CHAPTER 13
AMPLIFIER TUBES
TRIODES-RAISE WHISPERS TO SHOUTS

Lee DeForest, in placing a GRID between the cathode and the plate, completely changed the systems of communication. The addition of this THIRD ELEMENT made it possible to start out with a feeble signal and build it up to such strength that it could be heard by thousands.

This THREE-ELEMENT TUBE-the TRIODE-is not much different from the diode in physical structure.

Figure 103 is a cutaway section of a triode. This tube is an indirect-heater type. Notice that the cathode is a small cylinder. The new element-the GRID-is the FINE WIRE FENCE between the cathode and the plate.

The grid is formed by winding wires on two metal supports. Notice in figure 104 that the wires and supports look like the yard markers on a football field; hence the name, GRID. Some vacuum tubes have more than one grid.

THE GRID CONTROLS THE FLOW OF CURRENT

In the DIODE, the flow of electrons to the plate can be controlled by changing either the temperature of the filament or the voltage on the plate. In a triode, the grid is the most effective control on the flow of current from the cathode to the plate.

Recall an old familiar principle that you learned in d.c.-"Like charges repel each other?" In other words, if two negative-charged objects are brought close together, they will push away from each other. That's how the grid is able to control or regulate the flow of electrons.

To see how this principle applies, study the diagrams in figure 105. The three drawings are sections of a triode cut from top to bottom. The cathode is at the center. The small round circles represent the END VIEWS of the grid wires. The plate is indicated by the single line to the outside.

The GRID of figure 105B has been made SLIGHTLY NEGATIVE. Since the electrons are also negative, PART OF THE ELECTRONS that leave the cathode will be FORCED BACK toward the cathode. Only a few will have enough energy to "sneak by" the negative grid and get to the plate. You know-likes repel.

In figure 105C, the grid has been made VERY NEGATIVE. The FORCE OF REPULSION, offered by the grid, drives ALL of the electrons back toward the cathode. Thus the flow of current from the cathode to the plate is completely stopped. The NEGATIVE VOLTAGE ON THE GRID that is large enough to stop completely the flow of electrons from cathode to the plate is known as the CUT-OFF voltage of the tube.

THE GRID ACTS LIKE A VALVE

The grid in a triode may be compared to a valve in a water pipe. If the valve is wide open, it will not exercise any control on the flow. CLOSING the valve SLIGHTLY has the same effect as making the grid SLIGHTLY NEGATIVE. It will cause a SMALL REDUCTION in the flow. The MORE the valve is CLOSED, or the MORE NEGATIVE you make the grid, the GREATER will be the REDUCTION in the flow. When the valve of a pipe is CLOSED COMPLETELY, flow of water ceases. When the GRID is made EXTREMELY NEGATIVE, the flow of electrons to the plate is likewise stopped.

BIAS VOLTAGE

You have just seen that the number of electrons that will be able to get to the plate depends upon the AMOUNT OF NEGATIVE POTENTIAL ON THE GRID. The NEGATIVE VOLTAGE that is placed on the grid to REDUCE THE FLOW OF ELECTRONS is known as the BIAS VOLTAGE. It is important for you to understand that whenever the BIAS VOLTAGE

The reason for using a bias voltage on the grid is to permit the grid to control the flow of current to the plate on both positive and negative half-cycles of the signal. If no bias were used, the grid could control the flow of current on only the negative halves of the cycle.

COMBINED ABILITIES OF GRID AND PLATE VOLTAGES IN CONTROLLING THE FLOW OF PLATE CURRENT

Since the plate voltage also influences the flow of plate current, what will happen if the plate voltage is made greater? Will the grid be able to exercise the SAME CONTROL, or will the conditions be changed?

First, stop and think what effect a larger plate voltage alone will have on the movement of electrons to the plate. When the plate is made more positive, it will offer a GREATER FORCE OF ATTRACTION for the electrons. Thus, if the grid is kept at a constant bias voltage, increasing the positive potential of the plate will enable the plate to PULL MORE electrons past the grid.

WHICH DOES THE BETTER JOB IN CONTROLLING THE PLATE CURRENT?

Since both the grid and plate voltages are capable of controlling the flow of current to the plate, it is interesting and important to compare the two to see which does the better job.

Before you can make a comparison, it is necessary to set up some standard to be used in judging the performance of the two contestants. This is not a new situation. If you are to have a weight lifting contest, you will have certain standard weights that ALL contestants must lift. In a foot race, you will use a SET distance that all will have to run.

It is much the same in the contest between the grid and plate voltages. You don't have a distance that the two must run, or a weight they can lift-instead, you use a DEFINITE AMOUNT OF CURRENT THAT EACH MUST CONTROL

The first thing that you will do is to KEEP THE BIAS VOLTAGE CONSTANT. With the bias voltage constant, INCREASE THE PLATE VOLTAGE until the plate current has increased 10 milliamperes. After that trial run, you find that it was necessary to use 40 ADDITIONAL PLATE VOLTS to increase the plate current 10 milliamperes.

The score for the plate is now-

10 MILLIAMPERES--40 PLATE VOLTS

Return the plate voltage to the value it had before the contest started. Now DECREASE THE BIAS VOLTAGE-make it LESS negative-until the plate current has again increased 10 milliamperes. On this trial run, the GRID increased the current 10 milliamperes with a voltage change of only TWO VOLTS. The score board now reads-

10 MILLIAMPERES--40 PLATE VOLTS
10 MILLIAMPERES--2 GRID VOLTS

Which one won the contest? The score is so lopsided that you can't be mistaken. The GRID is by far the more effective in controlling the plate current. What is the ratio? That is simple; the GRID is-

40 / 2 = 20 times as effective as the plate in controlling the plate current.

The ratio of the effectiveness of the grid and plate voltages in controlling the plate current is given a definite name. It is called-

"Mu" or written in Greek symbol as μ It is pronounced as-"Me-u" or "Mew."

THE Mu OF A TUBE

"IT TELLS HOW MUCH A TUBE IS ABLE TO AMPLIFY A SIGNAL"

Actually, the Mu of a tube is much more than a mere interesting ratio. It tells you how much a vacuum tube is able to AMPLIFY a weak signal that is placed on the grid. Referring back to the contest just completed, two volts of grid change produced the same result as 40 volts

EFFECT OF TUBE CONSTRUCTION ON THE Mu OF THE TUBE

Now that you know that the grid voltage is able to exercise such a large control over the flow of current from the cathode to plate, you probably will think it to be a case of the "tail wagging the dog." That is not far from being the truth.

The reason why the grid is able to control the current so effectively lies in the construction of the tube. The GRID is placed MUCH CLOSER to the cathode than is the plate. Therefore, any change in grid voltage will exercise as much influence on the movement of electrons as a larger voltage change on the plate. In general, the nearer the grid is to the cathode, the HIGHER the Mu of the tube.

Typical triodes in common use have a wide range of amplification factors. Some of the older styles, such as the Type 27 tube, have a Mu of only 9, while the newer Type 6SF5 tube possesses a Mu of 100. Don't think that the tube with the highest amplification factor is the BEST for all purposes. There are many other factors that will influence the choice of the tube, but they are problems that belong to the more advanced technicians and designers.

A.C. PLATE RESISTANCE

The one form of resistance is the A.C. PLATE RESISTANCE. More commonly called just PLATE RESISTANCE. It takes into consideration the CHANGE in the PLATE VOLTAGE that is produced by the CHANGE IN PLATE CURRENT.

ΔI_p = 10 ma. or .01 ampere
ΔE_p = 40 volts

The Δ means "change."

To find the R_p of a tube you change Ohm's Law for resistance from-

R = E / I to ΔR_p = ΔE_p / ΔI_p

and substitute the values-

R_p = 40 / .01
R_p = 4,000 ohms

TRANSCONDUCTANCE

So far, you have learned of two characteristics of a vacuum tube, the amplification factor, Mu, and the internal plate resistance, R_p. A third factor-the TRANSCONDUCTANCE, obtained from the relationship of Mu and R_p -expresses how well a vacuum tube is able to do its work. It is a measure of the tube merit.

Don't let the word, transconductance, trouble you. It has a very simple meaning-

TRANS-Means to transport, carry, from one plate to the other.

CONDUCT-In electricity this means to carry ELECTRONS through a conductor.

Now put these two, words together. In a vacuum tube, what is carried or conducted from the cathode to the plate? It is electrons! So transconductance merely means-How WELL are the electrons CONDUCTED from the cathode to the plate?

The transconductance of a vacuum tube is a measure of HOW WELL the grid voltage is able to control the flow of current to the plate. It is expressed as the RATIO OF THE Mu TO THE R_p of the TUBE. In an equation, it will look like this-

TRANSCONDUCTANCE (G_m) = Mu / R_p (answer in MHOS-pronounced "Mose")

Here is an example of this-suppose that a triode has a Mu of 40, and an R_p of 20,000 ohms. Substituting in the equation and solving-

G_m = 40 / 20,000
G_m = 0.002 mho conductance

Because such a number as .002 mho is difficult to use, and the tranconductance of all tubes is small, it is a common practice to multiply the MHO by 1,000,000 and call the new number the MICROMHO. Thus 0.002 mho will become-

0.002 X 1,000,000 = 2,000 MICROMHOS conductance.

For most vacuum tubes, the transconductance is in the order of a few thousand micromhos. It is desired that the vacuum tube have a LARGE Mu and a SMALL R_p in order that it may have a high transconductance.

INTERELECTRODE CAPACITANCE

INTERELECTRODE CAPACITANCE is another high-sounding term, but it, too, has a simple meaning. You know that a CONDENSER is formed whenever two pieces of metal are brought near to each other. Within the triode, there are THREE small condensers formed, one between the

Figure 106 illustrates the interelectrode capacitance in a triode. The capacitance that is formed between the cathode and grid is the same as if a small condenser (A)

The capacitance between the plate and grid is the most important, because it is the cause of much trouble. The value of this capacity is small, usually less than 10 μμf for a triode, but large enough in a r.f. circuit to feed a considerable amount of energy from the plate circuit back into the grid circuit (feed back). This causes UNWANTED OSCILLATIONS.

These unwanted oscillations are so objectionable in r.f. amplifiers that it is necessary to use special connections known as NEUTRALIZING circuits to keep the OSCILLATIONS down. Neutralizing circuits add to the bulk and also reduce the overall efficiency of the set. You can see that this is objectionable, especially in small receivers.

THE TRIODE-GOOD, BUT-

The triode is a good vacuum tube, but it has its limitations. The more important are-

FIRST-The amplification factor of a triode is small. Therefore a triode requires a large DRIVING POWER. In other words, a triode must be fed a large signal before it can start the process of amplification.

SECOND-The most objectionable feature of the triode is its high INTERELECTRODE CAPACITANCE. This is not objectionable at AUDIO frequencies. But if the triode is used to amplify RADIO FREQUENCY signals, you'll have to use NEUTRALIZING CIRCUITS to overcome the high interelectrode capacitance.

WHAT DOES INTERELECTRODE CAPACITANCE DO?

The grid and plate of a vacuum tube can be compared to the plates of a condenser. To illustrate this, if the plate is made LESS POSITIVE by an increased flow of electrons, the grid will become MORE POSITIVE. And, if the plate is made MORE POSITIVE, the grid will become MORE NEGATIVE. Therefore, just as in a condenser, ANY CHANGE IN PLATE POTENTIAL WILL BE REFLECTED BACK AND CAUSE A CHANGE IN THE GRID POTENTIAL.

In an r.f. circuit, this feed-back SETS UP EXTRA OSCILLATIONS AT ITS OWN FREQUENCY. If your radio receiver or transmitter is a one-tube affair, these FEED-BACKS won't be serious, because the extra oscillations are weak. But a modern radio is not a one-tube job. All of the extra oscillations produced by one tube are amplified by

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