In 1959, Popular Electronics
magazine ran a 5-part series on test equipment usage. This final article is on the
use of a vacuum tube voltmeter (VTVM) for making accurate AC and resistance measurements.
Also in this edition is a construction article for RCA's
VoltOhmyst VTVM kit, so the two compliment each other. Author
Larry Klein discusses mainly the AC and ohmmeter functions, providing both functional
descriptions of the circuits and how to use them for making accurate measurements.
A very high input impedance is important to minimize the loading effect of the instrument
by keeping it from becoming a part of the circuit under test. FET-input digital
multimeters (DMMs) have nearly totally replaced VTVMs, but they can still be found
in some older electronics development labs and hobby benches.
May 1959 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is Part 4.
Test Instruments Part 5 - The Vacuum-Tube Voltmeter - A.C. and
By Larry Klein,
Last month we looked into a vacuum-tube voltmeter, examined the bridge circuit and
saw how it measured d.c. voltages. As a review, let's look at Fig. 5, a diagram
of the d.c. measurement circuit. (Figures 1 to 4 appeared in April.)
Fig. 5 - A simplified d.c. voltage measuring circuit showing
the range switch and bridge circuit.
Fig. 6 - Two typical voltage-doubling rectifiers used in
VTVMs. The diodes' contact potentials buck each other in (A) and the "A.C. Bal"
pot selects the zero point. In (B) the negative potential is bucked against B-plus
voltage tapped off the "A.C. Bal" pot.
Fig. 7 - Typical waveforms from a standard television
Fig. 8 - Note relationship between r.m.s. and P-P scales.
P-P scale is 2.83 times larger than r.m.s. scale.
Fig. 9 - Circuit diagram of an ohmmeter section of a vacuum-tube
Fig. 10 - Simplified input and range circuits of VTVM ohmmeter
The unknown d.c. voltage connected to the input terminals is applied across the
entire range switch voltage divider. Maximum on-scale reading is obtained by setting
the range switch at the proper voltage divider tap. The unknown d.c. voltage is
now applied to the input grid of the bridge - unbalance of the triodes results and
the meter deflects. So much for the d.c. bridge.
A.C. Voltage Measurement. What do we have to do to enable the
d.c. bridge to respond to a.c.? Why not simply rectify the unknown a.c. voltage
and then apply the resultant d.c. to the bridge input as we would any d.c. voltage?
That's actually what the standard VTVM does. Unfortunately, however, a number of
electronic bugs appear which prevent a simple diode circuit from being used, and
the circuits in actual practice usually look like those in Fig. 6. Why the
complications? Let's take a close look at Fig. 6 (A).
On one half of the cycle, the a.c. voltage to be measured is fed through capacitor
C1 to the cathode of one diode of the 6H6 tube, and thence to ground, The capacitor,
of course, gets charged in the process. On the positive-going part of the a.c. cycle,
no current flows through the first diode, C1 discharges and adds its voltage to
that developed across the three resistors connected to the plate of the second,
If we look carefully at the circuit, we'll recognize a type of voltage doubler.
Why a voltage doubler? Well, remember we need to get a d.c. voltage out of the rectifier
circuit which is at least as high as the a.c. input voltage. Taking into account
the voltage drop across the various components in the circuit, obviously some technique
is needed to soup up the d.c. output ... and that's what the doubler does.
Further circuit complications arise from a phenomenon called contact potential.
It seems that vacuum tubes, including diodes, tend to develop a small potential
between the elements. If allowed to remain, this slight voltage in the 6H6 would
cause a spurious reading on the low a.c. ranges. However, placing the a.c. balance
control between the two oppositely connected diodes, exact compensation can be made
by bucking out the opposing contact voltages.
Since the center contact of the a.c. balance potentiometer is also the take-off
point for the d.c. output, about half the d.c. developed across the three resistors
is lost by tapping off at this point. Actually, this is of small consequence, because
the d.c. voltage across the three resistors is equal to more than the peak of the
r.m s. a.c. input voltage, so we have volts enough to spare to provide an r.m.s.
R.M.S. and P-P. The key words in that last sentence were "r.m.s,
reading," which brings us to Fig. 6(B). Slightly more complicated than the
rectifier discussed above, this circuit also makes use of a doubler circuit.
Because of the low breakdown voltage of the 6AL5 tube, a voltage divider (in
addition to the one in the grid of the bridge tube) is needed to prevent the tube
from "arcing out" at the higher peak voltages. As shown, the a.c. input voltage
divider is part of the range switch and is, therefore, mechanically coupled to the
Perhaps you're wondering why the extra resistors at the a.c. input don't cause
a large difference in scale calibration between the a.c. and d.c. ranges. The VTVM
takes care of that by switching the last three bridge voltage divider resistors
out of the grid circuit when set up for an a.c. reading.
Whereas the job of the second diode in Fig. 6(A) is mainly to cancel out
the contact potential of the first diode, the second diode of Fig. 6(E) has
a different story to tell. Both diodes in Fig. 6(B) are used in a complete
voltage-doubler hookup which charges C2 to the full peak voltage of the incoming
waveform. Contact potential cancellation voltage is obtained from a tap across the
VTVM's B-plus supply.
The waveforms shown in Fig. 7 are taken from a standard TV set. You can
imagine the difficulties an r.m.s. calibrated a.c. meter would have translating
them to any sort of meaningful reading. Even putting a peak-to-peak reading scale
on the meter face (it would be the r.m.s. scale x 2.83) wouldn't help much because
the reading would still only be accurate for sine-wave inputs.
However, the P-P a.c. rectifier finds no difficulty in smoothing down these weird-looking
spikey TV waveforms into an exact d.c. equivalent and then feeding them to the bridge
circuit. The exact relationship between the P-P scales on a standard peak-reading
VTVM is shown in Fig. 8.
Resistance Measurement. One of the first things that hits your
eye in the ohmmeter section of the VTVM is the R x 1 meg. range switch position.
With the last scale division on the meter face marked 1000, this means that the
VTVM can read up to a 1000 x 1 million or a billion ohms!
The ohmmeter section of the average VTVM resembles the one shown in Fig. 9.
The string of seven resistors may differ in value somewhat depending on the exact
scales used and whether they are arranged in series, as shown, or switched individually.
But the principle of operation remains the same, as we shall see.
Suppose we redraw the range switch and input circuit of Fig. 9 into the
form of Fig. 10. We will use only one range resistor (Rrange) and
connect the resistor to be measured (Rx) to the VTVM's input terminals. The bridge
circuit remains the same and we will ignore it for now.
The first thing to do when using a VTVM ohmmeter is to "zero" it. Short the input
leads together and adjust the Zero Adj. control for a zero reading on the meter
scale. Then, unshort the leads of the VTVM and the needle will immediately swing
to the right-hand side of the meter face. Now adjust the meter to ∞ (infinite)
Let's see what the preceding adjustments have accomplished in terms of the internal
electronics of the VTVM.
Zero-adjusting the meter with the leads shorted has shorted out the battery through
resistor Rrange to ground and removed the voltage from the grid of the
bridge tube. Unshorting the test leads restores the battery voltage to the grid
and the meter swings full scale. The Ohms Adj. knob, which is in the same spot as
the A.C. and D.C. Cal. controls in the other circuits, adjusts the sensitivity of
the meter so that the applied battery voltage swings the meter needle exactly to
the infinite ohms scale marking on the meter face.
Suppose a 100-ohm resistor (Rx) is connected across the input leads and Rrange
is also set at 100 ohms. The voltage present at the grid of the bridge tube will
be exactly halved, and the meter will read half scale. Now if you look at the top
scale of the meter face shown in Fig. 8, you'll see that the center of the
scale indicates exactly 10.
If Rx were a 30-ohm resistor, for example, the shunting effect across Rrange
would be increased and even less voltage would reach the bridge tube. A higher value
resistor as Rx and a higher meter reading results. The only trick involved, and
the reason why it's so difficult for some home constructors to build their own ohmmeters,
is the scale calibration. As can be seen in Fig. 8, the scale divisions are
widely spaced at the right side of the meter face and narrow down towards the left.
A little thought as to how parallel resistors divide current will tell you why that
The Function Switch. In talking about the VTVM, we've left out
practically any reference to the function switch. Since these switches are so difficult
to show schematically in an understandable way without a prolonged discussion of
each switch position and what it accomplishes, we thought it best to save them till
The function switch is usually specially made for each manufacturer's VTVM and,
if analyzed, generally works out to be a five-pole, five-position unit. Some of
its jobs include switching the input jacks to the proper circuit, connecting in
the correct calibration control for each function, reversing the meter movement
connections for plus and minus d.c. and, in some cases, even turning the VTVM on
If you're curious, a complete schematic of the RCA "VoltOhmyst" VTVM kit is shown on page 79 of this issue and should
answer any questions you may have about the specific connections of the function
Next month we will put the VTVM "to work in an area in which it's practically
indispensable - repairing a hi-fi amplifier. The basic Williamson amplifier should
be a good subject, and we will learn how to troubleshoot one and what sort of measurements
the VTVM will turn up in working and non-working models.
Posted December 27, 2022
(updated from original
post on 3/18/2013)