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The Operation of Vacuum Tube Receptors

The Radio Amateur’s Hand Book, by A. Frederick Collins is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here: [LINK TO TABLE OF LINK]. Chapter XV: Operation of Vacuum Tube Receptors


From the foregoing chapters you have seen that the vacuum tube can be used either as a detector or an amplifier or as a generator of electric oscillations, as in the case of the heterodyne receiving set. To understand how a vacuum tube acts as a detector and as an amplifier you must first know what electrons are. The way in which the vacuum tube sets up sustained oscillations will be explained in Chapter XVIII in connection with the Operation of Vacuum Tube Transmitters.

What Electrons Are.–Science teaches us that masses of matter are made up of molecules, that each of these is made up of atoms, and each of these, in turn, is made up of a central core of positive particles of electricity surrounded by negative particles of electricity as shown in the schematic diagram, Fig. 69. The little black circles inside the large circle represent positive particles of electricity and the little white circles outside of the large circle represent negative particles of electricity, or electrons as they are called.

It is the number of positive particles of electricity an atom has that determines the kind of an element that is formed when enough atoms of the same kind are joined together to build it up. Thus hydrogen, which is the lightest known element, has one positive particle for its nucleus, while uranium, the heaviest element now known, has 92 positive particles. Now before leaving the atom please note that it is as much smaller than the diagram as the latter is smaller than our solar system.

What Is Meant by Ionization.–A hydrogen atom is not only lighter but it is smaller than the atom of any other element while an electron is more than a thousand times smaller than the atom of which it is a part. Now as long as all of the electrons remain attached to the surface of an atom its positive and negative charges are equalized and it will, therefore, be neither positive nor negative, that is, it will be perfectly neutral. When, however, one or more of its electrons are separated from it, and there are several ways by which this can be done, the atom will show a positive charge and it is then called a positive ion.

In other words a positive ion is an atom that has lost some of its negative electrons while a negative ion is one that has acquired some additional negative electrons. When a number of electrons are being constantly given by the atoms of an element, which let us suppose is a metal, and are being attracted to atoms of another element, which we will say is also a metal, a flow of electrons takes place between the two oppositely charged elements and form a current of negative electricity as represented by the arrows at A in Fig. 70.

When a stream of electrons is flowing between two metal elements, as a filament and a plate in a vacuum tube detector, or an amplifier, they act as carriers for more negative electrons and these are supplied by a battery as we shall presently explain. It has always been customary for us to think of a current of electricity as flowing from the positive pole of a battery to the negative pole of it and hence we have called this the direction of the current. Since the electronic theory has been evolved it has been shown that the electrons, or negative charges of electricity, flow from the negative to the positive pole and that the ionized atoms, which are more positive than negative, flow in the opposite direction as shown at B.

How Electrons are Separated from Atoms.–The next question that arises is how to make a metal throw off some of the electrons of the atoms of which it is formed. There are several ways that this can be done but in any event each atom must be given a good, hard blow. A simple way to do this is to heat a metal to incandescence when the atoms will bombard each other with terrific force and many of the electrons will be knocked off and thrown out into the surrounding space.

But all, or nearly all, of them will return to the atoms from whence they came unless a means of some kind is employed to attract them to the atoms of some other element. This can be done by giving the latter piece of metal a positive charge. If now these two pieces of metal are placed in a bulb from which the air has been exhausted and the first piece of metal is heated to brilliancy while the second piece of metal is kept positively electrified then a stream of electrons will flow between them.

Action of the Two Electrode Vacuum Tube.–Now in a vacuum tube detector a wire filament, like that of an incandescent lamp, is connected with a battery and this forms the hot element from which the electrons are thrown off, and a metal plate with a terminal wire secured to it is connected to the positive or carbon tap of a dry battery; now connect the negative or zinc tap of this with one end of a telephone receiver and the other end of this with the terminals of the filament as shown at A in Fig. 71. If now you heat the filament and hold the phone to your ear you can hear the current from the B battery flowing through the circuit.

Since the electrons are negative charges of electricity they are not only thrown off by the hot wire but they are attracted by the positive charged metal plate and when enough electrons pass, or flow, from the hot wire to the plate they form a conducting path and so complete the circuit which includes the filament, the plate and the B or plate battery, when the current can then flow through it. As the number of electrons that are thrown off by the filament is not great and the voltage of the plate is not high the current that flows between the filament and the plate is always quite small.

How the Two Electrode Tube Acts as a Detector.–As the action of a two electrode tube as a detector [Footnote: The three electrode vacuum tube has entirely taken the place of the two electrode type.] is simpler than that of the three electrode vacuum tube we shall describe it first. The two electrode vacuum tube was first made by Mr. Edison when he was working on the incandescent lamp but that it would serve as a detector of electric waves was discovered by Prof. Fleming, of Oxford University, London. As a matter of fact, it is not really a detector of electric waves, but it acts as: (1) a rectifier of the oscillations that are set up in the receiving circuits, that is, it changes them into pulsating direct currents so that they will flow through and affect a telephone receiver, and (2) it acts as a relay and the feeble received oscillating current controls the larger direct current from the B battery in very much the same way that a telegraph relay does. This latter relay action will be explained when we come to its operation as an amplifier.

We have just learned that when the stream of electrons flow from the hot wire to the cold positive plate in the tube they form a conducting path through which the battery current can flow. Now when the electric oscillations surge through the closed oscillation circuit, which includes the secondary of the tuning coil, the variable condenser, the filament and the plate as shown at B in Fig. 71 the positive part of them passes through the tube easily while the negative part cannot get through, that is, the top, or positive, part of the wave-form remains intact while the lower, or negative, part is cut off as shown in the diagram at C. As the received oscillations are either broken up into wave trains of audio frequency by the telegraph transmitter or are modulated by a telephone transmitter they carry the larger impulses of the direct current from the B battery along with them and these flow through the headphones. This is the reason the vacuum tube amplifies as well as detects.

How the Three Electrode Tube Acts as a Detector.–The vacuum tube as a detector has been made very much more sensitive by the use of a third electrode shown in Fig. 72. In this type of vacuum tube the third electrode, or grid, is placed between the filament and the plate and this controls the number of electrons flowing from the filament to the plate; in passing between these two electrodes they have to go through the holes formed by the grid wires.

If now the grid is charged to a higher negative voltage than the filament the electrons will be stopped by the latter, see A, though some of them will go through to the plate because they travel at a high rate of speed. The higher the negative charge on the grid the smaller will be the number of electrons that will reach the plate and, of course, the smaller will be the amount of current that will flow through the tube and the headphones from the B battery.

On the other hand if the grid is charged positively, see B, then more electrons will strike the plate than when the grid is not used or when it is negatively charged. But when the three electrode tube is used as a detector the oscillations set up in the circuits change the grid alternately from negative to positive as shown at C and hence the voltage of the B battery current that is allowed to flow through the detector from the plate to the filament rises and falls in unison with the voltage of the oscillating currents. The way the positive and negative voltages of the oscillations which are set up by the incoming waves, energize the grid; how the oscillator tube clips off the negative parts of them, and, finally, how these carry the battery current through the tube are shown graphically by the curves at D.

How the Vacuum Tube Acts as an Amplifier.–If you connect up the filament and the plate of a three electrode tube with the batteries and do not connect in the grid, you will find that the electrons which are thrown off by the filament will not get farther than the grid regardless of how high the voltage is that you apply to the plate. This is due to the fact that a large number of electrons which are thrown off by the filament strike the grid and give it a negative charge, and consequently, they cannot get any farther. Since the electrons do not reach the plate the current from the B battery cannot flow between it and the filament.

Now with a properly designed amplifier tube a very small negative voltage on the grid will keep a very large positive voltage on the plate from sending a current through the tube, and oppositely, a very small positive voltage on the grid will let a very large plate current flow through the tube; this being true it follows that any small variation of the voltage from positive to negative on the grid and the other way about will vary a large current flowing from the plate to the filament.

In the Morse telegraph the relay permits the small current that is received from the distant sending station to energize a pair of magnets, and these draw an armature toward them and close a second circuit when a large current from a local battery is available for working the sounder. The amplifier tube is a variable relay in that the feeble currents set up by the incoming waves constantly and proportionately vary a large current that flows through the headphones. This then is the principle on which the amplifying tube works.

The Operation of a Simple Vacuum Tube Receiving Set.–The way a simple vacuum tube detector receiving set works is like this: when the filament is heated to brilliancy it gives off electrons as previously described. Now when the electric waves impinge on the aerial wire they set up oscillations in it and these surge through the primary coil of the loose coupled tuning coil, a diagram of which is shown at B in Fig. 41.

The energy of these oscillations sets up oscillations of the same frequency in the secondary coil and these high frequency currents whose voltage is first positive and then negative, surge in the closed circuit which includes the secondary coil and the variable condenser. At the same time the alternating positive and negative voltage of the oscillating currents is impressed on the grid; at each change from + to – and back again it allows the electrons to strike the plate and then shuts them off; as the electrons form the conducting path between the filament and the plate the larger direct current from the B battery is permitted to flow through the detector tube and the headphones.

Operation of a Regenerative Vacuum Tube Receiving Set.–By feeding back the pulsating direct current from the B battery through the tickler coil it sets up other and stronger oscillations in the secondary of the tuning coil when these act on the detector tube and increase its sensitiveness to a remarkable extent. The regenerative, or feed back, action of the receiving circuits used will be easily understood by referring back to B in Fig. 47.

When the waves set up oscillations in the primary of the tuning coil the energy of them produces like oscillations in the closed circuit which includes the secondary coil and the condenser; the alternating positive and negative voltages of these are impressed on the grid and these, as we have seen before, cause similar variations of the direct current from the B battery which acts on the plate and which flows between the latter and the filament.

This varying direct current, however, is made to flow back through the third, or tickler coil of the tuning coil and sets up in the secondary coil and circuits other and larger oscillating currents and these augment the action of the oscillations produced by the incoming waves. These extra and larger currents which are the result of the feedback then act on the grid and cause still larger variations of the current in the plate voltage and hence of the current of the B battery that flows through the detector and the headphones. At the same time the tube keeps on responding to the feeble electric oscillations set up in the circuits by the incoming waves. This regenerative action of the battery current augments the original oscillations many times and hence produce sounds in the headphones that are many times greater than where the vacuum tube detector alone is used.

Operation of Autodyne and Heterodyne Receiving Sets.–On page 109 [Chapter VII] we discussed and at A in Fig. 36 is shown a picture of two tuning forks mounted on sounding boxes to illustrate the principle of electrical tuning. When a pair of these forks are made to vibrate exactly the same number of times per second there will be a condensation of the air between them and the sound waves that are sent out will be augmented. But if you adjust one of the forks so that it will vibrate 256 times a second and the other fork so that it will vibrate 260 times a second then there will be a phase difference between the two sets of waves and the latter will augment each other 4 times every second and you will hear these rising and falling sounds as beats.

Now electric oscillations set up in two circuits that are coupled together act in exactly the same way as sound waves produced by two tuning forks that are close to each other. Since this is true if you tune one of the closed circuits so that the oscillations in it will have a frequency of a 1,000,000 and tune the other circuit so that the oscillations in it have a frequency of 1,001,000 a second then the oscillations will augment each other 1,000 times every second.

As these rising and falling currents act on the pulsating currents from the B battery which flow through the detector tube and the headphones you will hear them as beats. A graphic representation of the oscillating currents set up by the incoming waves, those produced by the heterodyne oscillator and the beats they form is shown in Fig. 73. To produce these beats a receptor can use: (1) a single vacuum tube for setting up oscillations of both frequencies when it is called an autodyne, or self-heterodyne receptor, or (2) a separate vacuum tube for setting up the oscillations for the second circuit when it is called a heterodyne receptor.

The Autodyne, or Self-Heterodyne Receiving Set.–Where only one vacuum tube is used for producing both frequencies you need only a regenerative, or feed-back receptor; then you can tune the aerial wire system to the incoming waves and tune the closed circuit of the secondary coil so that it will be out of step with the former by 1,000 oscillations per second, more or less, the exact number does not matter in the least. From this you will see that any regenerative set can be used for autodyne, or self-heterodyne, reception.

The Separate Heterodyne Receiving Set.–The better way, however, is to use a separate vacuum tube for setting up the heterodyne oscillations. The latter then act on the oscillations that are produced by the incoming waves and which energize the grid of the detector tube. Note that the vacuum tube used for producing the heterodyne oscillations is a generator of electric oscillations; the latter are impressed on the detector circuits through the variable coupling, the secondary of which is in series with the aerial wire as shown in Fig. 74. The way in which the tube acts as a generator of oscillations will be told in Chapter XVIII.

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Collins, A. Frederick. 2002. The Radio Amateur’s Hand Book. Urbana, Illinois: Project Gutenberg. Retrieved April 2022, from

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