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Test Instruments Part 5
The Vacuum-Tube Voltmeter - A.C. and
Ohmmeter Ranges
May 1959 Popular Electronics
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May 1959 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
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Popular
Electronics 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. FET-input digital multimeters (DMMs) have largely
replaced VTVMs, but they can still be found in some older electronics
development labs and hobby benches.
See all articles from
Popular Electronics. Here is
Part
4.Test Instruments Part 5
The Vacuum-Tube Voltmeter - A.C.
and Ohmmeter Ranges
By Larry Klein,
Technical Editor
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 set.
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 voltmeter.
Fig. 10. Simplified input and range circuits of
VTVM ohmmeter section.
 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.)
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, conducting
diode.
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. reading.
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 bridge divider.
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) ohms.
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 is so.
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 last.
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 and off.
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 switch.
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 3/18/2013
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