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Test Instruments Part 4
The Vacuum-Tube Voltmeter - D.C.
April 1959 Popular Electronics
Electronics ran a 5-part series on test equipment usage. This installment
is on the use of a vacuum tube voltmeter (VTVM) for making DC measurements.
Don't pass over the article just because it refers to a vacuum tube
tester since there are lessons that apply to even the most modern
transistorized, computerized meter. Author Larry Klein discusses
mainly the DC functions, providing both functional descriptions
of the circuits and how to use them for making accurate measurements.
FET-input digital multimeters (DMMs) have largely replaced VTVMs,
but they can still be found in some older electronics development
labs and hobby benches.
April 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The Vacuum-Tube Voltmeter - D.C. Ranges
By Larry Klein
last three installments of this series were devoted to the volt-ohm-milliammeter.
Now let's take a look at its chief competition - the vacuum-tube
voltmeter (or VTVM).
Why put tubes in a voltmeter? To answer
this question, it is necessary to understand the exact meaning of
"sensitivity." As we found when checking out the VOM, the accuracy
voltmeter reading depends upon the extent to which it affects the
circuit under measurement. After the voltmeter is connected, the
voltage at a particular point in a circuit frequently no longer
has the value it had before the instrument was attached.
Fig. 1. Before measurement is taken, the voltage drops across
the two 100,000 ohm resistors are equal. Shunting effect
of meter causes an unequal voltage division and the meter
reads less than half the voltage applied across A and C.
For example, with two equal-value resistors connected across a 100-volt
source, the voltage across each resistor is 50 volts. (See Fig 1).
However, a voltmeter connected across either resistor will read
less than 50 volts. Why?
Well, if the VOM in Fig. 1 is a
1000-ohms/volt job, it has an internal resistance of 100,000 ohms
(on the 100-volt range). Therefore, when the meter is connected,
the total resistance between points Band C falls to 50,000 ohms.
Since this is only half as great as the resistance between points
A and B, the voltage divides unevenly and less voltage now appears
between points B and C.
Of the total 100 volts, 66 2/3 volts will appear from A to B, and
33 1/3 volts will appear from B to C. The voltmeter therefore reads
33 1/3 volts even though 50 volts was present before the measurement
Fig 2. Alternate versions of the VTVM bridge circuit. Their
basic difference is mainly in the placement of the meter
The higher the internal resistance of a voltmeter,
the less current it will drain from the circuit under test and the
more accurate the reading will be. This is the advantage of the
VTVM. Because of its very high input impedance, the VTVM provides
a reading practically identical to the voltage existing before the
test leads were connected.
Balanced Bridge. A check of the VTVM circuits shows that most current
models use one or the other of the two bridge amplifiers in Fig.
2. In both circuits vacuum tube V1 serves as a d.c. bridge whose
basic job is to effect an increase in the sensitivity of meter movement
M1. Either circuit can also be considered as a means of decreasing
the input current requirements for deflection of the meter-which
comes down to the same thing. More on this point later.
Zero adjust control balances the bridge circuit in the Precise
Model 9071 VTVM.
Range switch of the Eico Model 221 with precision divider
resistors wired directly to the switch terminals for rigidity.
Internal view of the Heathkit V7A VTVM. The range switch
is on the left, function switch on the right.
Four calibration controls of the Eico Model 221 are grouped
around the meter movement as seen from the top of the chassis.
The "active" elements in the VTVM. Standard models usually
have a lineup comprising (from left to right) a 50-ma. power
supply selenium rectifier, a 6AL5 a.c. rectifier and a 12AU7
bridge tube. An alternate arrangement consists of a 6X5
power rectifier, a 6H6 a.c. rectifier and a 6SN7 bridge
V1 is usually a 6SN7 or 12AU7 dual triode, and the indicating meter
(M1) is connected from plate to plate or from cathode to cathode
of the triodes. In both circuits, R1 adjusts for the normal differences
between the operating currents of the two triodes and appears on
the front panel of the VTVM as the Zero Adjust control.
control R2 is usually mounted inside the VTVM cabinet and adjusts
for the small changes in total tube current over long periods of
If you're wondering why the VTVM circuits is referred
to as a "bridge," as you can see Fig. 2 (B) can be easily redrawn
to the standard bridge configuration (Fig. 3) and may even be more
easily understood that way.
The theory of the bridge is quite
simple. Triode V1b is the "reference" triode - in that its grid
is grounded and the amount of current flowing through the tube is
determined by the bias developed across cathode resistor R4 and
the plate voltage of about 100 volts d.c. Triode V1a is operating
under the same conditions except that its input grid is connected
to a 5- or 6-resistor voltage divider which for the present discussion
we will treat as a single resistor and call R5.
control R1 varies the plate voltages to tube V1 in Fig. 2 (A) and
varies the cathode bias in the circuit of Fig. 2 (B). By adjustment
of R1, the voltage differences between the two plates or two cathodes
can be reduced to zero - and the meter therefore will also read
zero. We have a balanced condition.
Unbalancing the Bridge. Now that we have a stable balanced bridge,
how do we go about making practical use of it? Referring back to
Fig. 2, let's apply 1 volt d.c. between the input grid of V1a and
ground and see what happens. When the applied positive voltage reduces
the bias on V1a, the tube immediately reacts with an increase in
current flow through R3. This, in turn, as Ohm's law tells us, causes
an increased voltage drop across R3. Meter M1, up to this moment,
has been comparing the drops across R3 and R4 and - not finding
any difference - read "0." Now, though, M1 responds to the voltage
difference between the plates or cathodes of V1, swings upscale
- and we have a VTVM at work.
Fig. 3. The circuit of Fig. 2 (B) redrawn in the standard
Fig. 4. Voltage divider which sets the amount of the input
voltage reaching the grid of the input triode.
take a look at R5. In a practical VTVM, R5 appears as the voltage
divider shown in Fig. 4. The total voltage to be measured is always
applied across the complete string of resistors in the input circuit.
For higher voltage ranges, the input grid of V1 is tapped farther
down on the voltage divider circuit. Note that the input resistance
remains constant and equals the total resistance of the voltage
divider plus the isolating resistor in the d.c. probe. Adding up
the resistances in the divider, you can see that the sensitivity
of the VTVM is quite high.
The usual VTVM d.c. input resistance
is 11 megohms and can be as high as 25 megohms. This is most important
on the low voltage ranges of the VTVM which are often used for measurements
in high impedance circuits. Note that, unlike the VOM, the input
impedance of the VTVM remains constant regardless of which voltage
range is used.
Next month we will continue our investigations
and see what makes the VTVM able to respond to a million cycles
a.c. and read up to a billion ohms resistance.
Posted December 27, 2013
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