March 1930 Radio News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
You are taught early in your electronics career to be mindful of
the tendency for measurement equipment to affect the circuit it
is measuring, and therefore the indicated results. In the case of
high frequency circuits, even minute amounts of capacitance and/or
inductance can render results utterly unusable, but even in circuits
operating down to D.C. the simple internal resistance of a meter
can profoundly affect measurement accuracy. High impedance circuits
are particularly vulnerable to such "loading" effects by test equipment.
For example, consider a circuit being measured (device under test,
aka DUT) that has an impedance of 10 kΩ and the internal resistance
of the VOM is
(see diagram to left). If the open circuit "true" voltage level
is 11 V, then voltage division effected by the 100 kΩ
meter in series with the DUT's 10 kΩ internal resistance would
produce a VOM reading of 10 V (ten elevenths of 11 volts) -
clearly incorrect. In the days before FET (field effect transistor)
input multimeters, when most volt-ohm-milliammeters (VOMs) consisted
of a series of selectable voltage division circuits and a current-driven
d'Arsonval type meter movement, the internal impedance of a
meter was typically about 20 kΩ/V for many D.C. selector
ranges but only 1 kΩ/V for A.C. selector ranges (like
with the venerable
Simpson 260). Early VOMs had considerably lower internal resistances;
it was a real problem. The invention of the vacuum tube voltmeter
(VTVM) went a long way toward solving the issue, at least for circuit
impedances typical of the 1930s when the VTVM was invented. The
input impedance of a vacuum tube can be quite high, in the megohm
or tens of megohms range. Modern FET VOMs have input impedances
in the hundreds of megohms.
A New Tool for the Serviceman
By Joseph Heller
Joseph Heller, Chief Engineer of Wireless Egert Engineering,
Inc., holding the vacuum tube voltmeter of his design which
he describes here.
This vacuum-tube voltmeter has many uses such as testing the
perfection of and measuring loud-speaker reproduction and measuring
the gain of any receiver circuit
- The vacuum tube voltmeter will measure very feeble voltages
without drawing any current from the circuit.
- The meter is interchangeable on either a.c. or d.c. circuits,
and calibration on one range is accurate for all others.
- An accidental application of high voltage will neither injure
the instrument nor affect it in any way.
- The voltmeter also is useful for determining the performance
of an amplifier at different frequencies and with different
In the laboratory it is often necessary to measure the voltage
of a circuit which carries little current - if any - or we might
want to measure a potential and not a voltage drop. The usual type
of voltmeter is of course out of the question, for the same reason
that the old type of voltmeter is unsuited for measuring B-eliminator
voltages: i.e., the voltmeter itself draws current. When it is desired
to measure voltages where no current may be drawn from the circuit,
use is generally made of a vacuum-tube voltmeter.
Aside from the no-current advantage of this type of voltmeter,
is he added advantage that an accidental application of a high voltage
will not affect the instrument or injure it in any way.
The vacuum-tube voltmeter described here is equally accurate
on either d.c. or a.c. and on any frequency, either audio or radio,
up to 1500 kilocycles.
Most vacuum-tube voltmeters utilize what is known as the lower bend
of the grid voltage-plate current curve. The curves are obtained
in a rather simple way. All that is necessary is the simple circuit
shown in Fig. 1a. The tap on potentiometer R1 is turned until 4
volts of positive bias is indicated on voltmeter "V" and the reading
of milliammeter "A" is noted. This reading is then plotted at some
such point as "a" on the curve on Fig. 1b. R1 is then adjusted to
give a slightly lower voltage reading on voltmeter "V," and again
the current through milliammeter "A" is noted. This, when plotted,
will give some such point as "b' on the curve in Fig. 1. Continuing
in this manner we get points c, d, on the curve. At this point battery
"C" is reversed and the readings continued. We will then get such
points as f, g, h, etc., and draw the complete curve.
Back side of test set.
Many interesting facts can be derived from this curve. Suppose,
for instance, that the tube was left biased at point "g" which is
equivalent to a grid bias of -4 volts. If the circuit of Fig. 1a
is broken at the point "x," and an alternating current of three
volts impressed across the break, then the grid voltage would appear
as in Fig. 1c. The plate current would be very similar to this,
as shown in Fig. 1d. Since this curve is symmetrical, its average
value is shown by the dotted line. The last two curves mentioned
would be alike in shape, because the magnitude of the plate current
change would be practically equal whether the grid swung positively
or negatively. The condition we have at this time would compare
to that obtaining in a good amplifier stage.
If we should, however, bias our grid to point "j" on Fig. 1b,
which corresponds to about -9 volts, and again impress an alternating
voltage of three volts at point "X," we will get, as a result, curves
similar to those in Figs. 1e and 1f. It can be seen that the grid
voltage .curve is similar to that of the first case. A marked difference
is present, however, in the plate current curve. The upper half
of the wave remains practically the same, but the lower half has
been cut off. The voltage value shown bv the dotted line will, in
this case, be displaced from the axis. We have the condition, therefore,
of an alternating voltage causing an effective change in the plate
current. This will make itself evident by the higher reading of
the milliammeter "A." Continuing in this way it can be easily seen
that, if we should further increase the grid bias negatively, there
will be a change during the positive half cycle of the impressed
alternating voltage. but no sensible change during the negative
half; and his is exactly how the usual type of vacuum tube voltmeter
There are certain requirements which will both increase the ease
of operation and the reliability of such an instrument. To begin
with, a high negative bias does not mean that the plate current
has been entirely cut off. While the small current which does remain
would not make itself evident in a high range milliammeter, it will
make itself very troublesome in the sensitive micro-ammeter generally
used. The reading can be reduced to zero in the manner shown in
Fig. 2. By varying rheostat "R" we can send through the meter a
current from battery "K," equal and opposite to the current set
up by the plate battery. The meter will read zero and any change
in plate current will make itself evident by a deviation from zero.
In the actual construction of this instrument, the voltmeter, V1,
Fig. 2, should be included as well as a filament voltmeter V3. The
fixed resistance R2 in Fig. 2 is of the value of several megohms,
and is included in the circuit so that the voltmeter can be used
to measure the output from circuits coupled by condensers. If we
did not include this resistance and attempted to use the voltmeter
on a circuit isolated by a condenser, our voltmeter tube would not
receive any bias and would be useless for measurement work.
In Fig. 1a is shown the circuit which is employed to obtain
the characteristic curve of the tube to be used. The curve
thus obtained is shown in Fig. 1b. Figs. 1c and 1d show
the wave form of the grid potential and plate current, respectively,
when an alternating current of 3 volts is impressed on the
grid of the tube. In Figs. 1e and 1f the curves which would
result, if the grid were biased to about 9 volts, are shown.
Fig. 2 shows the circuit arrangement for obtaining zero
reading of the instrument. Fig. 3 shows the circuit for
amplifying and reading direct currents
In its present form this voltmeter is not useful for measuring voltages
below one volt without the addition of another instrument - a micro-ammeter
- while the voltmeter, without this instrument and without any changes,
is perfectly useful for voltages ranging from one volt upwards,
it would not be sensitive enough for voltages under one volt. If
it were necessary only to measure alternating voltages, a simple
amplifier would suffice. If, however, we want to make this instrument
as electrically flexible as possible, we should want it to measure
direct as well as alternating voltages. Since the ordinary amplifier
will not amplify a direct voltage it becomes necessary to make use
of what is known as a current amplifier. In most respects this amplifier
is exactly like that of the common type, with the important exception
that no condensers or transformers are permissible. To accomplish
our end in this matter we arrange a circuit as shown in Fig. 3.
It will be noted that the battery B1 is in an unusual position,
and that we have replaced the micro-ammeter by resistance R3. In
operation, a change of plate current through the voltmeter tube
will cause a voltage drop across R3. Since this changes the grid
voltage on the amplifier tube, the plate current through micro-ammeter
"A" will also change, and we have as a result a vacuum tube voltmeter
and an associated current amplifier which can be used to measure
either d.c. or a.c. voltages.
We might call attention at this point to the change in polarity
of meter A in Figs. 2 and 3. When connected as in Fig. 2 an impressed
voltage will cause an increase in the reading; but when the amplifier
tube is coupled to the voltmeter tube, the rise in plate current
through the latter will decrease the plate current through the ammeter.
Fig. 4 - The complete circuit diagram of the vacuum tube
voltmeter, an instrument which is fast gaining favor with
the more serious-minded servicemen and experimenters because
of the high degree of accuracy in measurement work which
can be attained by its use.
Now we will discuss the problem involved in the design and construction
of the apparatus for laboratory work. One of the first prerequisites
for such an instrument is the necessity of constancy of result.
If a reading is taken at one time, we shall want a reading repeated
at some later date to agree with the first. Also, since no one can
say for what purpose the apparatus will be used eventually, we shall
want a range from the lowest to the highest voltage that is likely
to be measured. A further desirability is the inclusion of some
method whereby the apparatus can be made ready for use without the
necessity of laboratory standards of any sort.
In order to make sure of the first condition it is necessary
only to make certain that all part are the finest obtainable, and
that the batteries and meters used are in good condition.
To make our instrument meet any reasonable demand for range,
we might make use of the input arrangement shown in Fig 4. This
is essentially a non-inductive potentiometer, and is so arranged
that the load upon the circuit to be measured is constant at 500,000
ohms. The values of separate resistances we used are indicated in
We can satisfy the last requisite by a novel arrangement for
setting the meters. If the reader will refer to Fig. 4, as he proceeds,
he should have no difficulty in following the discussion.
The first adjustment is that of filament voltage, accomplished
through the rotation of R1. With the filaments set at five volts
the plate voltmeter "V" is adjusted to read 130 volts by means of
rheostat R4. R5 is now turned until the plate current through the
amplifier tube is 2.5 milliamperes. This adjustment is the key to
the whole affair; it will be noticed that by adjusting the grid
bias of one tube, we automatically adjust the plate current through
both tubes to their correct value. The concluding adjustment is
carried out by adjusting "R" until the milliammeter "A" reads zero.
A study of the photographs will give a good idea of the method
used in constructing the model. A top panel is made of aluminum
1/4" thick, the edges were chamfered, and the top given a silvery
satin finish by fine emery and steel wool.
The vacuum tubes are slung beneath the panel, only a portion
of their tops protruding above. The shield caps which cover
them are in front of the case.
In order to make the apparatus as unvarying as possible, it was
wired with sturdy No. 12 round bus bar covered with spaghetti. Resistance
values as well as meter ranges are indicated in Fig. 4. Binding
posts were placed at point "m" These binding posts are short circuited
during normal use. If, however, we wish to cover a range such as
0-.1 volt with a full-scale deflection, the jumper is removed and
a zero to two hundred micro-ammeter substituted.
Since this meter can be used on either a.c. or d.c., we may calibrate
on d.c. and use it on a.c. A distinct advantage of the method used
to multiply the range is that it can be calibrated at anyone range
and will then be accurate for all ranges. It is of course understood
that the input resistances should be exact in values. A good d.c.
voltmeter of convenient range may be used in calibrating.
The finished instrument will come in handy in any experimental
laboratory where an exact knowledge of circuit behavior is desired.
It can be used to measure the gain of audio amplifiers, radio amplifiers,
total gain of sets, loud speaker performance, microphone characteristics,
and for a great many other measurements. Through its use we can
find out how an amplifier behaves at different frequencies and with
different circuit characteristics.
A future article will explain how some of these measurements
can be made.
Posted April 23, 2014