Dissecting Vacuum Tube Transmitters

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 XIX: The Operation of Vacuum Tube Transmitters

The three foregoing chapters explained in detail the design and construction of (1) two kinds of C. W. telegraph transmitters, and (2) two kinds of wireless telephone transmitters, the difference between them being whether they used (A) a direct current, or (B) an alternating current as the initial source of energy. Of course there are other differences between those of like types as, for instance, the apparatus and connections used (a) in the key circuits, and (b) in the microphone circuits. But in all of the transmitters described of whatever type or kind the same fundamental device is used for setting up sustained oscillations and this is the vacuum tube.

The Operation of the Vacuum Tube Oscillator.--The operation of the vacuum tube in producing sustained oscillations depends on (1) the action of the tube as a valve in setting up the oscillations in the first place and (2) the action of the grid in amplifying the oscillations thus set up, both of which we explained in Chapter XIV. In that chapter it was also pointed out that a very small change in the grid potential causes a corresponding and larger change in the amount of current flowing from the plate to the filament; and that if a vacuum tube is used for the production of oscillations the initial source of current must have a high voltage, in fact the higher the plate voltage the more powerful will be the oscillations.

To understand how oscillations are set up by a vacuum tube when a direct current is applied to it, take a look at the simple circuits shown in Fig. 94. Now when you close the switch the voltage from the battery charges the condenser and keeps it charged until you open it again; the instant you do this the condenser discharges through the circuit which includes it and the inductance coil, and the discharge of a condenser is always oscillatory.

Where an oscillator tube is included in the circuits as shown at A and B in Fig. 94, the grid takes the place of the switch and any slight change in the voltage of either the grid or the plate is sufficient to start a train of oscillations going. As these oscillations surge through the tube the positive parts of them flow from the plate to the filament and these carry more of the direct current with them.

To make a tube set up powerful oscillations then, it is only necessary that an oscillation circuit shall be provided which will feed part of the oscillations set up by the tube back to the grid circuit and when this is done the oscillations will keep on being amplified until the tube reaches the limit of its output.

The Operation of C. W. Telegraph Transmitters With Direct Current--Short Distance C. W. Transmitter.--In the transmitter shown in the wiring diagram in Fig. 76 the positive part of the 110 volt direct current is carried down from the lamp socket through one side of the panel cut-out, thence through the choke coil and to the plate of the oscillator tube, when the latter is charged to the positive sign. The negative part of the 110 volt direct current then flows down the other wire to the filament so that there is a difference of potential between the plate and the filament of 110 volts. Now when the 6-volt battery current is switched on the filament is heated to brilliancy, and the electrons thrown off by it form a conducting path between it and the plate; the 110 volt current then flows from the latter to the former.

Now follow the wiring from the plate over to the blocking condenser, thence to clip 3 of the tuning coil, through the turns of the latter to clip 2 and over to the filament and, when the latter is heated, you have a closed oscillation circuit. The oscillations surging in the latter set up other and like oscillations in the tuning coil between the end of which is connected with the grid, the aerial and the clip 2, and these surge through the circuit formed by this portion of the coil, the grid condenser and the filament; this is the amplifying circuit and it corresponds to the regenerative circuit of a receiving set.

When oscillations are set up in it the grid is alternately charged to the positive and negative signs. These reversals of voltage set up stronger and ever stronger oscillations in the plate circuit as before explained. Not only do the oscillations surge in the closed circuits but they run to and fro on the aerial wire when their energy is radiated in the form of electric waves. The oscillations are varied by means of the telegraph key which is placed in the grid circuit as shown in Fig. 76.

The Operation of the Key Circuit.--The effect in a C. W. transmitter when a telegraph key is connected in series with a buzzer and a battery and these are shunted around the condenser in the grid circuit, is to rapidly change the wave form of the sustained oscillations, and hence, the length of the waves that are sent out. While no sound can be heard in the headphones at the receiving station so long as the points of the key are not in contact, when they are in contact the oscillations are modulated and sounds are heard in the headphones that correspond to the frequency of the buzzer in the key circuit.

The Operation of C. W. Telegraph Transmitters with Direct Current.--The chief differences between the long distance sets which use a direct current, i.e., those described in Chapter XVI, and the short distance transmitting sets are that the former use: (1) a motor-generator set for changing the low voltage direct current into high voltage direct current, and (2) a chopper in the key circuit. The way the motor-generator changes the low- into high-voltage current has been explained in Chapter XVI.

The chopper interrupts the oscillations surging through the grid circuit at a frequency that the ear can hear, that is to say, about 800 to 1,000 times per second. When the key is open, of course, the sustained oscillations set up in the circuits will send out continuous waves but when the key is closed these oscillations are broken up and then they send out discontinuous waves. If a heterodyne receiving set, see Chapter XV, is being used at the other end you can dispense with the chopper and the key circuit needed is very much simplified. The operation of key circuits of the latter kind will be described presently.

The Operation of C. W. Telegraph Transmitters with Alternating Current--With a Single Oscillator Tube.--Where an oscillator tube telegraph transmitter is operated by a 110 volt alternating current as the initial source of energy, a buzzer, chopper or other interruptor is not needed in the key circuit. This is because oscillations are set up only when the plate is energized with the positive part of the alternating current and this produces an intermittent musical tone in the headphones. Hence this kind of a sending set is called a tone transmitter.

Since oscillations are set up only by the positive part or voltage of an alternating current it is clear that, as a matter of fact, this kind of a transmitter does not send out continuous waves and therefore it is not a C. W. transmitter. This is graphically shown by the curve of the wave form of the alternating current and the oscillations that are set up by the positive part of it in Fig. 95. Whenever the positive half of the alternating current energizes the plate then oscillations are set up by the tube and, conversely, when the negative half of the current charges the plate no oscillations are produced.

You will also observe that the oscillations set up by the positive part of the current are not of constant amplitude but start at zero the instant the positive part begins to energize the plate and they keep on increasing in amplitude as the current rises in voltage until the latter reaches its maximum; then as it gradually drops again to zero the oscillations decrease proportionately in amplitude with it.

Heating the Filament with Alternating Current.--Where an alternating current power transformer is used to develop the necessary plate voltage a second secondary coil is generally provided for heating the filament of the oscillation tube. This is better than a direct current for it adds to the life of the filament. When you use an alternating current to heat the filament keep it at the same voltage rather than at the same amperage (current strength). To do this you need only to use a voltmeter across the filament terminals instead of an ammeter in series with it; then regulate the voltage of the filament with a rheostat.

The Operation of C. W. Telegraph Transmitters with Alternating Current--With Two Oscillator Tubes.--By using two oscillator tubes and connecting them up with the power transformer and oscillating circuits as shown in the wiring diagram in Fig. 83 the plates are positively energized alternately with every reversal of the current and, consequently, there is no time period between the ending of the oscillations set up by one tube and the beginning of the oscillations set up by the other tube. In other words these oscillations are sustained but as in the case of those of a single tube, their amplitude rises and falls. This kind of a set is called a full wave rectification transmitter.

The waves radiated by this transmitter can be received by either a crystal detector or a plain vacuum-tube detector but the heterodyne receptor will give you better results than either of the foregoing types.

The Operation of Wireless Telephone Transmitters with Direct Current--Short Distance Transmitter.--The operation of this short distance wireless telephone transmitter, a wiring diagram of which is shown in Fig. 85 is exactly the same as that of the Direct Current Short Distance C. W. Telegraph Transmitter already explained in this chapter. The only difference in the operation of these sets is the substitution of the microphone transmitter for the telegraph key.

The Microphone Transmitter.--The microphone transmitter that is used to vary, or modulate, the sustained oscillations set up by the oscillator tube and circuits is shown in Fig. 84. By referring to the diagram at A in this figure you will readily understand how it operates. When you speak into the mouthpiece the sound waves, which are waves in the air, impinge upon the diaphragm and these set it into vibration--that is, they make it move to and fro.

When the diaphragm moves toward the back of the transmitter it forces the carbon granules that are in the cup closer together; this lowers their resistance and allows more current from the battery to flow through them; when the pressure of the air waves is removed from the diaphragm it springs back toward the mouth-piece and the carbon granules loosen up when the resistance offered by them is increased and less current can flow through them. Where the oscillation current in the aerial wire is small the transmitter can be connected directly in series with the latter when the former will surge through it. As you speak into the microphone transmitter its resistance is varied and the current strength of the oscillations is varied accordingly.

The Operation of Wireless Telephone Transmitters with Direct Current--Long Distance Transmitters.--In the wireless telephone transmitters for long distance work which were shown and described in the preceding chapter a battery is used to energize the microphone transmitter, and these two elements are connected in series with a microphone modulator. This latter device may be either (1) a telephone induction coil, (2) a microphone transformer, or (3) a magnetic modulator; the first two of these devices step-up the voltage of the battery current and the amplified voltage thus developed is impressed on the oscillations that surge through the closed oscillation circuit or the aerial wire system according to the place where you connect it. The third device works on a different principle and this will be described a little farther along.

The Operation of Microphone Modulators--The Induction Coil.--This device is really a miniature transformer, see A in Fig. 86, and its purpose is to change the 6 volt direct current that flows through the microphone into 100 volts alternating current; in turn, this is impressed on the oscillations that are surging in either (1) the grid circuit as shown at A in Fig. 89, and in Fig. 90, (2) the aerial wire system, as shown at B in Fig. 89 and Fig. 93. When the current from the battery flows through the primary coil it magnetizes the soft iron core and as the microphone varies the strength of the current the high voltage alternating currents set up in the secondary coil of the induction coil are likewise varied, when they are impressed upon and modulate the oscillating currents.

The Microphone Transformer.--This is an induction coil that is designed especially for wireless telephone modulation. The iron core of this transformer is also of the open magnetic circuit type, see A in Fig. 87, and the ratio of the turns [Footnote: See Chapter VI] of the primary and the secondary coil is such that when the secondary current is impressed upon either the grid circuit or the aerial wire system it controls the oscillations flowing through it with the greatest efficiency.

The Magnetic Modulator.--This piece of apparatus is also called a magnetic amplifier. The iron core is formed of very thin plates, or laminations as they are called, and this permits high-frequency oscillations to surge in a coil wound on it. In this transformer, see A in Fig. 88, the current flowing through the microphone varies the magnetic permeability of the soft iron core by the magnetic saturation of the latter. Since the microphone current is absolutely distinct from the oscillating currents surging through the coil of the transformer a very small direct current flowing through a coil on the latter will vary or modulate very large oscillating currents surging through the former. It is shown connected in the aerial wire system at A in Fig. 88, and in Fig. 93.

Operation of the Vacuum Tube as a Modulator.--Where a microphone modulator of the induction coil or microphone transformer type is connected in the grid circuit or aerial wire system the modulation is not very effective, but by using a second tube as a modulator, as shown in Fig. 90, an efficient degree of modulation can be had. Now there are two methods by which a vacuum tube can be used as a modulator and these are: (1) by the absorption of the energy of the current set up by the oscillator tube, and (2) by varying the direct current that energizes the plate of the oscillator tube.

The first of these two methods is not used because it absorbs the energy of the oscillating current produced by the tube and it is therefore wasteful. The second method is an efficient one, as the direct current is varied before it passes into the oscillator tube. This is sufficient reason for describing only the second method. The voltage of the grid of the modulator tube is varied by the secondary coil of the induction coil or microphone transformer, above described. In this way the modulator tube acts like a variable resistance but it amplifies the variations impressed on the oscillations set up by the oscillator tube. As the magnetic modulator does the same thing a vacuum tube used as a modulator is not needed where the former is employed. For this reason a magnetic modulator is the cheapest in the long run.

The Operation of Wireless Telephone Transmitters with Alternating Current.--Where an initial alternating current is used for wireless telephony, the current must be rectified first and then smoothed out before passing into the oscillator tube to be converted into oscillations. Further so that the oscillations will be sustained, two oscillator tubes must be used, and, finally, in order that the oscillations may not vary in amplitude the alternating current must be first changed into direct current by a pair of rectifier vacuum tubes, as shown in Fig. 93. When this is done the plates will be positively charged alternately with every reversal of the current in which case there will be no break in the continuity of the oscillations set up and therefore in the waves that are sent out.

The Operation of Rectifier Vacuum Tubes.--The vacuum tube rectifier is simply a two electrode vacuum tube. The way in which it changes a commercial alternating current into pulsating direct current is the same as that in which a two electrode vacuum tube detector changes an oscillating current into pulsating direct currents and this has been explained in detail under the heading of The Operation of a Two Electrode Vacuum Tube Detector in Chapter XII. In the C. W. Telegraph Transmitting Sets described in Chapter XVII, the oscillator tubes act as rectifiers as well as oscillators but for wireless telephony the alternating current must be rectified first so that a continuous direct current will result.

The Operation of Reactors and Condensers.--A reactor is a single coil of wire wound on an iron core, see Fig. 90 and A in Fig. 91, and it should preferably have a large inductance. The reactor for the plate and grid circuit of a wireless telephone transmitter where one or more tubes are used as modulators as shown in the wiring diagram in Fig. 90, and the filter reactor shown in Fig. 92, operate in the same way.

When an alternating current flows through a coil of wire the reversals of the current set up a counter electromotive force in it which opposes, that is reacts, on the current, and the higher the frequency of the current the greater will be the reactance. When the positive half of an alternating current is made to flow through a large resistance the current is smoothed out but at the same time a large amount of its energy is used up in producing heat.

But when the positive half of an alternating current is made to flow through a large inductance it acts like a large resistance as before and likewise smooths out the current, but none of its energy is wasted in heat and so a coil having a large inductance, which is called an inductive reactance, or just reactor for short, is used to smooth out, or filter, the alternating current after it has been changed into a pulsating direct current by the rectifier tubes.

A condenser also has a reactance effect on an alternating current but different from an induction coil the lower the frequency the greater will be the reactance. For this reason both a filter reactor and filter condensers are used to smooth out the pulsating direct currents.

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Collins, A. Frederick. 2002. The Radio Amateur's Hand Book. Urbana, Illinois: Project Gutenberg. Retrieved April 2022, from https://www.gutenberg.org/files/6934/6934-h/6934-h.htm#chap19

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