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Drawing of sailor in front of transmitter.

CHAPTER 15
JOBS OF A VACUUM TUBE
USES IN RADIOS

The vacuum tube is used in so many places, and in such a variety of electronic gear, that a complete listing of all its applications would fill a book larger than the New York telephone directory.

Fortunately, this multitude of uses can be divided into a relatively small number of classes. And by limiting the gear to radio alone, the number is reduced to four

Rectifiers
Amplifiers
Oscillators
Detectors

VACUUM TUBE RECTIFIERS

You learned in Chapter 12 how a diode vacuum tube changed a.c. into a pulsating d.c. Practically all receivers, and many transmitters, have one or more of these RECTIFIER TUBES.

 
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All radios require high d.c. voltages. Several devices, including batteries, are capable of supplying this d.c., but none has the convenience and efficiency of a rectifier, especially when a HIGH VOLTAGE with LOW CURRENT is desired.

The RECTIFIER TUBE, figure 110, is a PART of a circuit commonly called the POWER SUPPLY. In addition, the power supply also contains a TRANSFORMER and a FILTER.

Power supply schematic.
Figure 110.-Power supply.
The transformer has one primary and TWO secondary windings, S1, and S2. Winding S1, STEPS UP and S2 STEPS DOWN the line voltage. As an example, S1 may raise the primary voltage from 110 to 500 volts, while S2 will reduce the primary voltage to 5 volts. The stepped UP
Output of rectifier and filter drawings.
Figure 111.-Output of rectifier and filter.
 
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voltage is applied to the plate of the diode, and the stepped down voltage is used to heat the rectifier's filament.

The diode changes the a.c. into a pulsating d.c., and that's where the FILTER comes in figure 111. The FILTER CUTS OFF the PEAKS of the pulses and FILLS IN the GAPS between them. While the d.c. output of the filter looks rather bumpy in drawing B, it actually is much smoother, more like line C. The small amount of irregularities left in the d.c. is called the RIPPLE, which is seldom greater than 5 percent of the output voltage.

VACUUM TUBE AMPLIFIERS

The word AMPLIFY means to INCREASE in SIZE. In Chapter 13, you learned how one volt of grid change produced as large a change in the plate current as 10 volts applied to the plate.

When proper resistors and condensors are correctly connected to the vacuum tube, with the necessary voltages applied to the circuit, an a.c. of one volt can cause an a.c. of 10 volts to appear in the plate circuit. Thus the vacuum tube, with its other related parts, has AMPLIFIED the a.c. TEN TIMES.

Pulsating d.c. may also be amplified. If the voltage applied to the grid of a vacuum tube starts at zero and rises to a maximum of two volts, and then appears in the plate circuit pulsating between zero and 100 volts, it has been amplified 50 times.

A vacuum tube and its immediate related parts is called a STAGE. The amplification of a stage is the ratio of the voltage you put IN on the grid, to the voltage you get OUT. Thus, if 0.5 volt a.c. is put into a stage, and 200 comes out, the amplification of the stage is-

200 / 0.5 = 400

Some times you will hear the amplification of a stage called the GAIN of the stage.

The amplification of a stage depends upon the vacuum tube and the condensers, coils and resistors used with the

 
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vacuum tube. Some circuits using triodes have a very small gain-two, three, or four-while others using pentodes have gains of several hundred.

KINDS OF AMPLIFIERS

Radios use two kinds of amplifiers-AUDIO FREQUENCY and RADIO FREQUENCY. Each is designed to do its own work most efficiently.

Most RADIO FREQUENCY AMPLIFIERS use transformers to couple the stages together. A pentode tube is used with receivers and with low power stages in transmitters.

Two stage r.f. amplifier.
Figure 112.-Two stage r.f. amplifier.
The circuit in figure 112 is a two stage r.f. amplifier. Transformer L1 is used to couple the antenna to the grid of the first vacuum tube. Transformer L2 couples the first stage to the second amplifier stage. Notice that both vacuum tubes are pentodes.

Compare the two stage AUDIO FREQUENCY AMPLIFIER of figure 113 with the r.f. amplifier. Notice the coupling between the stages. In the a.f. amplifier C, R1, and R2

 
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form the coupling unit. This type of coupling is commonly called an R.C. COUPLING.

The input to the VT1 is from a microphone. An audio frequency transformer T1 couples the microphone to the grid of the tube. Notice that VT1 is a pentode and VT2 a triode. This indicates that BOTH pentodes and triodes are used in a.f. amplifiers.

Two stage a.f. amplifier.
Figure 113.-Two stage a.f. amplifier.
Figures 112 and 113 are placed in this chapter to give you an idea of how the two kinds of amplifiers look in a schematic diagram. You do not need to trouble yourself to find out how they work unless you wish. It is enough to be able to RECOGNIZE the difference when you see the two circuits.

VOLTAGE AND POWER AMPLIFIER

Both a.f. and r.f. circuits have voltage and power amplifiers. A voltage amplifier is designed to increase the voltage, and a power amplifier is designed to increase the flow of current.

The FIRST stages of receivers, transmitters and audio amplifiers, in which the input emf is weak, use VOLTAGE amplifiers. In the output stages of all three, POWER amplifiers are used to increase the flow of current.

69919P-46-11

 
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Pentodes and a few triodes are used in voltage amplifiers, while large triodes, beam power tubes, and specially designed pentodes are used in power amplifier stages.

If you wish to learn the types of tubes and the duties they perform, get some Electronics Technician's Mate to explain the tubes used in the specific receiver or transmitter that you are working with.

VACUUM TUBE AS AN OSCILLATOR

Back in Chapter 11 you read that an oscillator is just a high frequency a.c. generator, and that a COIL and CONDENSOR form the oscillator. Many times you will hear it stated that the VACUUM tube is an oscillator. That statement is not exactly correct, because NO PART of the vacuum tube OSCILLATES. It only SUPPORTS or REINFORCES the oscillations in the tank circuit.

A tank circuit, like the pendulum of a clock, must work against the opposition of all resistances that surround it. If it were not for these opposing forces, the oscillations, once started, would continue forever.

Since neither the pendulum nor a tank circuit is perfect, each requires that ENERGY be continually added to overcome the losses due to resistance. In the clock, a system of springs or weights provides the energy to keep the pendulum oscillating. In a tank circuit, a vacuum tube supplies the energy to keep the oscillations going.

While a vacuum tube is a necessary and essential pars of all oscillator circuits, it is NOT the part that oscillates It merely supplies the energy.

THE VACUUM TUBE AS A DETECTOR

In order that your message may travel from transmitter to a receiver, it is necessary to combine the AUDIO FREQUENCY SOUNDS with the RADIO FREQUENCY carries wave at the transmitter. This combining of waves is called MODULATION.

The MODULATED CARRIER wave is a garbled combination of the two frequencies that cannot be heard by the human ear. Therefore, before the message can be understood

 
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the audio frequency part of the carrier wave must be separated from the radio frequency components.

This separation of the a.f. and r.f. parts of the carrier wave is known by two names, DEMODULATION and DETECTION.

DIODE vacuum tubes are most commonly used as detectors in Navy receivers. The tube alone does not do the job. Additional coils, condensers and resistors are required to complete the circuits.

It is not necessary for you to know the exact action of the detector circuits. It is sufficient if you know that the a.f. portion of the carrier wave is sent on to the loud speakers or earphones, while the r.f. portion is discarded and cast aside.

OTHER USES OF VACUUM TUBES

You will find SPECIAL vacuum tubes used in a variety of places in both transmitters and receivers. One of the more frequent uses will be as a VOLTAGE REGULATOR. These tubes are special diodes containing a small amount of gas, and usually having cold cathodes.

A TUNING INDICATOR commonly called a TUNING EYE is used in some receivers. The RBO receiver uses one of these tubes. The purpose of the tube is to indicate the PRESENCE of a station, and also to indicate when the receiver is correctly tuned.

 
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