The A B C of vacuum tubes in radio reception; an elementary and practical book on the theory and operation of vacuum tubes as detectors and amplifiers. Explains non-mathematically the fundamental principles upon which all vacuum tube circuits are based. Includes practical circuits and practical questions and answers. It is written especially for the beginner. All electrical and radio terms are fully explained

Front matter

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Preface

The inauguration of radio-telephone broadcasting has quickly aroused an enormous general interest in radio reception of speech and music. The possibilities of this new method of broadcasting information of general interest and entertainment can hardly be estimated. The method is revolutionary and has been aptly compared to the invention of the printing press in this respect.

Vacuum tubes make possible the reception of radio-telephone speech and music and radio-telegraph signals with a degree of satisfaction not approached by prior devices.

The vacuum tube appears to the layman as a complicated form of incandescent electric lamp which operates in a very mysterious manner. In writing this little book the author has endeavored to overcome this impression and to clearly set forth the elementary principles of theory and operation. It is assumed that the reader has no knowledge of radio and very little, if any, of electricity. Mathematics has been especially avoided in the treatment of the subject and no technical terms are used without their meaning being made clear.

The functioning of the "lamps" is fascinating to a great many people and the author hopes that the desire for elementary information regarding it will be fulfilled in the text. No attempt has been made to set forth all the possible circuit arrangements but those shown may serve as suggestions to experimenters who desire to evolve their own circuits. The practical questions and answers, it is hoped, may be of value to those who are contemplating the installation of receiving

PREFACE

equipment or who are getting unsatisfactory results because of insufficient information.

Suggestions regarding improvement of the material presented and the manner of presentation will be gratefully received from readers.

April, 1922. The Author.

Contents

PAGE

Chapter I

Electric current — Electrical pressure or Electromotive force — Resistance— Direct current — Alternating current — Frequency — Inductance . — Capacitance — Resonance — Oscillations — The Reception of signals— Wave length — Tuning the receiving antenna — The Antenna as a complete circuit 15

CHAPTER II The Vacuum Tube as a Rectifier and Relay Without Moving

Parts

Tube construction — The Edison Effect — Electron emission from a heated filament — Dependence of plate current upon plate potential — Dependence of plate current upon filament current — Effect of grid potential upon plate current — Relay action — Rectification ... 38

CHAPTER VII Practical Questions and Answers Practical Questions and Answers . ......... 116 ~ List of Symbols ~

Battery — 1|| | |h-

Direct Current Dynamo --- Bh M ^B ca

Alternating \ ^-. recurrent f Xy~ or _JQ)

Generator ) ^— ^ v —

Fixed Capacitance " T"

(Condenser )

Variable Capacitance lb*

f l^br: Condenser) /\\

/vW Inductance _..^^7ffiff5ff\„

(Air Core Coil) VUVVVU

(Air Core Coil) y^ a

Variometer ( Variable Inductance) —

Fixed Co up ling of Co Us — \JHlfifiil/~~

(Fixed Inductive Coupling) ^TT\Kf^\^

Variable Coupling of Coils

t , "IS

Transtormer— gj

Iron Core Inductance ( or Reactance Coil )

-List of Symbols-

Ground

Wire Connections

V or or

No Connections of Wires Fixed Res is tctnce

Variable Resistance

Vacuum Tube —

Telephone Receivers

Voltmeter - •■

Ammeter

Galvanometer

— vww—

GALVO.

Crystal Detector

Chapter I

Electric current — Electrical pressure or Electromotive force — Resistance— Direct current — Alternating current — Frequency — Inductance — Capacitance — Resonance — Oscillations — The reception of signals — Wave length — Tuning the receiving antenna — The antenna as a complete circuit.

Before taking up the subject of vacuum tubes a brief outline of the most common electrical and radio phenomena together with the explanation of common electrical terms will be given. It is understood that for some this may be a review of knowledge previously gained. For others it may, however, serve to fix some very vague understandings and some familiarity with these fundamental facts is necessary in order to comprehend that which follows.

Electrical Current

It is very commonly known that a battery and a dynamo furnish what is called a "current of electricity" and that the electrical current will not flow unless a path is provided for it. The path is called the "electrical circuit" or simply the "circuit." For example, if it is desired to ring a door-bell it is necessary to connect the bell to a battery by means of insulated wires and if the path, or circuit, is broken at any point the bell will not ring. A push button may be used to keep the circuit broken until the button is pressed, whereupon an electric current flows through the circuit and the bell rings.

IS i6

Fig. I represents such a circuit which may be closed at will by pushing the button. Since no electricity flows when the circuit is open we may assume that it flows in all parts of the circuit when it is closed. Thus, if the current flows from the battery at "A" it must return to the battery at "B." The terminal of the battery from which the current flows is called

\Push Button

Contacts

Positive Terminal-'

B

Bell

- Negative

Terminal

Battery Fig. i — Simple electrical path or circuit.

the "positive terminal and the other the "negative" terminal.

To illustrate the flow of current, a closed circuit of pipe

may be used. A pump is located at "A" in Fig. 2 and the

Fig. 2 — Water analogy of simple electrical circuit.

pipe is considered to be full of water. When the pump is operated the water will be forced to flow, let us say, in the direction of the arrow. It flows from the pump through the pipe and returns to the pump and the same amount of water which leaves the pump in one second of time will return to it in the same length of time. In other words, there is a continuous flow of water until the pump stops. This corresponds to the electrical circuit and it may very easily be proven that the current is no stronger in one part than it is in any other part of the circuit.

We may measure the current or rate of flow of the water in gallons per second. That is, if there is one gallon flowing in one second past any given point in the pipe the rate of flow is one gallon per second. The current or rate of flow of electricity also may be measured and the unit is called the "ampere." The rate of flow of electric current is sometimes referred to as its "strength" and if the current strength is one ampere its rate of flow is one ampere, a certain fixed quantity of electricity per second.

It must not be assumed that electricity is actually a fluid like water. It is not. No one knows just what electricity is but its effects may be noted and its "flow" may be compared in many respects to that of water, i

Electrical Pressure or Electro-Motive Force

Electrical pressure is referred to as "electro-motive force" since in order to create a current of electricity it is necessary to provide some force to make it flow. In the case of the pipe full of water the pump must exert a force or pressure upon the water to push it through the pipe. In the case of the electrical circuit in Fig. 1 the battery furnishes the electrical pressure. Neither water nor electricity can flow unless a pressure is provided. If we wish either the water or the electric current to flow faster (at an increased rate) the pressure applied to it must be increased.

Water pressure may be measured in pounds per square inch but electrical pressure or electro-motive force is measured in volts. Sometimes electrical pressure is referred to as 'Voltage." E. M. F. is a convenient abbreviation to use for the long expression "electro-motive force" and it will be used hereafter in this text.

Resistance

When water flows through a pipe there is always a certain amount of friction between the water and the pipe. This hinders the flow of the water, that is, reduces the rate at which the water current flows. And the reduction in rate of flow depends directly upon the increase of the friction. Likewise there is friction present in the path of the electric current which acts as a "resistance" to the current flow. If the pressure or E. M. F. is kept constant the rate of current flow (current strength) increases as the resistance is decreased and vice versa. If, however, the resistance is kept constant it is natural to expect the current (rate of flow) of the electricity to increase with an increase of E. M. F. and vice versa. The current thus depends upon both the resistance and the E. M. F.

The unit of electrical resistance is called the "ohm" and if the resistance of a circuit is one ohm, one ampere of current will flow if the E. M. F. is one volt. This relation between current, E. M. F. and resistance is called "Ohm's Law" and is the first fundamental law of electrical circuits.

Direct Current

If the E. M. F. applied in a circuit is steady and only acts to force the current to flow in one direction through a circuit the resulting current is steady and is called a "direct current."

If the E. M. F. rises and falls but only acts in one direction the resulting current rises and falls and is called a "pulsating direct current" or a "pulsating current." Batteries produce a steady direct current in one direction through a circuit. Many dynamos, however, produce a pulsating E. M. F. which causes a pulsating current to flow (in one direction only) through a circuit. The periodical increases and decreases of E. M. F. and current may be made very small, however, so that they are hardly perceptible.

Alternating Current

Imagine that instead of a battery we have a dynamic generator of electrical pressure in a circuit and that the E. M. F. produced acts in first one direction, then the other, and that

^ Second of Time

Current Max. in Heg. Direction

Fig.

3 — One cycle of current changes and reversals.

per second.

Frequency 60 cycles while acting in each direction it increases to a maximum value and decreases to zero. The current created will also change its direction of flow and increase and decrease in strength periodically, as shown in Fig. 3. Such a current could be said to "alternate" in its direction of flow and is aptly called an "alternating current." These alternations or reversals of current flow, with their accompanying increases and decreases in rate of flow, take place very rapidly and at perfectly regular intervals. In this figure the current values or "rates of flow" are shown as increasing and decreasing in one direction above the horizontal line and increasing and decreasing in the opposite direction below that line. In order to distinguish the directions of flow from each other we may call one the "positive" direction and the other the "negative" direction, so that all current values along the vertical line and above the horizontal line are in the positive direction and all those along the vertical but below the horizontal are in the negative direction. During the period of time represented by the horizontal line the current has increased to a maximum value and decreased to zero in the positive direction then has reversed its direction and increased to a maximum and decreased to zero again. In increasing and decreasing, the current has passed through all values from zero to maximum in each direction. It has thus gone through one "cycle" of current changes and directions of flow. As long as the E. M. F. acts, the current will continue to go through these changes in a perfectly regular manner periodically.

Frequency

In the ordinary house lighting circuit, if it has alternating current, the periodical reversals of current flow usually take place at the rate of 60 cycles per second. It may be said then that the "frequency" of the alternating current is 60 cycles per second.

In radio work extremely high frequencies, varying approximately between 15,000 and 2,000,000 cycles per second, are also used. Such very high frequencies are called "radio frequencies." It is not within the scope of this little book tc discuss the various methods of producing alternating currents of high frequency but it may be said that all radio transmitters do produce them and that they are very essential.

Inductance

Inductance, or, more properly, "self inductance," is a property possessed to a greater or lesser extent by every electrical circuit. It has no effect upon a direct current but does have an effect, which will be discusssed, upon an alternating curk- I Cycle of Current J

Fig. 4 — Effect of inductance in an alternating current circuit.

rent. As shown in Fig. 4 it has the effect of causing the current to rise to its maximum value after the maximum value of E. M. F.

The current is said to "lag" behind the E. M. F.

"Reactance" is the name given to an "effective" resistance to alternating current which depends directly upon the value of the inductance and upon the frequency of the alternations.

In an alternating current circuit, if we for the moment disregard the actual resistance, the average value of current decreases as the reactance increases providing that the E. M. F. has a constant average value. That is, the reactance has the effect of a resistance although it is not true "ohmic' 'or actual resistance. If the circuit contains reactance and ohmic resistance, the average current value will decrease with an increase in either or both providing again that the average E. M. F. is constant.

A straight wire possesses inductance to some extent and the inductance of a wire increases with its length. Given a certain length of wire, its inductance and therefore its reactance may be very greatly increased by coiling it. Also, if more turns are added to the coil or if an iron core is placed inside of it, its inductance is increased and its reactance as well.

To distinguish this type of reactance from a type considered below we may call it "inductive reactance." To sum up, then, inductive reactance depends directly upon the inductance and frequency of current. The average value of alternating current in a circuit varies inversely with the reactance. *

Capacitance

Capacitance, or, as it is sometimes called, "capacity," has exactly the opposite effect of inductance upon an alternating current. Capacitance has the effect of causing the current to rise to its maximum value before the E. M. F. The current in this case is said to "lead" the E. M. F., as is graphically shown in Fig. 5.

* The reciprocal of the product of capacitance and fre-

* If the product were 2, the reciprocal would be l/2. The reciprocal of

3 is 1-3, of 2x 100 it is ■ 1— and so forth.

quency determines the value of a reactance which in this case may be called a "capacitive reactance." The capacitive reactance, however, does not have the effect of a resistance to the alternating current but exactly the opposite. The average

/ Cycle of Current p\

I I

^ I Cycle ofE.M.F. >j

Fig- 5 — Effect of capacitance in an alternating current circuit.

current value depends directly upon the capacitive reactance in a circuit, providing the average value of E. M. F. is kept constant. The average value of the alternating current then increases with an increase in either the average E. M. F. or the capacitive reactance and decreases with an increase of actual resistance. Capacitive reactance decreases with an increase of either capacitance or frequency, and vice versa.

A straight wire has some capacitance which increases with the length of the wire. Coiling the wire will also increase its capacitance. But since inductance is the predominant property the capacitance of a wire or coil is very small in comparison.

If two metallic plates are assembled with a sheet of glass between them there is produced what is known as a ' 'condenser." This arrangement would have a very appreciable capacitance. To increase the capacitance of this condenser we could (i) increase the area of the glass and the metallic plates; (2) decrease the thickness of the glass (the distance separating the metallic plates) ; (3) use a substance between the plates which has a higher "dielectric constant.' ' The material between the plates, whatever it may be, is called the "dielectric" and although it may not be an absolute insulator it does have a very high resistance, usually, to the passage of a current. Air is assumed to have a "dielectric constant" of unity (or one) and if a condenser with the same dimensions has a dielectric which gives five times the capacity of the air condenser, the "dielectric constant" of the other material is five. Some dielectric constants in the order of their increase are: Air — 1, paraffine — 2; hard rubber — 2.5; glass — 6 to 8; mica — 8.

It would be very convenient at times to have a variable capacitance whose value could be adjusted at will. If the maxi-

.'Terminafs--.

* si

Stationary Plates-*

<- Movable Plates

Fig. 6 — Scheme of a variable condenser. (If the movable plates are made stationary it is a "fixed" condenser.)

mum value is not necessarily very large, an air dielectric may be used. A number of movable plates could be arranged so that they interleave with a number of stationary plates without touching them. All the stationary plates are connected together and to a terminal and the movable plates are connected together and to a second terminal. There will be an increase in the maximum capacitance obtainable if more plates are used because this will increase the total plate area, the dielectric being the air between the stationary and movable plates. Fig. 6 shows how the plates are interleaved and Fig. 7 illustrates a variable air condenser with and without its case.

The question may be asked, "How does an electric current flow through a condenser if the dielectric is a good insulator

Connecting „ , Posts

Scale in Degrees^

Man die to Rotate Movable Plates

Shaft upon which Movable Plates are

Mounted

'Insulating Buttons .. ,, n, j ) or Washers between

Movable Plates- Plates

Fig. 7 — Typical variable condenser with air dielectric.

and prevents the passage of current?" A direct current will not flow through a condenser and neither will an alternating current. But the alternating current will appear to flow through because in all parts of the circuit on either side of the dielectric it rises and falls in rate of flow back and forth between the dielectric and the source of E. M. F. To make the matter more clear, suppose an analogy is considered. In Fig. 8 a pipe full of water is represented with an elastic partition at "A" and a pump at "B." The pump is so arranged that the to-and-fro motion of the piston exerts a pressure periodically in one direction then the other and causes the water, therefore, to flow in each direction periodically. The water does not flow past the elastic partition at any time. But, the elasticity of the partition allows it to stretch and during the time the current is forced in the direction of the full line arrow, it stretches and exerts a back pressure upon the water exactly equal to the pressure of the pump at every instant. When the pump pushes the water in the opposite direction the partition stretches in the opposite direction and again exerts a back pressure equal at every instant to the pump pressure. There is thus a surge of water back and forth through the pipe, causing the partition to stretch in first one direction then the other, within the limit of its elasticity. And, then, as far as the pipe is concerned, there is an alternating current of water in it in spite of the partition. The dielectric of a condenser is the "partition" and it must have electrical elas-

Elash'c Partition-

Piston Rod-'

B Pump

Fig. 8 — Water analogy of alternating current in a circuit containing a

condenser.

ticity, otherwise it might be ruptured in the same manner that the partition in the pipe would be, if the pump pressure became too great. It is thus readily seen that while an alternating current does not actually flow through the dielectric, the current does alternate in the circuit in spite of the dielectric.

INTRODUCTORY 2J

Resonance

Suppose two tuning forks are tuned to the same musical pitch and placed a foot or so apart. If one is struck a light blow it vibrates and a sound is heard which is the musical pitch dependant upon the rate of vibration of the fork. The fork is setting up waves in the air which reach the ear and are translated by the brain into a musical tone which is recognized. What is perhaps more interesting, however, is that the second tuning fork also vibrates in sympathy with the first. If the first fork is grasped in the hand to stop its vibration, the second will still vibrate for a short time and emit a musical tone exactly of the same pitch as the first one. It is evident that the second fork is vibrating at the same rate as the first before it was grasped. It is said to be "in tune" or "in resonance" with the first.

Suppose, however, that the first tuning fork is adjusted so that, when struck, it vibrates at a different rate. This time the second one will not vibrate in sympathy as it would if it also had been adjusted to vibrate at the same rate. The two forks are then not in tune with each other and the second \% "out of resonance" with the first.

In the first case the second fork was forced to vibrate at the same mechanical frequency (of vibration) as the first fork, because it was in resonance. In the second case there was no resonance, so the second fork was unaffected.

An electrical circuit in which an alternating current flows does not vibrate mechanically, but it may be thought of as vibrating electrically. Also, one electrical circuit may be adjusted so that it is "in resonance" with another and so that an alternating current flowing in one will produce an alternating current in the other of the same frequency. In this case of "electrical resonance," the second circuit is caused to 'Vibrate electrically" in sympathy with the first.

Oscillations

Alternating electrical currents of extremely high frequency are said to be alternating at "radio frequencies," as has been noted. And such high frequency alternating currents are

-f

Zero Current'

<- /Cycle

*l

-Amplitude

Jn£o5{tlve_ I

Direction '

of Flow

•Amplitude in Negative Direction of Flow

Fig. 9 — Continuous oscillations.

called "Radio frequency alternating currents." Instead of using this long name a shorter one has been selected for practical use, and it is said that such alternating currents are "oscillations." A definition of oscillations may then be, "radio frequency alternating current."

The maximum value of current strength attained in each direction of current flow during each cycle is called the "amplitude." If successive amplitudes are of equal values as long as the current flows, the oscillations are said to be "continuous oscillations." An alternating E. M. F. of constant amplitude is, of course, necessary to the production of continuous oscillations. Fig. 9 may assist the reader to understand this term. An ordinary, low frequency, alternating current also has constant amplitudes.

If an E. M. F. has constantly decreasing amplitude it will produce a current of constantly decreasing amplitude. Also, if the amplitudes of the E. M. F. decrease rather rapidly, the current will cease to flow after a short period of time because of the absence of E. M. F. to force it to flow. If, then, the successive amplitudes of oscillations decrease, the

•t

^■5

^t Cycle ->\

t± 1 Train of Oscillations — ->j k — I Train of Oscillations — >|

.-Amplitudes in Positive Direction \

Zero Current ''

Is

''^Amplitudes in

Negative Direction or Flow

Fig. 10 — Damped oscillations.

oscillations may be said to be "damped," and there is produced what are called "damped oscillations." Sometimes continuous oscillations are referred to as "undamped oscillations," because their successive amplitudes are not damped. When damped oscillations finally cease they are said to be "damped out." Fig. 10 will assist in understanding this matter. Two series, or groups, of damped oscillations are shown, and each series may be referred to as a "train" of damped oscillations.

If an electrical shock were given to a circuit containing inductance and capacitance, that is, if an electrical pressure

3o A B C OF VACUUM TUBES

were suddenly applied and immediately withdrawn, the circuit would vibrate electrically in the same manner as the struck tuning fork vibrated mechanically. An alternating current would be produced in the circuit, providing its resistance were not too great, but the successive amplitudes would decrease and the oscillations would die out just as the vibrations of the tuning fork die out a short while after the fork is struck. In the second circuit, just as with the second fork, even though there is resonance the vibrations die out after a short time.

If, however, the first tuning fork were tapped very lightly at the end of each swing it would continue to vibrate as long as the tapping took place at the proper times and in the proper amounts to maintain these vibrations (or swings). The regular periodical tapping may be compared to a regular periodical E. M. F. applied in an electrical circuit. If the E. M. F. is applied at the proper times and always in the same amounts, the oscillations will not decrease in amplitude but will be "continuous." Now, in the case of the tuning forks, if the vibrations of the first one are continuous (due to continuously applied taps), the vibrations of the second one will continue and be of constant amplitude. Likewise, in a second electrical circuit, if the oscillations in the first are continuous those in the second will also be continuous. Continuous oscillations in a second circuit are then the result of a continuously applied E. M. F. in that circuit due, in turn, to a continuously applied E. M. F. (and therefore oscillations of continuous amplitude) in the first circuit.

The action of producing oscillations in a second circuit by creating oscillations in a primary circuit is called "induction," and we may say that when oscillations are produced in a secondary circuit they are "induced" therein.

The Reception of Signals

It should now be quite clear that if oscillations are produced in a suitable circuit at a transmitting station oscillations will be produced by induction in a suitable circuit at a receiving station, when the two circuits are in resonance with each other. Each of the circuits is called an "antenna," or sometimes, an "aerial," since each is frequently erected so that it extends some distance from the earth up into the air.

When vibrations are induced in a tuning fork by another, the air between them is set in motion and it is this air which transmits the motion from one fork to the other. That is, the first fork sets the air into vibration and the vibrating air sets the second fork into vibration. Another illustration would be the dropping of a stone into a quiet pond of water. Water waves radiate from the point where the stone strikes the water until they reach a cork located a distance away. The cork is set into motion and bobs up and down, or oscillates. But the cork does not move further away from or closer to the point where the oscillations were created in the water. This proves that the water itself does not travel from one place to another but that the oscillating motion is transmitted or radiated from the point where the disturbance occurs. In the case of either the air or the water it is the oscillatory or vibratory motion which is transmitted.

This throws some light upon electrical induction, But in the case of oscillations produced in a secondary electrical circuit neither air nor Water is necessary to transmit the motion. In fact, electrical oscillations in a circuit have no effect upon either air or water. There must be some medium, however, which is capable of being set into vibration by the electrical oscillations and will, in turn, create electrical oscillations in a distant circuit. This medium is called the "ether," and is one which pervades all space. It is present in the entire universe where water and air may be entirely absent. It is, in fact, the ether which is set into vibration by the oscillations in a transmitting antenna. These vibrations spread out in circles just like the ripples in the water after a stone is dropped and when they reach a receiving antenna, create electrical oscillations in that circuit. The whole purpose of "radio" is to create ether vibrations and to translate them at a distant point.

Now, assume that continuous oscillations or recurrent trains of damped oscillations are produced in a transmitting antenna and either continuous or recurrent trains of damped oscillations are induced in the receiving antenna. We still lack, a device to make the received oscillations perceptible to one of our senses. But in a following chapter means for accomplishing this necessary result are considered.

Wave Length

The ether vibrations or oscillations have the same frequency as the electrical oscillations which create them. Moreover, this oscillatory motion of the ether is projected through it with the speed of light, light being only ether vibration at considerably different frequencies. This rate of speed is 300,000,000 meters (approximately 186,000 miles) per second of time. Almost too rapid a rate to conceive — in fact, ether oscillations caused by electrical oscillations travel over seven times around the earth in one second.