March 1930 Radio News
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. All copyrights (if any) are 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.
See all available vintage
Radio News articles.
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 circuit
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
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
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
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 Fig. 4.
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"
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
A future article will explain how some of these measurements can
Posted April 23, 2014