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¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
You have seen the steam escaping from a pan of boiling water. But, do you know what steam IS and what CAUSED it to escape from the water? In the first place, steam is water - but vaporized. It's a cloud of water molecules separated from each other by air. And if these water molecules were brought together again (condensed), you'd have droplets of water. The molecules escaped from the pan of water BECAUSE OF HEAT. Whenever heat is added to a substance the molecules and electrons pick up speed. Their energy - KINETIC energy - is increased by heat.
In the case of boiling water, this is what happened. The molecules picked up kinetic energy from the heat. When the energy of anyone molecule became great enough, it "boiled" out of the water surface and shot into the air. The result was steam. When the molecule cooled off, it lost its excess energy and dropped back to the water surface or combined with other water molecules to make droplets. Result - water again. Something very similar to this happens in a hot wire. When heat is applied, the electrons, in their orbits, pick up speed (kinetic energy). They whirl around the nucleus - their speed ever increasing until - zing - they pop out of the conductor and shoot into the air around the wire.
Figure 204. - Thermionic emission.
Every electron in the wire cannot behave like this, because every lost, or EMITTED, electron leaves a positive charge in the wire. This positive charge is an attraction on the remaining electrons. The positive charge also pulls many emitted electrons back to the wire. But as long as the wire is heated it continues to emit some electrons, and the HOTTER the wire, the GREATER the number of emitted electrons. The cloud of electrons around the hot wire creates a SPACE CHARGE. The space charge is NEGATIVE because electrons are negative. It acts exactly like the static negative charge on a comb. Figure 204 gives a complete picture of heat, or THERMIONIC, emission of electrons. A shows thermionic emission from a wire that is heated DIRECTLY by sending a current through it. B is a wire being heated INDIRECTLY by a heater unit - filament or cathode. Notice the names applied to the parts of these elements. The two terms, FILAMENT and CATHODE, are often used interchangeably.
Heat is not the only way to make electrons boil out of a conductor. For some kinds of material, light will do it. Electrons, emitted by light, are called PHOTOELECTRONS. Fast moving electrons striking other molecules will knock electrons out of the second object. When this second object is solid, the process is termed, SECONDARY EMISSION. When the second object is a gas the process is called IONIZATION.
How about proton emission, is it possible? Yes, but it takes far more energy than electron emission. Protons are heavy, but if enough energy is supplied, positive particles CAN be forced out of a material. It is easiest to produce positive particles in a gas. As a matter of fact, every time an electron is emitted by gas ionization, the particle remaining is a positive ion.
Some special tubes employ these methods of emission. But by far the most useful emission, is thermionic emission - direct or indirect. Almost all vacuum tubes use a thermionic source of electrons.
When a cathode is to be used as a source of electrons, air must be removed from around the emitting element. If air remains, the air molecules clog up the space around the filament. Ionization of the air results, and, instead of a smooth and steady emission of electrons, a garbled mess results. Either the air is removed and the cathode operated in a vacuum, or, an inert gas, like argon, is used. Inert gases do not interfere with the electrons boiling out of the cathode.
TUBE OR ENVELOPE
The cathode and certain other elements are enclosed in a TUBE or ENVELOPE of glass or steel. If a vacuum is to be used, the air is pumped out of the tube. If argon or some other suitable gas is to be used, the air is removed and the gas is put into the tube at low pressure.
DIODES are vacuum tubes containing TWO electrodes - a cathode and an ANODE. The cathode may be either of the two types - filament or heater. Regardless of type, THE CATHODE IS SURROUNDED BY THE SPACE CHARGE OF NEGATIVE ELECTRONS. The anode is a metal plate and is connected to the external, or load, circuit. Figure 205 shows a typical diode tube. Trace the circuits through this diagram. The cathode or filament is heated by battery A, and is surrounded by a space charge caused by the electron cloud. The B battery makes the cathode NEGATIVE. Don't confuse the A battery connections with the kind of potential on the cathode. The cathode might be indirectly heated or it might be heated with a.c. Regardless of how it is heated, it would have to be NEGATIVE because of its B battery connection. The current through the heating battery, A, and the cathode, C, is traced by solid arrows.
Now examine the anode, or plate, circuit. The plate is connected to the POSITIVE side of battery B. Therefore, the plate has a POSITIVE potential and you can trace this circuit just like any other electrical circuit. The electrons of the cathode space charge are attracted by the positive plate drift through the tube and land on the plate. Passing through the load, they go through the battery and return to the cathode. The heat of the cathode gives 'em another shot of energy-they bounce through the cathode surface and are on their way to the plate again. You can trace the plate current by following the broken arrows through figure 205.
Figure 205. - Diode tube circuit.
NOW FOR YOUR QUESTIONS -
1. Is the circuit through the filament a special one? No! It's a normal circuit containing a source (generator, battery, or line), and a load (the hot filament). It's traced like any circuit is traced - from negative to positive.
2. Is the circuit through the plate a special kind? No! Current flows from the negative side of the battery, across the tube through the plate and load, and back to the positive side of the battery.
3. Is the current through the tube different from currents through a wire? YES and NO. Yes, because the current is not contained in a metal conductor. Yes, because a special kind of switch can be used for control. No, because this current is like any other current when it is in a magnetic field. No, because the strength of this current is controlled by the potential difference. between cathode and anode -just like any current is controlled by voltage.
4. How does potential control the current in a vacuum tube? If the positive potential of the plate is increased, the potential difference (p.d.) between cathode and plate is raised. The plate has more attraction for the space charge electrons - and more flow results. When the plate is sufficiently positive to attract ALL the emitted electrons, the tube is operating at SATURATION POTENTIAL.
Does the combination of two circuits - HEATER or CATHODE, and LOAD or ANODE - make a special problem? No, because you can ELIMINATE the heater circuit from your thinking. ALL IT does is heat the cathode. The important circuit is through the tube, load, and source.
What happens when the anode, or plate, is not positive? Suppose you reverse the battery connection of B in figure 205 and find out what happens. The circuit would look like figure 206. Can you trace it? Try it! You came up against a brick wall! You got as far as the plate and stopped-or you SHOULD have stopped. The plate is cold and does not emit electrons. Therefore, there is NO electron cloud at the plate to furnish electrons for a current across the tube. And the filament electrons cannot drift across to the plate-the plate is negative and repels them.
A tube with a negative plate acts like an OPEN CIRCUIT. No CURRENT FLOWS. Therefore, the vacuum tube is a ONE WAY CIRCUIT. It will carry current only from the cathode to the plate. And then only when the plate is positive in respect to the negative space charge of the cathode's electron' cloud. Here is another way of looking at it. Liken the tube to a pipe. At one end is the cathode with plenty of electrons. These electrons are pushed around by their own negative charges. Repelling each other, they want to move. The plate is at the other end of the pipe. If it is POSITIVE, it wants electrons - it draws them to it by attraction. Current flows in the pipe (or tube). BUT if the plate is NEGATIVE - it has plenty of electrons of its own. Then the plate's own electrons will repel any attempt of the cathode's electrons to land. No CURRENT FLOWS.
Figure 206. - Diode with negative plate.
Changing a.c. to d.c. - that's what RECTIFYING means. And the diode tube is a good rectifier. If instead of a battery, a source of a.c., either an alternator or the secondary of a transformer, is' connected in the plate circuit-the plate is alternately positive and negative. Look at figure 207. A shows the circuit of the rectifier. B is the a-c VOLTAGE curve of the source; and this voltage is impressed on the plate. C is the CURRENT flowing in the plate and load circuit. Notice the difference between the a-c VOLTAGE and the plate CURRENT. ALTERNATING voltage is impressed but DIRECT current flows. When the voltage is negative, NO current flows because the plate is negative. When the voltage is positive, the plate is also positive and the flow of current is in direct proportion to the voltage.
Figure 207. - Diode rectifier.
Figure 208. - Full wave rectification.
One diode, as a rectifier, uses only one half of the a.c. This is called a HALF-WAVE RECTIFIER. Two diodes or a tube with two plates can be connected as a FULL WAVE RECTIFIER. Figure 208 is a graph of the d.c. produced by a full-wave rectifier. Notice that it's a PULSATING current. Rectifiers are often used as battery chargers. In fact, a rectifier can be used to' convert a.c. for almost any use requiring d.c.
Figure 209. - Triode.
TRIODES are tubes containing THREE electrodes. They are like the diode except for the addition of a third electrode, called a GRID. Figure 209 shows the construction of a triode. Notice the grid's position BETWEEN the cathode and anode. Although there are a number of different types and arrangements of the triode (see figure 210), they all place the grid between the cathode and the anode. Notice, in figure 210, that the grid is like a screen between the filament and plate. This construction means that all the electrons moving from the cathode to the anode must pass through the grid.
Up to now, current has been controlled by switches, rheostats, breakers, and fuses. But here is a new type of control - the grid of a triode. This is how it works. Imagine a triode with no voltage on the grid. Current flows normally from cathode to anode - it passes right through the grid. Now impress a small negative voltage on the grid (as in figure 209). The grid has a negative charge of its own and will repel the electrons which try to get through it on their way to the plate. The more negative the grid, the fewer electrons which can get through. Reverse this and, as the grid becomes LESS NEGATIVE (more positive), it permits more and more electrons to get through to the plate. The grid is like a gate or valve controlling the current through the tube.
Figure 210. - Triode construction.
Perhaps an example would help in understanding the grid's action. The grid is like the valve on a fire hydrant. A fire hydrant has enough water pressure to knock a man down. And you certainly can't control the water flowing in a fire hose by putting your hand over the nozzle. But you can control the water in a fire hose by VERY LITTLE EFFORT on the VALVE. The current in a vacuum tube is like . the water in a fire hose. There's lots of it and it has a high potential. But very little potential on the grid (valve) controls the heavy tube current.
If the grid should become positive, it would act like an anode and attract the cathode electrons to itself. There would be reduced flow to the plate and the grid would lose control of the plate current. For this reason, grids are normally operated at a negative potential.
TRIODES AS AMPLIFIERS
The voltage in electrical signals of radio, radar, telephone -and fire control systems is extremely small. It may be as low as 3 or 4 millionths of a volt. The received signals must be AMPLIFIED. Amplifying simply means increasing the strength.
For example, say that a fire control signal has a strength of 0.01 volt. This signal is to control a switch which in turn controls a turret drive motor. BUT the switch will not operate on less than 0.1 volt. In short, the original signal is only one - tenth the strength required to throw the motor. switch. The signal must be amplified ten times - and a triode will do the job.
Figure 211 is the triode circuit used as an amplifier. The weak signal - 0.01 v. - is fed into the grid current. In the grid, it controls the plate current. Remember that the cathode is surrounded by electrons and the plate is positive. Just how many electrons get to the plate from the cathode depends on the grid's potential. And this grid potential is controlled by the signal. As long as the grid is negative, it retards current flow to the plate - but - the AMOUNT of retarding at every instant is determined by the negativeness of the grid. As the voltage on the grid becomes MORE NEGATIVE the current to the plate is reduced. But, as the voltage on the grid NEARS ZERO the CURRENT SURGES THROUGH. The amount of current flowing to the plate follows the pattern of the sine wave of voltage of the signal.
Okay for the negative half of a-c voltage, but, when the grid is positive - it acts like an anode. It collects the electrons to itself and loses its control of the plate current. This would give you a plate current which was uncontrolled by the grid voltage. The plate current would not follow the pattern of the signal's sine wave. To prevent the grid losing control, add a battery at C. Notice that the NEGATIVE .side of the C battery is connected to the grid. Now you have two voltages on the grid-the impressed a.c. (the signal) and the negative C battery voltage, called a GRID-BIAS. The effect of the bias is this. When the a.c. on the grid becomes positive, the bias is just strong enough to cancel the positive a.c. and keep the grid NEGATIVE. The grid does not lose control. You can say that the positive a.c. - makes the grid LESS NEGATIVE and that the negative a.c. makes the grid MORE NEGATIVE. In fact, the negative a.c. makes the grid so negative, that it almost cuts off the plate current. Now you have MAXIMUM CURRENT flowing when the grid is on the maximum POSITIVE a.c. - and the MINIMUM CURRENT when the grid is maximum NEGATIVE a.c.
Figure 211. - Triode amplifier.
Say it this way - the positive a.c. cancels the negative bias producing a surge of plate current. But the negative a.c. adds to the negative bias, cutting the plate current almost to zero.
Plate current, then, is a changing or pulsating d.c. Notice, in figure 211, how strong this PLATE D.C. is compared to the WEAK A.C. from the signal. This is because A VERY SMALL VOLTAGE ON THE GRID WILL CONTROL A HEAVY CURRENT TO THE PLATE. Amplification has taken place. The a.c. was only 0.01 volt but the pulsating d.c. in the plate circuit will produce ten times 0.01 volt.
Now, how does the plate CURRENT produce this VOLTAGE? Why, by feeding it into a mutual induction circuit - a transformer. And you know that pulsating d.c. produces a.c. in a transformer.
Look at the plate load in figure 211 - it has an alternating voltage of 0.1 volt - the same sine wave that was on the grid, but ten times as strong. This is the kind of amplification that enables you to control heavy circuits with tiny electrical signals.
Review the complete picture - go back through figure 211. The circuit may seem complicated, but it's the best way to increase the strength of electrical impulses. Remember how it works. The grid gets the signal and controls the plate current. The plate gets whatever current the grid lets through. It then feeds it to a transformer for conversion to a.c.
Many times a SINGLE STAGE amplifier does not boost the signal up high enough. Then you'd use a two or three stage job. A second stage is COUPLED to the first by connecting the first amplifier to the primary of a transformer. Then the second stage (second amplifier circuit) grid is connected to the transformer's secondary. With this connection, the 0.01 volt signal is amplified to 0.1 volt in the first stage. And the 0.1 volt is amplified to 1 volt in the second stage.
REPLACEMENT FOR "C" BATTERY
The C battery, used to bias the grid is troublesome. It wears out, it's heavy, and it's fragile. Let's get rid of it! A condenser and a resistor in the grid circuit will do the bias job. Here is how they work. A condenser is made up of a number of conducting plates separated by insulators. Half of the plates are connected to one side of the line and half are connected to the other side. Figure 212 shows this construction. Note that there is NO electrical path through the condenser. All the plates of one terminal are separated from the plates of the other terminal by insulators.
Figure 212. - Condenser construction.
Remember the Leyden jar? It was a condenser but it only had two plates. Remember what it did? It stored an electrical charge - electrons. A MANY plate condenser is like a Leyden jar except that many plates increase the capacity for STORING ELECTRONS.
Figure 213. - Condenser in the grid circuit.
Now put a condenser in the grid circuit and you'll see how it works. Connect the condenser as shown in figure 213.
Figure 214 shows what happens at the condenser. When the impressed a.c. on the grid is negative, electrons pile up on plate 1.
Figure 214. - Condenser action.
This pile up gives plate 1 a negative charge and this negative charge forces electrons out of plate 2 and onto the grid. (A of figure 214.) Safar so good - NEGATIVE impressed voltage produces a NEGATIVE grid. But when the impressed voltage becomes positive, B of figure 214 shows that electrons are drained out of plate 1, leaving it positive. Plate 1, being positive, attracts electrons-it pulls them out of the grid and onto plate 2. The grid, by losing 'electrons becomes POSITIVE and acts like an anode. It collects electrons from the cathode. Now, when the impressed voltage AGAIN becomes NEGATIVE, as in C, of figure 214, the grid has TWO negative charges. One from the condenser and the other from the electrons picked up by the grid when 'it was positive. This process goes on and on. Each time the grid is positive it collects a little more negative, charge. Finally the NEGATIVE CHARGE is as strong as the IMPRESSED POSITIVE. Does this sound familiar? It should-exactly the same thing happened when you added a C battery to bias the grid. The grid in either case gets an ADDITIONAL NEGATIVE CHARGE. The charge comes from either a battery or from a condenser. In either case, the grid has a NEGATIVE BIAS.
There is only one fault in this circuit. The process of piling up electrons on the grid is too good. It goes too far! The grid becomes too husky a negative. It shuts off practically all the plate current and no signal gets through. A GRID LEAK is the answer. It is used to pass some of the grid electrons around the condenser. The grid leak is a high resistance shunt made of nichrome, carbon, or some other high resistance material. Whenever the impressed voltage is positive, a few electrons leak off the grid and onto plate 1 of the condenser. This leaking off of electrons keeps the grid from acquiring too high a negative charge.
REPLACEMENT FOR "B" BATTERY
Getting rid of the B battery is a cinch. All the B does, is provide a positive potential for the plate. Any source of positive potential will do that. How about regular 110 volt outlets? They won't work - too high a voltage. But if the outlets are d.c., you can use a resistor to cut the voltage down to about 45 volts. This would work fine. But if the 110 volts is a.c., you have a job. A.C. will not do for anode connections because the anode must be PERMANENTLY positive.
How about a rectifier? It will convert a.c. to d.c. And a rectifier is just what's used. But one thing must be done to the a.c. before it is fed to the rectifier. The voltage must be reduced to about 45 volts in a transformer. Then it is ready for feeding into a rectifier for conversion to d.c.
Figure 215. - Filter choke coil system.
The rectifier produces 45 volts PULSATING d.c. But you can't use a pulsating current. The grid must be a STEADY positive. Now, to get rid of the pulsations. To do this job you'll have to take some of the tops off the pulsating current and fill up its valleys. This will give you a steady potential so that the anode voltage is constantly and steadily positive. The problem is whipped by a FILTER CHOKE COIL.
The filter choke coil is connected as in figure Actually, you'll note, the coil is not alone. It is connected with two condensers across the line. Here is how the circuit works. Electrons come from the rectifier in steady beats or pulsations. The first condenser fills up and current starts to trickle through the choke coil. TRICKLE is the word be-cause the voltage of self induction HOLDS THIS CURRENT BACK. As the pulsation slacks off, the condenser begins unloading its store of electrons-feeding them into the coil. When the pulsation nears zero, the voltage of self induction AIDS the weakening pulse, draws electrons out of the con-denser plates, and keeps the current moving. Thus, the condenser, aided by the coil's self-induced voltage, keeps a steady current moving simply by alternately charging and discharging.
Figure 216. - Filtering pulsating d.c.
The condenser is like a reservoir tank. It fills up on the strong pulses and unloads when the pulses become weak. The coil's self-induced voltage is like a control pump. Whenever the pulsation increases, the induced voltage keeps it down and when the pulse weakens the induced voltage gives it a helping hand. Even so, the output is not entirely smooth. A second condenser takes care of this. This second condenser stores the little humps of current which still get through the choke coil, and unloads them when the little valleys Come through. The final product is a smooth steady 45 volt d.c.
Look at figure 216. It shows the transformation of voltage in a filter. First the alternating voltage from a line source. Second, the pulsating d.c. from the rectifier. Third, the smoothed out d.c. from the choke coil and condenser. And the final product, a steady d.c.
Figure 217. - Complete amplifier tube circuit.
Now lets look over the entire circuit. Grid signal, amplifier tube, load, grid condenser and leak, rectifier, and filter choke coil are all shown in figure 217.
MORE OF THE SAME
You have the BASIS of vacuum tubes, but not their complete circuits. In radio, fire control, radar, and telephony, vacuum tubes are used for specific jobs. And each job uses a special combination of circuits.
Each rate requires knowledge of its own special circuits. So-for knowledge about a special circuit, you use the book for your specific rating.
Chapter 19 Quiz