September 1942 QST
[Table
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged. 
It's
hard for most people alive today to imagine a time when vacuum tubes
were the only means of amplification and rectification available.
The discovery and application of semiconductors as replacements
was a huge step forward for all but the highest power applications
like megawatt power amplifiers. Equally hard to imagine is having
to design circuits without the aide of computers  or at least a
digital calculator. Parameter tables and slide rules were de rigueur
for the day. Power supplies in the hundreds of volts were commonplace
and printed circuit boards were a platform of the future. Pointtopoint
wiring ruled the day.
Other than for special cases like traveling
wave tubes (TWTs) and microwave magnetrons, there are not many engineers
left that design tubes. As with a lot of the vintage methods and
equipment, it is amateur hobbyists who keep the art of tube circuits
alive. The Internet is full of projects and articles on tube design.
I have accumulated a few resources over the years right here on
RF Cafe.
NOTE: References to the Handbook
in the Assignments are for the ARRL Handbook of the day.
Sorry, I don't have that available.
See all available
vintage QST articles.
A Course in Radio Fundamentals
Lessons in Radio Theory for the Amateur
By George
Grammer, W1DF
No.4  VacuumTube Fundamentals
EXPERIMENTS designed to show comprehensively the operation of
the vacuum tube as an amplifier require a fairly elaborate array
of test apparatus. Finding the gainfrequency characteristic of
an audio amplifier, for example, requires the use of a calibrated
source of variable frequency over the audiofrequency range, plus
a calibrated attenuator and means for measuring voltage with readings
independent of frequency, while distortion cannot readily be observed
without an oscilloscope. Such equipment is expensive and satisfactory
substitutes cannot readily be constructed at home.
However,
simple experiments designed to show the properties of vacuum tubes
readily can be performed with the gear described in the preceding
installments. As a convenience in setting up apparatus, a tube board
such as is shown in Fig. 1 can be added. It consists simply of a
baseboard on which is placed a square piece of Bakelite in which
is mounted an ordinary octal socket, connections being brought out
from the socket prongs to machinescrew terminals. This permits
chanting tube connections without soldering. The heater terminals
are permanently connected to a terminal strip mounted at the back
of the board; this strip also has terminals for "B" supply, one
negative and two positive. The latter take care of separate plate
and screen voltages when a tetrode or pentode is used. A pushbutton
mounted on the board provides a means of closing the plate (or screen)
circuit when the milliammeter in the test instrument is being used
for other measurements.
In using the plate power supply
with its variable voltage divider it should be remembered that only
a limited current can be taken through the divider taps for more
than very short periods of time. The variable resistor, in particular,
is rated at only a few watts, and if the output current is more
than 15 milliamperes or so the time during which current flows must
be kept to a minimum. Since a reading can be taken in a matter
of seconds this is no handicap, but if the supply is used for continuous
output the resistor arm should be set at the end connected to the
transformer center tap (see Fig. 4, p. 65, August QST), or else
a switch should be provided for shorting between the negative output
terminal and the wire connected to the centertap of the power transformer.
Tube Characteristics
Some amplification
of the Handbook material dealing with tube constants may be helpful
in connection with the experimental work. In the paragraph on "Characteristics"
(§ 32), for instance, plate resistance is defined as "the ratio,
for a fixed grid voltage, of a small plate voltage change to the
plate current change it effects." This can be written in the form
of an equation:
where r_{p} stands for plate resistance, ΔE_{p}
for the change in plate voltage, and ΔI_{p} for the corresponding
change in plate current. The sign Δ indicates that we are concerned
not with one value but with the difference between two values. (In
other respects the equation is simply the familiar statement of
Ohm's Law.) The other two constants, amplification factor and mutual
conductance, also can be defined in formulas instead of words:
By simple substitution in these formulas it is found that the
three constants are related in this way:
The values of the constants can be found by plotting characteristic
curves and measuring the change which occurs in one quantity when
the other is changed any arbitrary amount. However, this method
must be used with some caution when the characteristic curve does
not turn out to be a straight line. If the line bends, the "constant"
is not actually always the same, but varies with the point on the
curve at which it is measured. For example, suppose that Fig. 2
represents a curve showing the variation of plate current as the
plate voltage is varied, and from it we want to determine the plate
resistance. We arbitrarily select A as the point from which to start
and, also arbitrarily, decide to make the plate current change,
I_{p} 2 milliamperes. A 2milliampere increase brings us
to point B on the curve. Then the corresponding change in plate
voltage, E_{p}, is the difference between the plate voltages
which cause 1 and 3 milliamperes to flow. Thus E_{p} = 70
 30 = 40 volts. Then
Suppose that instead of 2 milliamperes for I_{p} we had selected
1 milliampere. This would bring us to point C on the curve, and
now E_{p} = 53  30 = 23 volts. Substituting these new values
in the equation gives us
Because of the curvature of the characteristic the value of
the "constant" r_{p} as measured by this method will depend
considerably upon the value of Δ selected. As the value of Δ is
made smaller and smaller the value of the ratio ΔE_{p}/ΔI_{p
}approaches the ratio AD/DE, where the line FE is drawn tangent
to the curve at point A (that is, the line FE touches but does not
intersect the curve at point A). In Fig. 2 this ratio is
which is the value of the plate resistance at point A on the
curve. If points B or C had been selected instead of A as the starting
place (point at which plate resistance is to be determined) different
values of plate resistance would be obtained, since it is obvious
that tangents drawn through these points would not coincide with
the tangent EF.
In determining the values of the tube constants
from the curves, therefore, the preferred procedure is to draw a
tangent to the curve at the point at which the value of the constant
is to be measured, and then use the tangent line as a basis for
measurement of ΔE_{p} and ΔI_{p} (or whatever pair
of quantities is represented by the curve).
Fig. 2
While there is bound to be some inaccuracy in drawing the tangent,
in general the results will be nearer the truth than if two points
on the curve itself are selected. Of course if the curve is straight
the curve and its tangent coincide, so that in the special case
of a straightline curve points can be taken directly from the curve.
Caution
In the diagrams of the various
setups for the experiments to follow, milliammeters and voltmeters
are indicated where measurements are to be made. If enough separate
instruments are at hand, they may be used as shown. However, if
only the single combination test instrument is available for measuring
currents and voltages, extreme care should be used to see that the
proper range is selected before making voltage measurements. In
alternate switching from current to voltage it is only too easy
to leave the range switch on 01 ma. when connecting the instrument
across three or four hundred volts  with consequences easy to imagine.
Such an error bad enough normally  would be practically fatal
now, with instrument replacements or repairs virtually impossible.
Watch that range switch!
ASSIGNMENT 11
Study Handbook Sections 31 and 32, starting page
42. Perform Exps. 21, 22 and 23.
Questions
1) How does conduction take place in a thermionic
vacuum tube?
2) What is the space charge?
3) What is the purpose of the grid in a triode?
4)
Name the three fundamental tube characteristics and define them.
5) Why is a "load" necessary if a vacuum tube
is to perform useful work?
6) What are tube characteristic
curves?
7) Why is amplification possible with a triode
tube?
8) What is meant by the term "interelectrode
capacity"?
9) What is the difference between static
and dynamic characteristic curves?
10) In what form is the
power supplied to the platecathode circuit of a tube dissipated?
11) What is the purpose of tube ratings?
12) What
is meant by the term "plate current cutoff point or"?
13)
What is grid bias, and why is it used?
14) Define
saturation point.
15) What is rectification?
ASSIGNMENT 12 Study Handbook
Sections 33 and 34, starting page 44. Perform Exp. 24. Questions
1) Name three forms which the plate load
for a triode amplifier may take.
2) Define voltage
amplification; power amplification. What is the essential difference
between amplifiers designed for the two purposes?
3)
What determines the choice of operating point for an amplifier?
4) Define plate efficiency. How does it vary
with different types of operation (Class A, B and C)?
5)
What is harmonic distortion and how is it caused?
6)
Describe ClassA amplifier operation.
7) What
is feedback? What is the result of application of positive feedback?
Of negative feedback?
8) How is the input capacity
of a triode amplifier affected by its operating conditions?
9) What is driving power?
10) What is the
phase relationship between the alternating voltage applied to the
grid of an amplifier having a resistance load and the amplified
voltage which appears in the plate circuit?
11) What is
the effect of the value of load resistance on the amplification
obtainable with a given tube?
12) If a certain power amplifier
circuit delivers 3.5 watts when a signal voltage of 20 peak volts
is applied to the grid, what is the power sensitivity of the amplifier?
13) Describe ClassB amplifier operation.
14) What
is the definition of a decibel?
15) If the power level at
one point in an amplifier is 0.25 watt and at a later point is 4
watts, what is the gain in db.?
16) What are the distinguishing
characteristics of a ClassC amplifier?
17) What is the
difference between parallel and pushpull operation?
18)
A certain circuit provides an attenuation of 15 db.
What
is the ratio of power levels in the circuit?
19) If a signal
of 0.6 volt is applied to an amplifier having a voltage amplification
of 125, what is the output voltage?
20) In a certain amplifier
an input voltage of 0.01 volt produces an output voltage of 50 across
500 ohms. The input resistance of the amplifier is 0.1 megohm. What
is the gain of the amplifier in db.?
ASSIGNMENT
13 Study Handbook Sections 35 and 36, beginning
page 48. Perform Exp. 25.
Questions
1) What is the purpose of the screen grid in a tetrode
or pentode tube intended for use as a radiofrequency amplifier?
2) Does the shielding afforded by the screen
grid have to be as complete in a tetrode or pentode designed for
audio frequency amplification as in one designed for radiofrequency
amplification?
3) Describe secondary emission.
4)How may the effects of secondary emission be reduced in
a screengrid tube?
5) What is the difference
between a "variableµ" and "sharp cutoff H tube?
6)
Why is a mercuryvapor rectifier preferred to a highvacuum rectifier
when the rectifier tube must handle a considerable amount of power?
7) How does a mercuryvapor gridcontrol rectifier
differ from a highvacuum triode? Could such a gas triode" be used
for amplification in the ordinary sense of the word?
8)
Identify five general types of multipurpose tubes.
9)
What is a beam tube?
10) Name the two general types of cathodes
used in thermionic vacuum tubes.
11) What is the advantage
of the unipotential cathode?
12) What is the purpose of
centertapping the filament supply of a tube whose cathode is heated
by alternating current?
13) A certain r.f. power amplifier
requires a negative grid bias of 200 volts for ClassC operation.
The d.c, grid current is to be 16 milliamperes under operating conditions.
If the bias is to be obtained entirely from grid leak action, what
value of gridleak resistance is required?
14) A triode
amplifier requires a negative grid bias of 30 volts, at which bias
the plate current is 45 milliamperes. What value of cathode resistance
will give the required bias? If the amplifier is to be used at audio
frequencies as low as 100 cycles, what value of bypass capacity
should be shunted across the resistor to minimize negative feedback?
15) What value of cathode bias resistance should be provided
for a 6F6 used as a ClassA pentode audio amplifier with 250 volts
on the plate? (Use published operating conditions.) What value of
bypass condenser should be used to prevent negative feedback at
frequencies down to 80 cycles?
16) A pushpull r.f. power
amplifier requires 400 volts bias and a d.c, grid current of 15
milliamperes per tube under rated operating conditions. If 130 volts
of fixed bias is to be provided by batteries, what id leak resistance
should be used?
ASSIGNMENT 14
Study Handbook Section 37, beginning page 50. Perform Exp. 26.
Questions 1)
How may a vacuumtube circuit be made to generate selfsustained
oscillations?
2) Can oscillations be set up
in a circuit in which the feedback is negative?
3)
What is negative resistance?
4) Define series
feed; parallel feed .
5) Draw two circuits utilizing
magnetic feedback.
6) How can the amount of
feedback be controlled in the Colpitts circuit?
7)
Draw a simple triode crystal oscillator circuit. Which of the ordinary
oscillator circuits does it resemble most closely?
8)
Define the plate efficiency of an oscillator.
9)
Name four factors which can affect the frequency of oscillation.
10) What is a multivibrator? Name one of the uses for this
type of oscillator.
11) How can the effect of plate voltage
variations on frequency of oscillation be minimized?
12)
Draw three oscillator circuits with capacity feedback, and describe
how the feedback may be controlled in each.
13) What is
the usual method of obtaining grid bias in an oscillator circuit?
Why is it used in preference to other methods?
14) How can
frequency drift in an oscillator be reduced?
15) A 25microhenry
coil is available for use in an oscillator circuit which is to operate
at approximately 2000 kc, What capacity will be required to tune
the coil?
ASSIGNMENT 15 Study
Handbook Sections 38 and 39, beginning page 55.
Questions 1) What is
a fluorescent screen?
2) Describe the construction
and operation of a simple cathode ray oscilloscope tube. 3)
By what methods may an electron beam be deflected?
4)
Define deflection sensitivity.
5) How is the
intensity of the fluorescent spot controlled?
6)
What is the purpose of the sweep circuit in an oscilloscope?
7) Name two common forms of sweep. What are the
advantages and disadvantages of each?
8) What
is an electron gun?
9) Why is it desirable to
use amplifiers for the deflection voltages for a cathode ray tube?
10) Why should the time of the return trace in a linear
sweep circuit be as short as possible?
11) Explain the method
by which patterns are formed on the fluorescent screen. Construct
a pattern, using a. linear sweep with return trace time equal to
1/20 of the total time of the sweep cycle, for two cycles of a sine
wave applied to the vertical plates. Construct a pattern, using
the same two sinewave cycles applied to the vertical plates, but
with a single sine wave for the horizontal sweep. Compare with the
linear sweep.
12) Describe the operation of a gastriode
linear sweep generator.
Fig. 3
EXPERIMENT 21
Diode Characteristics
Apparatus:
This experiment uses the plate power supply, tube board, test
set, vacuumtube voltmeter, and three 1 watt resistors, 25,000,
50,000 and 100,000 ohms. The circuit arrangement is shown in
Fig. 3. Measurements must be made of the voltage applied to
the tube and the current flowing in its platecathode circuit;
the single test instrument can be used for both purposes by
being shifted back and forth for each pair of readings. However,
the small current consumed by the instrument when used as a
voltmeter will cause the actual output voltage to be lower when
the voltage is being measured than when the instrument is shifted
to read plate current. Unless a separate voltmeter which can
be left permanently in the circuit is available, it is advisable
to use the v.t. voltmeter, thus avoiding the loading effect.
The test instrument is therefore shifted between the plate circuit
of the tube being tested and the plate circuit of the voltmeter
tube.
The tube to be tested may be a 6H6, the diode
section of a combination diodeamplifier tube, or simply a small
triode such as the 6J5 with the grid and plate connected together
to act as a single plate.
Procedure:
The object of the experiment is to plot characteristic curves,
plate voltage vs. plate current for the tube alone (static characteristic)
and with various values of load resistance in series with the
plate circuit (dynamic characteristics). Starting at zero plate
voltage, increase the plate voltage in small steps, taking plate
current readings at each voltage step. With no load resistor
in the circuit, take readings at intervals of voltage which
will give current intervals of about 1 milliampere so that
enough points will be secured to give a smooth curve when the
points are plotted. In the case of the 6H6 tube, using one plate
and cathode only, onevolt intervals are suitable. Proceed similarly
when the load resistance is inserted in the circuit; in this
case larger voltage intervals (5volt steps, for instance) can
be used.
In using the single test set for all measurements.
the pushbutton should be closed while the voltage measurement
is being made so that the voltage can be adjusted to the proper
value with plate current flowing. If the plate circuit is not
closed at the time the voltage is adjusted, the voltage will
drop when the milliammeter is connected in the plate circuit
of the tube to measure plate current. It is not necessary to
make provision for closing the plate circuit of the v.t.v.m.
when the meter is being used elsewhere.
The observed
data should be plotted in the fashion shown in Fig. 4, which
gives characteristic curves taken on a 6H6. With no load the
current is quite high, reaching 10 milliamperes with about 7.5
volts applied. Other types of tubes may give considerably different
plate current values without load, but should approximate the
load curves given since the current which flows at a given voltage
is principally determined by the load resistance rather than
the tube. As is to be expected, the current decreases, at a
given applied voltage, as the load resistance is increased.
If the noload curve is inspected carefully, it will
be observed that it is not a straight line, particularly near
the lowvoltage end. The lamp in Exp. 10 was another example
of a nonlinear circuit, although for a different reason. In
the present case, the nonlinearity arises from the fact that
the number of electrons drawn to the plate is not strictly proportional
to the voltage applied between plate and cathode, The d.c. resistance
of the diode at any voltage is equal to that voltage divided
by the current which it forces through the tube. In practice
the behavior of the tube when an alternating voltage is applied
is of more interest, in which case the a.c, plate resistance,
or resistance effective to small changes in applied voltage,
is important. The value of this plate resistance is found as
described in the introduction to this installment.
When
a load resistance is inserted in the plate circuit the linearity
of the circuit consisting of the resistance and the tube is
better than that of the tube alone. This improvement, which
increases as the load resistance is increased, is because the
load resistor tends to reduce the effect of variations in the
resistance of the tube. For example, if the resistance of the
tube varies between 1000 and 3000 ohms with a
Fig 4
certain range of applied voltage the resistance change is
2000 ohms, or an increase of 200%, using the smaller number
as a base, If a 10,000ohm resistor is connected in series,
the minimum resistance becomes 11,000 ohms and the maximum resistance
13,000 ohms, so that the increase in resistance is now only
2000/11,000, or 18%. With 100,000 ohms in series, the increase
is from 101,000 to 103,000 ohms, so that the percentage increase
is now 2%. In the curves of Fig. 3 the addition of the load
resistance makes all the points fall on a line which is practically
straight except at the low voltage end where the tube resistance
has its highest value. The higher the load resistance the less
marked does this slight curvature become.
In taking
data it will be observed that a small current flows in the plate
circuit even at zero plate voltage. This
current is
the result of the fact that some electrons are emitted from
the cathode with sufficient velocity to reach the plate even
though there is no positive charge on the plate to attract them.
For complete cutoff of plate current
Fig. 5
it would be necessary to make the plate a volt or two negative
with respect to the cathode, thus repelling these high energy
electrons from the plate. Since the current in any case is very
small  a very small fraction of a milliampere  it can be neglected
in most applications of the tube. However, in flowing through
an external load resistance of high value a volt or two may
be developed across the load, which may need to be taken into
account in some cases.
Experiment 22
Triode Static Characteristics
Apparatus: The setup for this experiment is
shown in Fig. 5. Insofar as the plate circuit of the triode
is concerned. the arrangement is practically the same as that
used for diode measurements, :Fig. 3, except that it is possible
to measure plate voltage with the test instrument rather than
the v.t. voltmeter. This is because larger plate voltage steps
may be used so that a high range (500 volts or the nearest provided
on the test instrument), which will have a resistance of a half
megohm or so, will give sufficient accuracy for all measurement.
The bias supply is incorporated in the
Fig. 6
setup to provide variable grid bias, and its voltage output
also may be measured by the test instrument on the condition
that the voltmeter resistance is 25,000 ohms or so (25volt
scale). Be sure that the positive output terminal of the bias
supply is connected to the grounded side of the 115volt line,
using the lamp provided for checking as described in July QST.
In using a single instrument in place of the three indicated,
the pushbutton should be closed each time the plate voltage
is measured so that the voltage will be that existing when plate
current flows. The resistor R shown in Fig. 5
is not needed in this experiment, so the pushbutton may be
connected directly to the plate. Procedure:
The object of the experiment is to determine the relationship
between plate voltage, plate current and grid voltage of a small
triode. One quantity is held constant throughout a run, the
second is varied, and corresponding measurements of the third
are made. A receiving triode such as the 6J5 is suitable. Three
sets of characteristics can be taken; the first, with the plate
voltage held fixed while the behavior of plate current with
varying grid voltage is observed, is called the "grid voltage
plate current" characteristic. When a series of such data is
taken with several fixed values of plate voltage, a "family"
of curves results. A typical gridvoltage platecurrent family
taken in this way on a 6J5 is shown in Fig. 6. The plate voltage
was set at 50 volt intervals from 50 to 400 volts (the maximum
output voltage of the power supply described in August QST),
Fig. 7
enough points being taken at each plate voltage to permit
smooth curves to be drawn. Notice that for each value of plate
voltage the curve bends at the higher values of negative grid
voltage (as the plate current decreases toward the cutoff point)
but that the curvature decreases as the grid bias becomes less
negative. The curves eventually straighten out and become practically
parallel, and the distances between the 50volt intervals also
approach equality. The dashed line shows the value of plate
current at which the plate dissipation (plate voltage multiplied
by plate current) is equal to the maximum rated value for the
tube; above this line the plate dissipation is exceeded.
The "plate family," shown plotted from experimental data in
Fig. 7, is obtained by holding the grid bias constant at selected
values and measuring the plate current as the plate voltage
is varied. These curves show the same general tendency to bend
when the plate current is near cutoff, and to straighten out
at higher values of plate current. The plate family is frequently
more useful than the set of grid voltageplate current curves
represented by Fig. 6. When the remaining quantity,
plate current, is held constant while the grid voltage is varied
(the plate voltage being adjusted for each value of grid bias
to give the selected value of plate current) the set of curves
shown in Fig. 8 results, again plotted from experimental data
on a 6J5. These "constant current" curves show the relative
effect of grid voltage and plate voltage on plate current. The
curves are nearly straight lines for all except very small values
of plate current, showing that the amplification factor is practically
constant for a given platecurrent value regardless of the plate
and grid voltages. The fact that, with the exception of the
curve for a plate current of 0.1 milliampere, the curves are
very nearly parallel indicates that the amplification factor
also is nearly independent of the plate current so long as the
latter is not near the cutoff point.
The values of
amplification factor, µ, plate resistance, r_{p}, and
mutual conductance, g_{m}, can be measured from these
three sets of curves. The mutual conductance, ΔI_{p}ΔE_{g}
can be found from the curves of Fig. 6 since these curves show
the relationship between grid voltage and plate current. The
plate resistance, ΔI_{p}/ΔE_{p}, can be measured
from the curves of Fig.: 7, which relate plate current to plate
voltage for various values of grid bias, while the amplification
factor ΔI_{p}ΔE_{g}, can be taken from the curves
of Fig. 8. The method of making these measurements is described
in the introduction to this installment. Since these "constants"
are a function of three variables a large number of graphs
would be required to give their behavior even partially completely,
but one special case is shown in Fig. 9. This graph shows the
variation in µ, r_{p} and g_{m} as a function
of grid bias when the plate voltage is held constant at 250
volts, the normal rated operating voltage for the tube, and
is a plot of values measured at 250volt points on each of the
three sets of curves in Figs. 6, 7 and 8. It is plain that the
amplification factor changes relatively little compared to the
changes in the other two quantities. Increasing negative grid
bias causes
Fig. 8
the mutual conductance to decrease, which means that the
amplification obtainable from the tube also decreases since
amplification is proportional to mutual conductance, other things
being equal. On the other hand, the plate resistance increases
with increasing negative grid bias. As a check on the accuracy
of measurement, the three curves should satisfy the relationship
within reasonable limits of accuracy, for any given value
of grid bias. If published average curves for the type of
tube measured are available, it will be of interest to compare
them to the curves determined experimentally. Exact duplication
of the published curves is not to be expected, of course, because
of slight variations in manufacture.
EXPERIMENT
23
Triode Dynamic Operation
Apparatus: Same equipment as
for Exp. 22, with the addition of the following resistors: 5000,
10,000, 25,000, 50,000 and 100,000 ohms. Resistors of 1watt
rating will be satisfactory. Procedure:
The object of this experiment is to plot dynamic grid voltageplate
current characteristics for representative values of plate load
resistance. Using a fixed value of platesupply voltage, insert
a resistor at R, Fig. 5, and measure the plate current as the
grid bias is varied in steps of 2.5 volts or so. Each time the
grid bias is changed, readjust the platesupply voltage (measured
across the
Fig. 9
supply terminals, not from plate to cathode of the tube
being investigated) with the pushbutton closed so that the
voltage under load will be the actual value selected. The voltage
will need to be reset as the plate current increases, because
of voltage drop in the power supply. When a complete set of
data has been obtained with one value of plate load resistance,
change to another value and take another run. When finished
with all values of resistance, plot the data in the form of
curves showing plate current against grid bias.
A typical
set of such curves, taken on a 6J5 with the plate voltage constant
at 300, is shown in Fig. 10. As the plate load resistance is
made larger the maximum plate current (at zero grid bias) becomes
smaller, as is to be expected. The plate current cutoff point,
however, occurs at approximately the same value of negative
grid bias in each case, since the plate voltage is fixed and
at zero current there is no voltage drop in the load resistor.
As in the case of the diode which was the subject of Exp. 21,
increasing the value of load resistance has the effect of straightening
out the curve, so that the curves taken with high values of
load show less bending than curves with no load or small values
of load resistance.
The effect of the load resistance
on the amplification obtainable from the tube, and also the
distortion it introduces, can be found graphically from curves
such as these. In Fig. 11, as an illustration, the curve for
R = 10,000 ohms has been plotted singly for the purpose of showing
the relationship between varying grid signal voltage and the
corresponding variations in plate current. An operating point
should be chosen somewhere near the middle of the relativelystraight
part of the curve, such that the product of the plate current
by the voltage between plate and cathode
Fig. 10
Fig. 11
will not exceed the rated plate dissipation of the tube.
In Fig. 11 the operating point selected is the point A, at 7.5
volts grid bias, making the nosignal plate current slightly
less than 8 milliamperes. The dashed line extending downward
from A is the axis of grid voltage, and the line extending to
the right is the axis of plate current. On the grid voltage
axis a sine wave is plotted as the assumed signal voltage (the
actual shape of the signal wave is not highly important, but
the sine wave is representative of a single frequency) as a
function of time, one complete cycle being represented. In Fig.
11 the signal has a maximum amplitude of 5 volts, so that the
instantaneous grid voltage swings between the limits of 2.5
volts and 12.5 volts about the fixed grid bias of 7.5 volts.
A corresponding time scale is applied to the plate current axis
so that the plate current corresponding to the grid voltage
at a given instant can be plotted.
Fig. 12
At zero time (beginning of the cycle) the grid voltage is
7.5 and the plate current 7.8 ma., approximately, Oneeighth
cycle later (point B) the grid signal voltage has risen to 71
% of its maximum value so that the instantaneous grid voltage
is 4 volts. The plate current, C, at that same instant is 12.3
milliamperes, and this value is plotted at D, oneeighth cycle
from zero time on the platecurrent axis. Points for other instants
are similarly obtained until enough are plotted to permit drawing
a smooth curve. When the cycle is complete it can be compared
for shape to the original grid signal. As Fig. 11 shows, the
two halves of the plate current cycle are not exactly the same
shape, as they were in the grid signal. This difference in shape
represents distortion, and the greater the difference the more
distortion there is present. As is obvious from the drawing,
the distortion is caused by the curvature of the tube characteristic,
since if the characteristic were perfectly straight the plate
current would be proportional to the grid voltage. Plotting
similar graphs from dynamic curves taken with different values
of load resistance readily will show the effect of the load
resistance on distortion. The gain of the tube
as an amplifier can also be found from the graph of Fig. 11
or from the curves of Fig. 10. Referring to Fig. 12, it can
be seen that with fixed plate supply voltage, E_{b},
the current flowing in the plate circuit will cause a voltage
drop across the load resistance, this drop being equal to I_{p}R,
where I_{p} is the value of the plate current and R
the resistance. The voltage actually between plate and cathode
of the tube is the platesupply voltage minus the voltage drop
in the resistance. When an a.c. signal is applied to the grid,
the plate current varies at the same frequency, hence a corresponding
a.c. voltage is developed across the load resistor. This a.c,
voltage is the useful output of the tube. The maximum drop in
the resistor occurs when the plate current is maximum, corresponding
to the most positive value of instantaneous grid voltage, and
the minimum drop occurs when the plate current is minimum, corresponding
to the most negative value of instantaneous grid voltage. In
Fig. 11 these platecurrent values are 14.5 milliamperes for
an instantaneous grid voltage of 2.5, and 3.0 ma, for a grid
voltage of 12.5. Since the plate load resistance is 10,000
ohms, the maximum voltage drop is 0.0145 X 10,000, or 145 volts,
and the minimum drop is 0.003 X 10,000, or 30 volts. The difference,
145  30, or 115 volts, is the total change in voltage across
the load corresponding to a total change in grid voltage of
10 volts. Hence the voltage gain is 115/10, or 11.5. The same
information could be obtained from the curves of Fig. 10 by
finding the currents corresponding to any chosen change in grid
voltage, and then proceeding as above to find the voltage output.
From such information a curve can be plotted showing the variation
of amplification with load resistance.
EXPERIMENT
24
ClassA Amplification
Apparatus: The power supply,
bias supply, v.t. voltmeter and tube board are used in this
experiment, together with a potentiometer or volume control
and the resistors specified in Exp. 23. Almost any potentiometer
resistance may be used, although values higher than about 100,000
ohms should be avoided if possible. The circuit arrangement
is shown in Fig. 13. The heater voltage for the tubes is used
as a source of a.c. voltage for the grid of the tube being tested,
the value of voltage applied to the grid being adjusted by means
of the potentiometer. The a.c. voltage in either the grid or
plate circuit is measured by the vacuum tube voltmeter, the
input circuit of which is connected to the circuit being measured
through the 0.01µfd. condenser. This condenser blocks the d.c.
voltages present and permits only the a.c, to be measured.
Before performing the experiment the v.t. voltmeter
should be calibrated on a.c, A source of variable a.c, voltage
can most conveniently be obtained by making a slight change
in the bias supply so that its voltage divider can be connected
directly across the a.c, line. Referring to Fig. 2
Fig. 13
Fig. 14
page 56, July QST, disconnect the top end of R, from the
filter and connect it to the a.c. output terminal. Then proceed
to calibrate the voltmeter by the same method used in making
the d.c, calibration, using the 0.01µfd. blocking condenser
in the "hot" voltmeter lead. Connect the 1µfd. condenser, C_{3},
to the cathode of the voltmeter tube (Fig. 6, page 66, August
QST). The calibration will be in terms of r.m.s. voltages, since
the test set calibration is r.m.s. The a.c, calibration will
resemble that taken on d.c., except that the curve above about
40 volts on the high range may show considerable departure from
linearity. If so, use only the linear part of this scale. This
effect is attributable to the fact that with a capacity of only
1 µfd. at C_{3} the time constant of the circuit is
too small at 60 cycles to permit the cathode bias to build up
to a value sufficient to prevent grid current from flowing at
the higher applied voltages. In performing the experiment care
should be taken to keep the maximum voltage to be measured within
the linear part of the highrange curve.
Procedure:
The purpose of this experiment is to confirm by measurement
the results of the gain calculations carried out as described
in Exp. 23. Adjust the grid bias (restore the voltage divider
connection to the filter after completing the a.c, calibration)
and plate voltage to the values used in the calculations, using
the same tube. These were 7.5 and 300 volts respectively in
our example, using a 6J5. Set the potentiometer so that the
voltage applied to the grid is about 2 volts r.m.s. as measured
between grid and cathode (Fig. 13). Insert a resistor in the
plate circuit of the tube at R, and adjust the platesupply
voltage to the selected value (300 in this illustration) with
plate current flowing (pushbutton closed). Shift the v.t.v.m.
to the plate circuit and measure the a.c, output voltage, keeping
the pushbutton closed. Repeat for various values of plate load
resistance, using two resistors in series to make up values
intermediate to those available in the single units. The results
of a typical
Fig. 15
set of measurements are given below, for 2 volts r.m.s.,
applied to the grid:
The gain of the amplifier will be equal to the output voltage
divided by the input voltage, or just half (input voltage =
2) the figures above. Plot the data in the form of a curve,
as shown in Fig. 14.
Note that the gain rises 8B the
plate load resistance is increased, but eventually a point is
reached where a considerable increase in load resistance causes
only a negligibly small increase in gain. The gain obtainable
is proportional to the amplification factor and also to the
ratio of the plate load resistance to the sum of the plate load
resistance and the a.c, plate resistance of the tube, and when
the plate load resistance is large compared to the tube resistance
this ratio changes very slowly. Hence the amplification tends
to level off as the plate load resistance is increased. From
the curves of Fig. 9 the tube plate resistance is seen to be
about
Fig. 16
7500 ohms. When the plate load resistance is about 5
times the plate resistance, or approximately 40,000 ohms, the
amplification increases very slowly with further increases in
load resistance. Hence a load in the vicinity of 50,000 ohms
is a suitable value for this tube as a resistancecoupled voltage
amplifier.
At 10,000 ohms, the value used in the illustration of
Exp. 23, the measured gain is about 13.5 as compared to the
calculated value of 11.5. The percentage difference, while fairly
large, is to be expected in view of unavoidable errors in measurement
and in plotting and reading the curves. Also, the resistance
was assumed to be exactly 10,000 ohms in the calculations, while
the manufacturing tolerances on the resistors is 10%. Ohmmeter
measurement of the resistor actually used in the experiment
showed the resistance to be on the high side of 10,000 ohms.
EXPERIMENT 2G
Pentode Characteristics
Apparatus: The apparatus setup used
in this experiment is shown in Fig. 15. The power supply, bias
supply, tube board and test instrument are required. In taking
one set of data it is necessary to maintain the screen grid
at constant voltage, preferably the rated value, and for this
purpose a VR10530 is substituted in the power supply for the
VR15030 previously specified. The tube tested can be a small
receiving pentode such as the 6J7. In making voltage
measurements, the highest voltage range on the test instrument
which will permit reasonably accurate reading should be used
so that the effects of voltage regulation will be minimized.
The 500volt scale for plate voltage and 25volt scale for grid
voltage will be satisfactory (or nearest equivalent ranges provided
on the actual instrument).
Fig. 17
Procedure: In this experiment curves equivalent
to those plotted for the triode (Exp. 22) are to be obtained,
for the purpose of determining the relationships between plate
current and grid and plate voltages in a pentode. It is advisable
to take data for the platevoltageplate current family first.
Using a 6J7, first set the grid bias at zero and then vary the
plate voltage, taking plate current readings at each value of
plate voltage selected. From a plate voltage of 100 up to the
maximum available from the supply (about 400) 50volt steps
will be satisfactory, Below 100 volts it is suggested that readings
be taken at 10, 25, 50 and 75 volts. Each time the plate voltage
is adjusted be sure the pushbutton in the plate circuit is
closed so that the voltage will be set to the proper value with
plate current flowing. When a set of measurements
has been made with zero grid bias, increase the bias to 1 volt
negative and repeat, Continue at 1volt intervals in bias until
a set of measurements has been taken for 6 volts. At higher
bias the plate current will be cut off, or else so small in
value as to be negligible. Plot the data in curves such as are
shown in Fig. 16. Comparing these curves to the
equivalent triode family in Fig. 7 shows a tremendous difference
in the behavior of plate current with varying plate voltage.
In the triode case (Fig. 7) the plate current is very markedly
dependent upon the plate voltage. On the other hand, except
for the region of plate voltage lower than the screen voltage,
the plate current of the pentode is practically unaffected by
the plate voltage. The curves begin to droop as the plate voltage
is reduced below 100, but the dropoff is not really marked
until the plate voltage is quite low. The fact that the plate
voltage has relatively little effect on plate current while
the grid voltage has a very great effect indicates that the
amplification factor, ΔE_{p}/ΔE_{g}, is very
high.
The cause of this behavior is the screen grid.
Since the
Fig. 18
screen grid is an electrostatic shield, it prevents the
electric field set up by the plate from penetrating to the region
occupied by the cathode and control grid, hence electrons in
this region are unaffected by the plate potential. The control
grid. however, has just as much effect on the electron stream
as it does in a triode. Electrons passing through through the
control grid are attracted to the screen because the latter
is operated at a positive potential, but many of them have sufficient
velocity to pass between the screengrid wires without being
caught by the screen grid itself. These electrons then come
under the influence of the electric field set up by the plate
, and are attracted to it, forming the plate current. Since
the plate can attract only the electrons which get through the
screen, it is obvious that the plate current will be determined
almost wholly by the screen potential and the structure of the
screen grid. The effect of the screen grid on
plate current can be found by holding the plate voltage at a
fixed value and varying the screen voltage (for a fixed value
of grid bias) while observing the plate current. A slight modification
of the experimental setup of Fig. 15 is necessary. Connect
the screen grid to the variable tap on the power supply as shown
in Fig. 17, and tap the plate connection on the powersupply
voltage divider so that the plate voltage will be about 250
volts. The first tap below maximum will be satisfactory. If
the plate voltage varies slightly during a run no harm will
be done since the plate current is only slightly affected by
the plate voltage so long as it is appreciably higher than the
screen voltage. Vary the screen voltage in small enough steps
so that smooth, curves can be plotted from the data. Do this
for several values of gridbias voltage. Typical experimental
Fig. 19
curves obtained by this method are shown in Fig. 18, taken
on a 6J7. These curves have essentially the same nature as the
curves of Fig. 7, which is to be expected from the explanation
of the operation of the screengrid tube given above.
Since the plate voltage has relatively little effect on
the plate current, a singlegrid voltageplate current curve
will suffice for practically all plate voltages above the screen
voltage, so long as the latter is not changed. Such a characteristic
can be taken by holding the plate and screen voltages fixed,
reading plate current while varying the grid bias. An experimental
curve on a 6J7 is shown in Fig. 19. Although in the triode case
the corresponding curves (Fig. 6) had to be drawn for several
values of plate voltage, in this case such a series would lie
so close together as to merge into one curve, for all practical
purposes. It can be seen, however, . that the curve has the
same general characteristics as those typical of triodes, and
if the mutual conductance is measured it will be found to be
approximately the same as for a triode of the same size. The
plate resistance is obviously high, since a large change in
plate voltage is required to make a comparatively small change
in plate current. Both plate resistance and amplification factor
are very difficult
Fig. 20
to measure with any reasonable accuracy because in each
case the ratio of the two quantities involved is so high that
the probable error in measuring the smaller of the two reflects
a large error in the ratio.
Further experimental work
may be done with the tube by plotting a series of grid voltageplate
current curves for different values of screen voltage. Also,
the effect of secondary emission may be investigated by running
a series of plate voltageplate current curves, corresponding
to those of Fig. 16, but with the suppressor grid connected
to plate instead of cathode. The characteristics of a variableµ
tube of the same general type, such as the 6K7, also may be
taken and compared to the sharp cutoff 6J7.
EXPERIMENT 26
Oscillator Operation
Apparatus: The power supply,
v.t. voltmeter and tube board are needed for this experiment,
together with the additional parts indicated in the diagram
of Fig. 20. The Hartley oscillator circuit is indicated in this
diagram, with parallel feed in both plate and grid circuits.
The radiofrequency chokes are 2.5millihenry piewound units,
and the blocking capacities are midget mica condensers. Provision
should be made for changing the gridleak resistance and for
using different values of load resistance. The 1watt resistors
used in previous experiments will be satisfactory in both cases.
Procedure: The object of this experiment
is to show the effect of gridleak resistance on oscillator
plate current, grid current, and r.f. output voltage, the plate
voltage being fixed at some convenient value and other circuit
conditions left unchanged. In the circuit of Fig. 20 the tuned
circuit is formed by one of the condensers and coils on the
circuit board, the whole 35turn coil being used with the cathode
of the oscillator tube (a 6J5) tapped on the coil 10 turns from
the grid end. The v t. voltmeter is connected between the cathode
and plate of the tube (through the plate blocking condenser)
to measure the r.f. plate voltage. The 1µfd. bypass condenser
in the v.t.v.m. cathode circuit (C_{3}) should not be
used.
With the plate voltage at some value which prevents
excessive plate current, such as 100 volts, insert a 5000ohm
resistor as a grid leak and measure the plate current, grid
current, and r.f. plate voltage. Adjust the plate voltage to
the chosen value with the plate circuit closed so that the tube
draws plate current. There should be no load on the oscillator
on the first run. Change the grid leak to 10,000 ohms and repeat,
then continue with successively higher values of gridleak resistance
up to 100,000 ohms. Connect a 25,000ohm resistor across the
v.t.v.m. input circuit as a load and repeat the measurements.
Continue with lower values of load resistance until the circuit
refuses to oscillate. The data may then be plotted in graphical
form.
Typical results of such measurements are shown
in the curves of Fig. 21. Curves for no load and for a load
of 10,000 ohms are shown for comparison, although if several
values of load resistance are used it would be better to use
separate sheets for each, to avoid confusion. With no load the
variation in r.f. output voltage over the whole range of gridleak
resistance is relatively small. The plate current is low and
decreases somewhat as the gridleak resistance is increased.
The grid current at the lowest gridleak resistance is relatively
high, but decreases with increasing gridleak resistance. The
grid bias  product of grid current by gridleak resistance
 shows comparatively little variation, indicating the selfregulating
properties of the oscillator in this respect; that is "the grid
current regulates itself so as to develop about the same bias
over a wide range of grid resistance,
When the circuit
is loaded the plate current shows a pronounced increase. This
is partly because the load reduces the Q of the tuned circuit,
thus lowering its parallel impedance and hence allowing more
plate current to flow, much in the same way that the plate current
increased in the curves of Fig. 10 with lower load resistance
for a fixed value of grid bias. At the same time the r.f. output
voltage decreases while the internal voltage drop in the tube
increases. This effect is comparable to the decrease in amplification
with lower load resistance which was observed in Exp. 24. The
platecurrent increase is exaggerated in the case of the oscillator
because the decrease in r.f. plate voltage is accompanied by
a proportional decrease in r.f. grid voltage, since the r.f,
grid voltage is obtained from the plate circuit. Hence the grid
bias also decreases, if the gridleak resistance and feedback
coupling are fixed. With lower grid bias more plate current
will flow, and to some extent the amplification increases so
that the r.f. output voltage tends to become greater. Thus two
tendencies working in opposite directions are present, but
with the net result that there is a decrease in both r.f. output
voltage and grid bias and an increase in plate current. Increasing
the value of gridleak resistance again results in selfregulating
action with respect to grid bias, while r.f. output voltage
and plate current decrease together.
The experiment
can be extended by making a similar set of observations with
a new value of feedback, obtained by changing the position
of the cathode tap on the coil. It is also of interest to compare
the operation of the various oscillator circuits which can be
made up from the coils and condensers on the circuit board.
Fig. 21
ANSWERS TO PROBLEMS IN INSTALLMENT 3
If no answer is given to a question, it is to
be found in the appropriate Handbook section or in the description
of the experiment or experiments accompanying that section.
Assignment 8: Q.2  10
volts; 500 volts; 500 volts.
Q.6  125; 55,000 ohms.
Q.7  Neglecting internal resistance: 11.4; 39.6 ohms.
Including internal resistance: 10.4; 38.7 ohms.
Q.94.55 µh.; 114 µµfd.
Q.10  The curve should go through
the following points:
50 µµfd.  41.4 µh.
100 µµfd.  20.7 µh.
150 µµfd.  13.8 µh.
200 µµfd.  10.4 µh.
250 µµfd.  8.3 µh.
Q.11  The curves should
go through the following points:
Q.12  3760 kc.
Q.13  a) 7120 kc.
b) 224.
c) 100,000 ohms.
d) 224 volts.
e) 1.12 volts; 0.56 amp.; 0.0025 amp.; 224  Q.
f) 7400 ohms; error = 8.1 % (could be neglected); 160%.
g) Neglecting internal resistance: 0.56 amp.; 0.0312 amp.; 17.9.
Including internal resistance: 0.557 amp.; 0.0338 amp.; 16.5.
Q.14  2.99 µh.; 42 µµfd.
Q.18  Same in both
cases.
Q.19 10 ohms.
Q.20  63.3 µµfd.; 157,000
ohms. Assignment 9:
Q.7  135 µh.; 3.7 µµfd.; no; 1.35 µh.; 370 µµfd.; tap load
down on coil.
Q.8  (For a frequency of 7120 kc.):
Capacity values for circuit A are maximum, for circuit B
minimum; fairly wide range of values can be used with circuit
C.
Assignment 10:
Q.2  85.7
meters; 281 feet.
Q.9450 kc., 4450 kc.; 3901.5 kc.,
3898.5 kc.; 1000 cycles, 14,299 kc.
Q.14  19 µµfd.
or higher.
Q.15  32 µfd. or higher.
Q.16  1.1
millihenry or higher.
Q. 17  Yes (47,000 ohms); no
(5650 ohms).
Related Pages on RF Cafe  Tube
Base Diagrams  A Course in
Radio FundamentalsVacuum Tubes Sep 1942 QST 
Electrons on Parade
Tube Production Video by RCA c.1942 
NEETS Module 6  Vacuum Tubes
 Vacuum Tube Vendors
Posted 2/21/2011
