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. Point-to-point 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 - Vacuum-Tube 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 gain-frequency characteristic of an audio amplifier,
for example, requires the use of a calibrated source of variable frequency
over the audio-frequency 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 machine-screw 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 push-button 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 center-tap
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" (§ 3-2), 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 2-milliampere
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 straight-line
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 0-1 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 3-1 and 3-2, 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 3-3 and 3-4, 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 Class-A 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 Class-B 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 Class-C amplifier?
17) What is the difference between parallel
and push-pull 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
3-5 and 3-6, 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 radio-frequency 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 radio-frequency amplification?
3) Describe secondary emission.
4)How may
the effects of secondary emission be reduced in a screen-grid tube?
5) What is the difference between a "variable-µ"
and "sharp cut-off H tube?
6) Why is a mercury-vapor
rectifier preferred to a highvacuum rectifier when the rectifier tube
must handle a considerable amount of power?
7)
How does a mercury-vapor grid-control rectifier differ from a high-vacuum
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 center-tapping 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 Class-C 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 grid-leak 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 by-pass capacity
should be shunted across the resistor to minimize negative feed-back?
15) What value of cathode bias resistance should be provided
for a 6F6 used as a Class-A pentode audio amplifier with 250 volts on
the plate? (Use published operating conditions.) What value of by-pass
condenser should be used to prevent negative feed-back at frequencies
down to 80 cycles?
16) A push-pull 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
3-7, beginning page 50. Perform Exp. 26.
Questions
1) How may a vacuum-tube circuit be made to
generate self-sustained oscillations?
2) Can oscillations
be set up in a circuit in which the feed-back is negative?
3)
What is negative resistance?
4) Define series feed;
parallel feed .
5) Draw two circuits utilizing magnetic
feed-back.
6) How can the amount of feed-back 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 feed-back, and describe how the feed-back 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 25-microhenry 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 3-8 and 3-9, 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 sine-wave 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 gas-triode linear sweep generator.
Fig. 3
EXPERIMENT 21
Diode CharacteristicsApparatus:
This experiment uses the plate power supply, tube board, test set,
vacuum-tube 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 plate-cathode 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 diode-amplifier 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, one-volt intervals
are suitable. Proceed similarly when the load resistance is inserted
in the circuit; in this case larger voltage intervals (5-volt 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 no-load curve is inspected carefully, it will be
observed that it is not a straight line, particularly near the low-voltage
end. The lamp in Exp. 10 was another example of a non-linear 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 10,000-ohm 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 cut-off 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 22Triode Static
Characteristics Apparatus:
The set-up 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
set-up 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 (25-volt scale). Be
sure that the positive output terminal of the bias supply is connected
to the grounded side of the 115-volt line, using the lamp provided
for checking as described in July QST. In using a single instrument
in place of the three indicated, the push-button 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 push-button
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 grid-voltage plate-current
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 50-volt 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 cut-off,
and to straighten out at higher values of plate current. The plate
family is frequently more useful than the set of grid voltage-plate
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 plate-current 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 cut-off
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 250-volt 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 1-watt rating will be satisfactory.
Procedure: The object of this experiment is to
plot dynamic grid voltage-plate current characteristics for representative
values of plate load resistance. Using a fixed value of plate-supply
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 plate-supply voltage (measured
across the
Fig. 9
supply terminals, not from plate to cathode of the tube being
investigated) with the push-button closed so that the voltage under
load will be the actual value selected. The voltage will need to
be re-set 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 relatively-straight 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 no-signal 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 plate-current
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 plate-supply 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 plate-current
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 Class-A 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 0.01-µfd. 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 0.01-µfd. blocking condenser in the "hot" voltmeter lead.
Connect the 1-µfd. 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 high-range 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 plate-supply 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 push-button 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 resistance-coupled 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 set-up 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 VR-105-30 is
substituted in the power supply for the VR-150-30 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 500-volt scale for plate voltage and 25-volt
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 plate-voltage-plate 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) 50-volt 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 push-button
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 1-volt 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 drop-off
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 screen-grid
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 set-up
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 power-supply 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 grid-bias
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 screen-grid tube given above.
Since the plate voltage
has relatively little effect on the plate current, a single-grid
voltage-plate 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 voltage-plate current curves
for different values of screen voltage. Also, the effect of secondary
emission may be investigated by running a series of plate voltage-plate
current curves, corresponding to those of Fig. 16, but with the
suppressor grid connected to plate instead of cathode. The characteristics
of a variable-µ tube of the same general type, such as the 6K7,
also may be taken and compared to the sharp cut-off 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 radio-frequency chokes
are 2.5-millihenry pie-wound units, and the blocking capacities
are midget mica condensers. Provision should be made for changing
the grid-leak resistance and for using different values of load
resistance. The 1-watt resistors used in previous experiments will
be satisfactory in both cases.
Procedure:
The object of this experiment is to show the effect of grid-leak
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 35-turn 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 1-µfd. by-pass 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 5000-ohm 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 grid-leak resistance up to 100,000 ohms. Connect
a 25,000-ohm 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 grid-leak
resistance is relatively small. The plate current is low and decreases
somewhat as the grid-leak resistance is increased. The grid current
at the lowest grid-leak resistance is relatively high, but decreases
with increasing grid-leak resistance. The grid bias - product of
grid current by grid-leak resistance - shows comparatively little
variation, indicating the self-regulating 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 plate-current
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 feed-back 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 grid-leak resistance again results in self-regulating
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 feed-back,
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.9-4.55 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.9-450 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).
Posted 2/21/2011