there is another screening grid between plate and cathode in the pentode. The result of this high screening action is to make the amplification factor of the

BASIC TUBE TYPES 9 pentode extremely high, of the order af 1,000 or more, and to give the tube a high effective resistance in the plate circuit. The pentade is, in fact, very nearly a constant-current device. The variation of plate current with grid voltage, which is measured by a factor known as the

"grid-plate transconductance" or, more commonly, the "mutual con- ductance" of the tube, is about the same as in the triode. Only about one-tenth of the high amplification factor of the pentode can be realized

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in amplifier operation. However, the reduced plate-control-grid capacity makes the pentacle a better tube in voltage-amplifier applications.

The pentode is a versatile tube. It can be connected to give diode, triode, and screen-grid as well as pentode action. It is available in constant- and variable-mu forms. It is probably the most extensively used tube in low-power applications. There are probably more pentocles in use today than any other type of electron tube. A cutaway drawing of a pentade showing the electrode structure is given in Fig. 2.5.

2.5. Beam-power Tube. The beam-power tube is a special type of tetrode. It is designed so that the electrons move from cathode to plate in dense sheets. This effect is achieved by making the control grid and screen grid of the same pitch and aligning the grid wires. The electrode structure of the tube is shown in Fig. 2.6.