HANDBOOK

Thermionic Emission

6CB6

Figure 4

CUT-AWAY DRAWING OF A 6CB6 PENTODE

1Л Ш DC Ы Q.

5 < -I

Ш

Ь < -I a.

Ю 20 30 40

D.C. PLATE VOLTS

surface forces of the material and hence of the energy required of the electron before it may escape), and of the constant A which also varies with the emitting surface. The relationship between emission current in amperes per square centimeter, /, and the above quantities can be expressed as:

Secondary The bombarding of most metals Emission and a few insulators by electrons will result in the emission of other electrons by a process called secondary emission. The secondary electrons are literally knocked f rom the surface layers of the bombarded material by the primary electrons which strike the material. The number of secondary electrons emitted per primary electron varies from a very small percentage to as high as 5 to 10 secondary electrons per primary.

The phenomena of secondary emission is undesirable for most thermionic electron tubes. However, the process is used to advantage in certain types of electron tubes such as the image orthicon (TV camera tube) and the electron-multiplier type of photo-electric cell. In types of electron tubes which make use of secondary emission, such as the type 931 photo cell, the secondary-electron-emitting surfaces are specially treated to provide a high ratio of secondary to primary electrons. Thus a high degree of current amplification in the electron-multiplier section of the tube is obtained.

The Space As a cathode is heated so that

Charge Effect it begins to emit, those electrons which have been discharged into the surrounding space form a negatively charged cloud in the immediate vicinity of the cathode. This cloud of electrons around the cathode is called the space charge. The electrons comprising the charge are continuously changing, since those electrons making up the original charge fall back into

Figure 5

AVERAGE PLATE CHARACTERISTICS OF A POWER DIODE

the cathode and are replaced by others emitted by it.

The Diode

If a cathode capable of being heated either indirectly or directly is placed in an evacuated envelope along with a plate, such a two-element vacuum tube is called a diode. The diode is the simplest of all vacuum tubes and is the fundamental type from which all the others are derived.

Characteristics When the cathode within a of the Diode diode is heated, it will be found that a few of the electrons leaving the cathode will leave with sufficient velocity to reach the plate. If the plate is electrically connected back to the cathode, the electrons which have had sufficient velocity to arrive at the plate will flow back to the cathode through the external circuit. This small amount of initial plate current is an effect found in all two-element vacuum tubes.

If a battery or other source of d-c voltage is placed in the external circuit between the plate and cafhode so that it places a positive potential on the plate, the flow of current from the cathode to plate will be increased. This is due to the strong attraction offered by the positively charged plate for any negatively charged particles (figure 5).

Space-Chorge Limited Current

At moderate values of plate voltage the current flow from cathode to anode is limited by the space charge of electrons around the cathode. Increased values

OXIDE COATED

TUNGSTEN

TUNGSTEN nLA MENT

point OF MAXIMUM SPACE-CHARGE -LIMITED EMISSION

PLATE VOLTAGE

Figure 6

MAXIMUM SPACE-CHARGE-LIMITED EMISSION FOR DIFFERENT TYPES OF EMITTERS

of plate voltage will tend to neutralize a greater portion of the cathode space charge and hence will cause a greater current to flow.

Under these conditions, with plate current limited by the cathode space charge, the plate current is not linear with plate voltage. In fact it may be stated in general that the plate-current flow in electron tubes does not obey Ohms Law. Rather, plate current increases as the three-halves power of the plate voltage. The relationship between plate voltage, E, and plate current, f, can be expressed as:

1 = К еу

where К is a constant determined by the geometry of the element structure within the electron tube.

Plate Current As plate voltage is raised to Saturation the potential where the cath-

ode space charge is neutralized, all the electrons that the cathode is capable of emitting are being attracted to the plate. The electron tube is said then to have reached saturation plate current. Further increase in plate voltage will cause only a relatively small increase in plate current. The initial point of plate current saturation is sometimes called the point of Maximum Space-Charge-Limited Emission (MSCLE).

The degree of flattening In the plate-voltage plate-current curve after the MSCLE point will vary with different types of cathodes. This effect is shown in figure 6. The flattening is quite sharp wirh a pure tungsten emitter. With thoriated tungsten the flattening is smoothed somewhat, while with an oxide-coated cathode the flattening is quite gradual. The gradual saturation in emission with an oxide-coated emitter is generally considered to result from

o- в +6

Figure 7

ACTION OF THE GRID IN A TRIODE

(A) shows the triode tube wifb cutoff bias on the grid. Note that all the electrons emitted by the cathode remain inside the grid mesh.

(B) shows the same tube with an irtermediate value of bias on the grid. Note the medium value of plate current and the fact that there is a reserve of electrons remaining within the grid mesh. (C) shows the operation with a relatively small amount of bias which with cerfoin tube types will allow suhstarrtially all the electrons emitted by the cathode to reach the plate. Emission is said to be saturated in this cosa In a maiority of tube types о high value of positive grid volfage is required before plate-current saturation takes place.

a lowering of the surface work function by the field at the cathode resulting from the plate potential.

Electron Energy The current flowing in the Dissipation plate-cathode space of a con-

ducting electron tube represents the energy required to accelerate electrons from the zero potential of the cathode space charge to the potential of the anode. Then, when these accelerated electrons strike the anode, the energy associated with their velocity is immediately released to the anode structure. In normal electron tubes this energy release appears as heating of the plate or anode structure.

The Triode

If an element consisting of a mesh or spiral of wire is inserted concentric with the plate and between the plate and the cathode, such an element will be able to control by electrostatic action the cathode-to-plate current of the tube. The new element is called a grid, and a vacuum tube containing a cathode, grid, and plate is commonly called a triode.

Action of If this new element through which the Grid the electrons must pass in their course from cathode to plate is made negative with respect to the cathode, the nega-

HANDBOOK

Т riode С haracteri sti cs

0 100 200 300 400 5

PLATE VOLTS (Ep) Figure 8

NEGATIVE-GRID CHARACTERISTICS(Ip VS. Ep CURVES) OF A TYPICAL TRIODE

Average plate characteristics of this type are most commonly used in determining the Class A operating characteristics of a triode amplifier stage.

tive charge on this grid will effectively repel the negatively charged electrons (like charges repel; unlike charges attract) back into the space charge surrounding the cathode. Hence, the number of electrons which are able to pass through the grid mesh and reach the plate will be reduced, and the plate current will be reduced accordingly. If the charge on the grid is made sufficiently negative, all the electrons leaving the cathode will be repelled back to it and the plate current will be reduced to zero. Any d-c voltage placed upon a grid is called a bias (especially so when speaking of a control grid). The smallest negative voltage which will cause cutoff of plate current at a particular plate voltage is called the value of cutoff bias (figure 7).

Amplification The amount of plate current in a Factor triode is a result of the net field

at the cathode from interaction between the field caused by the grid bias and that caused by the plate voltage. Hence, both grid bias and plate voltage affect the plate current. In all normal tubes a small change in grid bias has a considerably greater effect than a similar change in plate voltage. The ratio between the change in grid bias and the change in plate current which will cause the same small change in plate current is called the amplification factor or fi of the electron tube. Expressed as an equation:

with ip constant (A represents a small increment).

The /i can be determined experimentally by making a small change in grid bias, thus slightly changing the plate current. The plate current is then returned to the original value by making a change in the plate voltage. The ratio of the change in plate voltage to the change in grid voltage is the fx of the tube under the operating conditions chosen for the test.

Current Flow In a diode it was shown that in a Triode the electrostatic field at the cathode was proportional to the plate potential, Ep, and that the total cathode current was proportional to the three-halves power of the plate voltage. Similiarly, in a triode it can be shown that the field at the cathode space charge is proportional to the equivalent voltage (Eg + Ep/), where the amplification factor, fi, actually represents the relative effectiveness of grid potential and plate potential in producing a field at the cathode.

It would then be expected that the cathode current in a triode would be proportional to the three-halves power of (Eg +Ер/д). The cathode current of a triode can be represented with fair accuracy by the expression:

Cathode current = К Eg +-j

where К is a constant determined by element geometry within the triode.

Plate Resistance The plate resistance of a vacuum tube is the ratio of a change in plate voltage to the change in plate current which the change in plate voltage produces. To be accurate, the changes should be very small with respect to the operating values. Expressed as an equation:

Rn = --- Eg = constant, Л = small

increment

f = -

The plate resistance can also be determined by the experiment mentioned above. By noting the change in plate current as it occurs when the plate voltage is changed (grid voltage held constant), and by dividing the latter by the former, the plate resistance can be determined. Plate resistance is expressed in Ohms.

Tronsconductonce The mutual conductance, also referred to as trans-conductance, is the ratio of a change in the plate current to the change in grid voltage which brought about the plate current change, the plate voltage being held constant. Expressed as an equation:

(Ip) <

20 30 40 M 0 70 Ю 90 100 GRID VOLTAGE (Eg)

Figure 9

POSITIVE-GRID CHARACTERISTICS (Ip VS. Eg) OF A TYPICAL TRIODE

Piote chorocterfstics af fhls type ore most commonfy used In determining the pulse.slgnal operating characteristics of a triode amplifier sfage. Note the forge emission capabifity of the oxide-coated heater cathode in tubes of the general type of the 6J5.

Ep = constant, Д = small

-g increment

The trans conductance is also numerically equal to the amplification factor divided by the plate resistance. = ;x/Rp.

Transconductance is most commonly expressed in microreciprocal-ohms or micro-mhos. However, since transconductance expresses change in plate current as a function of a change in grid voltage, a tube is often said to have a transconductance of so many milliamperes-per-volt. If the transconductance in milliamperes-per-volt is multiplied by 1000 it will then be expressed in micromhos. Thus the transconductance of a 6A3 could be called either 5-25 ma./volt or 5250 micromhos.

Characteristic Curves of о Triode Tube

The operating characteristics of a triode tube may be summarized in three sets of curves: The Ip vs. Ep curve (figure 8), the Ip vs. Eg curve (figure 9) and the Ep vs. E curve (figure 10). The plate resistance (Rp) of the tube may be observed from the Ip vs. Ep curve, the transconductance (Gm) may be observed from the Ip vs. Eg curve, and the amplification factor (ц) may be determined from the Ep vs. Eg curve.

The Load Line A load line is a graphical representation of the voltage on the plate of a vacuum tube, and the current

I- -I

§

-ZO -15 -10 -5 0 +5 +10 +!5 +2

GRID VOLTS (Ей)

Figure 10 CONSTANT CURRENT (Ep VS. Eg) CHARACTERISTICS OF A TYPICAL TRIODE TUBE

Tbis type of graphical representation Is used for Class С amplifier calculations since the operating characteristic of a Class С amplifier Is a straight line when drawn upon a constant current- graph.

passing through the plate circuit of the tube for various values of plate-load resistance and plate-supply voltage. Figure 11 illustrates a triode tube with a resistive plate load, and a supply voltage of 300 volts. The voltage at the plate of the tube (Cp) may be expressed as:

ep = Ep -(ip x RlJ

where Ep is the plate supply voltage, ip is the plate current, and Rl is the load resistance in ohms.

Assuming various values of ip flowing in the circuit, controlled by the internal resistance of the tube, (a function of the grid bias) values of plate voltage may be plotted as shown for each value of plate current (ip). The line connecting these points is called the load line for the particular value of plate-load resistance used. The slope of the load line is equal to the ratio of the lengths of the vertical and horizontal projections of any segment of the load line. For this example it is:

.01 - .02 1

Slope = -= -.0001 = --

100 - 200

10,000

The slope of the load line is equal to -1/Rl- At point A on the load line, the voltage across the tube is Zero. This would be true for a perfect tube with zero internal voltage drop, or if the tube is short-circuited from cathode to plate. Point В on the load line corresponds to the cutoff point of the tube, where no plate current is flowing. The operating range of the tube lies between these two extremes. For additional information re-

HANDBOOK

Triode Load Line

iii[iiip

Ep=3oov

Figure 11

The static load line for a typical triode tube with a plate load resistance of 10,000 ohms.

garding dynamic load lines, the reader is referred to theRadiotron Designers Handbook, 4th edition, distributed by Radio Corporation of America.

Application of Tube Characteristics

As an example of the application of tube characteristics, the constants of the triode amplifier circuit shown in figure 12 may be considered. The plate supply is

M iM

:Rl = 8K

Ec=4v.i- Ep=300V.

Figure 12

TRIODE TUBE CONNECTED FOR DETERMINATION OF PLATE CIRCUIT LOAD LINE, AND OPERATING PARAMETERS OF THE CIRCUIT

300 volts, and the plate load is 8,000 ohms. If the tube is considered to be an open circuit no plate current will flow, and there is no voltage drop across the plate load resistor, Rl. The plate voltage on the tube is therefore 300 volts. If, on the other hand, the tube is considered to be a short circuit, maximum possible plate current flows and the full 300 volt drop appears across Rl- The plate voltage is zero, and the plate currentis 300/8,000, or 37.5 milliamperes. These two extreme conditions define the load line on the Ip vs. Ep characteristic curve, figure 13.

For this application the grid of the tube is returned to a steady biasing voltage of -4 volts. The steady or quiescent operation of the tube is determined by the intersection of the load line with the -4 volt curve at point Q, By projection from point Q through the plate

Figure 13

APPLICATION OF Ip VS. Ep CHARACTERISTICS OF VACUUM TUBE

J 200 о

f f PLATE VOLTS (Ep)

I I

4 VOLT plate swing

Eg -4

d.c. bias level (Ec)

Ip +12.75

steady state (j \ plate cuhrent-J

1----

4Cg-p I

I

Figure 15 SCHEMATIC REPRESENTATION OF INTERELECTRODE CAPACITANCE

comes apparent. A voltage variation of 8 volts (peak-to-peak) on the grid produces a variation of 84 volts at the plate.

steady state (r ,\ plate voltage

Figure 14

POLARITY REVERSAL BETWEEN GRID AND PLATE VOLTAGES

current axis it is found that the value of plate current with no signal applied to the grid is 12.75 milliamperes. By projection from point Q through the plate voltage axis it is found that the quiescent plate vpltage is 198 volts. This leaves a drop of 102 volts across which is borne out by the relation 0.01275 x 8,000 = 102 volts.

An alternating voltage of 4 volts maximum swing about the normal bias value of -4 volts is applied now to the grid of the triode amplifier. This signal swings the grid in a positive direction to 0 volts, and in a negative direction to -8 volts, and establishes the operating region of the tube along the load line between points A and B. Thus the maxima and minima of the plate voltage and plate current are established. By projection from points A and В through the plate current axis the maximum instantaneous plate current is found to be 18.25 milliamperes and the minimum is 7.5 milliamperes. By projections from points A and В through the plate voltage axis the minimum instantaneous plate voltage swing is found to be 154 volts and the maximum is 240 volts.

By this graphical application of the Ip vs. Ep characteristic of the 6SN7 triode the operation of the circuit illustrated in figure 12 he-

Polarity Inversion When the signal voltage applied to the grid has its maximum positive instantaneous value the plate current is also maximum. Reference to figure 12 shows that this maximum plate current flows through the plate load resistor R, producing a maximum voltage drop across it. The lower end of Rl is connected to the plate supply, and is therefore held at a constant potential of 300 volts. With maximum voltage drop across the load resistor, the upper end of Rl is at a minimum instantaneous voltage. The plate of the tube is connected to this end of Rl and is therefore at the same minimum instantaneous potential.

This polarity reversal between instantaneous grid and plate voltages is further clarified by a consideration of Kirchhoffs law as it applies to series resistance. The sum of the IR drops around the plate circuit must at all times equal the supply voltage of 300 volts. Thus when the instantaneous voltage drop across Rl is maximum, the voltage drop across the tube is minimum, and their sum must equal 300 volts. The variations of grid voltage,plate current and plate voltage about their steady state values is illustrated in figure 14.

Interetectrode Capacitance always exists be-Capacitance tween any two pieces of metal separated by a dielectric. The exact amount of capacitance depends upon the size of the metal pieces, the dielectric between them, and the type of dielectric. The electrodes of a vacuum tube have a similar characteristic known as the interetectrode capacitance, illustrated in figure 15- These direct capacities in a triode ate: grid-to-cathode capacitance, grid-to-plate capacitance, and plate-to-cathode capacitance. The inter-electrode capacitance, though very small, has a coupling effect, and often can cause unbalance in a particular circuit. At very high

HANDBOOK

Tetrodes and Pentodes

< -I

о 100 гоо 300 400 soo

VOLTS (Ep)

Figure 16

TYPICAL Ip VS. Ep TETRODE CHARACTERISTIC CURVES

a. e

<

100 200 300 400 500

VOLTS (Ep)

Figure 17 TYPICAL Ip VS. Ep PENTODE CHARACTERISTIC CURVES

frequencies (v-h-f), interelectrode capacities become very objectionable and prevent the use of conventional tubes at these frequencies. Special v-h-f tubes must be used which are characterized by very small electrodes and close internal spacing of the elements of the tube.

4-4 Tetrode or Screen Grid Tubes

Many desirable characteristics can be obtained in a vacuum tube by the use of more than one grid. The most common multi-element tube is the tetrode (four electrodes). Other tubes containing as many as eight electrodes are available for special applications.

The Tetrode The quest for a simple and easily usable method of eliminating the effects of the grid-to-plate capacitance of the triode led to the development of the screen-grid tube or tetrode. When another grid is added between the grid and plate of a vacuum tube the tube is called a tetrode, and because the new grid is called a screen, as a result of its screening or shielding action, the tube is often called a screen-grid tube. The interposed screen grid acts as an electrostatic shield between the grid and plate, with the consequence that the grid-to-plate capacitance is reduced. Although the screen grid is maintained at a positive voltage with respect to the cathode of the tube, it is maintained at ground potential with respect to r.f. by means of a by-pass capacitor of very low reactance at the frequency of operation.

In addition to the shielding effect, the screen grid serves another very useful purpose. Since the screen is maintained at a positive potential, it serves to increase or accelerate the flow of electrons to the plate. There being large openings in the screen mesh, most of

the electrons pass through it and on to the plate. Due also to the screen, the plate current is largely independent of plate voltage, thus making for high amplification. When the screen voltage is held at a constant value, it is possible to make large changes in plate voltage without appreciably affecting the plate current, (figure 16).

When the electrons from the cathode approach the plate with sufficient velocity, they dislodge electrons upon striking the plate. This effect of bombarding the plate with high velocity electrons, with the consequent dis-lodgement of other electrons from the plate, gives rise to the condition of secondary emission which has been discussed in a previous paragraph. This effect can cause no particular difficulty in a triode because the secondary electrons so emitted are eventually attracted back to the plate. In the screen-grid tube, however, the screen is close to the plate and is maintained at a positive potential. Thus, the screen will attract these electrons which have been knocked from the plate, particularly when the plate voltage falls to a lower value than the screen voltage, with the result that the plate current is lowered and the amplification is decreased.

In the application of tetrodes, it is necessary to operate the plate at a high voltage in relation to the screen in order to overcome these effects of secondary emission.

The Pentode The undesirable effects of secondary emission from the plate can be gready reduced if yet another element is added between the screen and plate. This additional element is called a suppressor, and tubes in which it is used are called pentodes. The suppressor grid is sometimes connected

to the cathode within the tube; sometimes it is

brought out to a connecting pin on the tube base, but in any case it is established nega-

remote cut-off grid

i-OR ID

CATHODE

SHARP CUT-OFF GRID

Figure 18

REMOTE CUTOFF GRID STRUCTURE

- GRID VOLTS 0 Figure 19 ACTION OF A REMOTE CUTOFF GRID STRUCTURE

tive with respect to the minimum plate voltage. The secondary electrons that would travel to the screen if there were no suppressor are diverted back to the plate. The plate current is, therefore, not reduced and the amplification possibilities are increased (figure 17).

Pentodes for audio applications are designed so that the suppressor increases the limits to which the plate voltage may swing; therefore the consequent power output and gain can be very great. Pentodes for radio-frequency service function in such a manner that the suppressor allows high voltage gain, at the same time permitting faiirly high gain at low plate voltage. This holds true even if the plate voltage is the same or slightly lower than the screen voltage.

Remote Cutoff Remote cutoff tubes (variable Tubes mu) are screen grid tubes in

which the control grid structure has been physically modified so as to cause the plate current of the tube to drop off gradually, rather than to have a well defined cutoff point (figure 18). A non-uniform control grid structure is used, so that the amplification factor is different for different parts of the control grid.

Remote cutoff rubes are used in circuits where it is desired to control the amplification by varying the control grid bias. The characteristic curve of an ordinary screen grid tube has considerable curvature near the plate current cutoff point, while the curve of a remote cutoff tube is much more linear (figure 19). The remote cutoff tube minimizes crosstalk interference that would otherwise be produced. Examples of remote cutoff tubes are: 6BD6, 6K7, 6SG7 and 6SK7.

Beam Pbwer A beam power tube makes use Tubes of another method for suppressing

secondary emission. In this tube there are four electrodes: a cathode, a grid, a screen, and a plate, so spaced and placed that secondary emission from the plate is suppressed without actual power loss. Because

of the manner in which the electrodes are spaced, the electrons which travel to the plate are slowed down when the plate voltage is low, almost to zero velocity in a certain region between screen and plate. For this reason the electrons form a stationary cloud, or space charge. The effect of this space charge is to repel secondary electrons emitted from the plate and thus cause them to return to the plate. In this way, secondary emission is suppressed.

Another feature of the beam power tube is the low current drawn by the screen. The screen and the grid are spiral wires wound so that each turn in the screen is shaded from the cathode by a grid turn. This alignment of the screen and the grid causes the electrons to travel in sheets between the turns of the screen so that very few of them strike the screen itself. This formation of the electron stream into sheets or beams increases the charge density in the screen-plate region and assists in the creation of the space charge in this region.

Because of the effective suppressor action provided by the space charge, and because of the low current drawn by the screen, the beam power tube has the advantages of high power output, high power-sensitivity, and high efficiency. The 6L6 is such a beam power tube, designed for use in the power amplifier stages of receivers and speech amplifiers or modulators. Larger tubes employing the beam-power principle are being made by various manufacturers for use in the radio-frequency stages of transmitters. These tubes feature extremely high power-sensitivity (a very small amount of driving power is required for a large output), good plate efficiency, and low grid-to-plate capacitance. Examples of these tubes are 813, 4-250A, 4X150A, etc.

Grid-Screen The grid-screen mu factor (.figg) Mu Factor is analcous to the amplification factor in a triode, except that the screen of a pentode or tetrode is sub-

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Mixer and Converter Tubes

stituted for the plate of a triode. fig denotes the ratio of a change in grid voltage to a change in screen voltage, each of which will produce the same change in screen current. Expressed as an equation:

- Isg = constant, Д = small increment

The grid-screen mu factor is important in determining the operating bias of a tetrode or pentode tube. The relationship between control-grid potential and screen potential determines the plate current of the tube as well as the screen current since the plate current is essentially independent of the plate voltage in tubes of this type. In other words, when the tube is operated at cutoff bias as determined by the screen voltage and the grid-screen mu factor (determined in the same way as with a triode, by dividing the operating voltage by the mu factor) the plate current will be substantially at cutoff, as will be the screen current. The grid-screen mu factor is numerically equal to the amplification factor of the same tetrode or pentode tube when it is triode connected.

Current Flow The following equation is the in Tetrodes expression for total cathode cur-ond Pentodes rent in a triode tube. The expression for the total cathode current of a tetrode and a pentode tube is the same, except that the screen-grid voltage and the grid-screen /i-factor are used in place of the plate voltage and ц of the triode.

Cathode current = К (Eg + - ]

Cathode current, of course, is the sum of the screen and plate current, plus control grid current in the event that the control grid is positive with respect to the cathode. It will be noted that total cathode current is independent of plate voltage in a tetrode or pentode. Also, in the usual tetrode or pentode the plate current is substantially independent of plate voltage over the usual operating range- which means simply that the effective plate resistance of such tubes is relatively high. However, when the plate voltage falls below the normal operating range, the plate current falls sharply, while the screen current rises to such a value that the total cathode current remains substantially constant. Hence, the screen grid in a tetrode or pentode will almost invariably be damaged by excessive dissipation if the plate voltage is removed while the screen voltage is still being applied from a low-impedance source.

Tiie Effect of The current equations show how Grid Current the total cathode current in triodes, tetrodes, and pentodes is a function of the potentials applied to the various electrodes. If only one electrode is positive with respect to the cathode (such as would be the case in a triode acting as a class A amplifier) all the cathode current goes to the plate. But when both screen and plate are positive in a tetrode or pentode, the cathode current divides between the two elements. Hence the screen current is taken from the total cathode current, while the balance goes to the plate. Further, if the control grid in a tetrode or pentode is operated at a positive potential the total cathode current is divided between all three elements which have a positive potential. In a tube which is receiving a large excitation voltage, it may be said that the control grid robs electrons from the output electrode during the period that the grid is positive, making it always necessary to limit the peak-positive excursion of the control grid.

Coefficients of In general it may be stated Tetrodes and that the amplification factor Pentodes of tetrode and pentode tubes

is a coefficient which is not of much use to the designer. In fact the amplification factor is seldom given on the design data sheets of such tubes. Its value is usually very high, due to the relatively high plate resistance of such tubes, but bears little relationship to the stage gain which actually will be obtained with such tubes.

On the other hand, the grid-plate transcon-ductance is the most important coefficient of pentode and tetrode tubes. Gain per stage can be computed directly when the Сщ is Icnown. The grid-plate transconductance of a tetrode or pentode tube can be calculated through use of the expression:

with Esg and Ep constant.

The plate resistance of such tubes is of less importance than in the case of triodes, though it is often of value in determining the amount of damping a tube will exert upon the impedance in its plate circuit. Plate resistance is calculated from:

with Eg and Egg constant.

4-5 Mixer and Converter Tubes

The superheterodyne receiver always in-

cathode

oscillator grid

-screen grid

PLATE

-metal shell

.aIe

filament

-suppressor and shell

i-signal grid

Figure 20 GRID STRUCTURE OF 6SA7 CONVERTER TUBE

eludes at least one stage for changing the frequency of the incoming signal to the fixed frequency of the main intermediate amplifier in the receiver. This frequency changing process is accomplished by selecting the beat-note difference frequency between a locally generated oscillation and the incoming signal frequency. If the oscillator signal is supplied by a separate tube, the frequency changing tube is called a mixer. Alternatively, the oscillation may be generated by additional elements within the frequency changer tube. In this case the frequency changer is conmion-ly called a converter tube.

Conversion The conversion conductance{G Conductance is a coefficient of interest in the case of mixer or converter tubes, or of conventional triodes, tetrodes, or pentodes operating as frequency changers- The conversion conductance is the ratio of a change in the signal-grid voltage at the input frequency to a change in the output current at the converted frequency. Hence G,. in a mixer is essentially the same as transconductance in an amplifier, with the exception that the input signal and the output current are on different frequencies. The value of Gj. in conventional mixer tubes is from 300 to 1000 micromhos. The value of G, in an amplifier tube operated as a mixer is approximately 0.3 the Gni of the tube operated as an amplifier. The voltage gain of a mixer stage is equal to GcZl where is the impedance of the plate load into which the mixer tube operates.

The Diode Mixer The simplest mixer tube is the diode. The noise figure, or figure of merit, for a mixer of this type is not as good as that obtained with other more complex mixers; however, the diode is useful as a mixer in u-h-f and v-h-f equipment where low interelectrode capacities are vital to circuit operation. Since the diode impedance is

О О

о о

7Г

-р-Г

Ее Cgk- г=; \

i----l--;

Figure 21

SHOWING THE EFFECT OF CATHODE LEAD INDUCTANCE

The c/egenerotive action af cathode lead in-ductance tends to reduce the effective grld-to-cathode voltage with respect to the voltage available across the input tuned circuit. Cathode lead inductance also Introduces undesirable coupling between the Input and the output circuits.

low, the local oscillator must furnish considerable power to the diode mixer. A good diode mixer has an overall gain of about 0.5.

The Triode Mixer A triode mixer has better gain and a better noise figure than the diode mixer. At low frequencies, the gain and noise figure of a triode mixer closely approaches those figures obtained when the tube is used as an amplifier. In the u-h-f and v-h-f range, the efficiency of the triode mixer deteriorates rapidly. The optimum local oscillator voltage for a triode mixer is about 0.7 as large as the cutoff bias of the triode. Very little local oscillator power is required by a triode mixer.

Pentode Mixers and Converter Tubes

The most common multi-grid converter tube for broadcast or shortwave use is the penta grid converter, typified by the 6SA7, 6SB7-Y and 6BA7 tubes (figure 20). Operation of these converter tubes and pentode mixers will be covered in the Receiver Fundamentals Chapter.

4-6 Electron Tubes at Very High Frequencies

As the frequency of operation of the usual type of electron tube is increased above about 20 Mc, certain assumptions which are valid for operation at lower frequencies must be reexamined. First, we find that lead inductances from the socket connections to the actual elements within the envelope no longer are negligible. Second, we find that electron