Test Instruments Part 6: The Vacuum Tube Voltmeter
July 1959 Popular Electronics
to the advent of FET-input digital multimeters (DMMs), the vacuum
tube voltmeter (VTVM) was the primary instrument for use when high
input impedance was required. For the newcomer to electrical measurements,
high input impedance for the measuring instrument is needed when
measuring high impedance circuits so as not to load down the circuit
under test and cause an erroneous result. For instance, suppose
you are trying to measure the voltage across a 100 kΩ resistor that
is in series with a 50 kΩ and your voltmeter has a 100 kΩ input
impedance. The parallel combination of the two 100 kΩ resistances
(resistor and voltmeter) is 50 kΩ. If the supply voltage is 3 volts,
the voltage across the 100 kΩ resistor would actually be 2 volts,
but with the voltmeter across it, the reading would be 1.5 volts.
If the input impedance of the voltmeter was 10 MΩ instead, the parallel
resistance across the 100 kΩ resistor would be about 99 kΩ, which
would only introduce a very small error. I'll try to get the previous
5 parts of this article.
July 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.
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Test Instruments Part 6
THE VACUUM-TUBE VOLTMETER
Signal Tracing in a
By Larry Klein
checking out the vacuum-tube voltmeter in the last two installments
of Test Instruments, we discovered that one of the most important
reasons for using a VTVM was "sensitivity." In practical terms,
the sensitivity of a measuring instrument determines how it affects
the circuit under test. Using a low ohms/voltmeter in a high impedance
circuit is like trying to gauge a person's strength with a 10-ton
weight. What you're trying to measure crumbles under the load.
One area where the VTVM comes into its own is in signal
voltage measurement in hi-fi amplifiers. For not only can the VTVM
measure the a.c. signal voltage without knocking it to its knees,
but it will perform the measurement in the presence of d.c. at any
frequency in the amplifier's range. If there's a large enough signal
at a tube pin - your VTVM will read it.
What's so important
about signal voltage? Well, signal voltage is what your magnetic
phono cartridge (or tape head or tuner) supplies to your amplifier
to be passed on to your speaker. Your amplifier is not just a passive
element, but is more like an electronic Charles Atlas that builds
up the weakling input signals into the sort of powerful currents
that can move, if not mountains, at least speaker cones. If we take
a look at the signal voltage at each stage of its development, we
can get a good idea of exactly what contribution is made by each
tube in the circuit.
The Guinea Pig. Let's take a standard hi-fi amplifier
as our guinea pig and put it through its paces using an audio generator
to supply the signal voltage and a VTVM to measure it.
Fig. 11. Interconnections between test instruments and amplifier
when making gain and wattage checks.
Fig. 12. Slight (A) and extreme (B) clipping which occurs when
output stage of amplifier is overdriven.
Fig. 13. Voltage divider setup which can be used to supply
a signal in the millivolt range to amplifier under test.
Fig. 14. Eico HF-12 amplifier circuit broken down into its separate
sections for the purpose of discussion in the text. Reading
from top to bottom, sections are: preamplifier, tone control
and power amplifier.
amplifier we will work with - the Eico HF-12 - is not only well
designed but, in addition, has the advantage for us that it shares
a number of its circuit configurations with amplifiers of similar
and higher wattage. The same tests and techniques we use to check
out the Eico unit can therefore also be applied to other mono and
Let's take a quick look at the HF-12's
schematic in Fig. 14. The preamp stage uses a
tube (V1) with plate-to-grid feedback equalization. The two triodes
of a 12AU7 tube (V2) comprise the Baxandall negative feedback tone
control circuit which in turn feeds a modified Williamsontype power
amplifier comprising a dualtriode driver/inverter (V3) and a pair
of push-pull pentode outputs (V4 and V5).
For the tests
we have in mind, we'll need a VTVM and an audio oscillator. Any
of the small fixed-frequency transistorized sinewave generators
will do for gain tests, but for tone control check-out a variable
frequency audio generator such as the Heath, Eico, or Knight-Kit
units is required.
First, the test instrument setup. A vital
item in our check-out procedure is a 16-ohm "load" resistor connected
across the amplifier's 16-ohm output terminals. Since the Eico at
full output will throw a good 12 watts into the load resistor, we
better use one with at least a 20-watt rating and expect it to
get pretty hot. Several series/parallel resistors will do as well
as a single wire-wound job as long as the resistance and wattage
demands are met.
Set up the amplifier with the tone controls
at flat, volume control on full, and the selector switch to Aux.
Do not plug the amplifier in yet.
Watts = E2/R.
Switch on the audio generator and set it up for 1 kc. with the output
control at minimum, and the output lead plugged into the amplifier's
Set your VTVM to the 15-volt, a.c., or higher
range, and connect the test leads across the 16-ohm load resistor.
Now plug in the amplifier, let it warm up, and slowly turn up the
output level of the audio generator.
Watch the VTVM meter
needle climb up scale - when it reaches about 14 volts, we know
the amplifier is putting out a healthy 12 watts. How do we know?
Ohm's law tells us so - using the formula: W=E2/R; where
E is the voltage developed across the 16-ohm (R) load resistor and
W, of course, is watts output.
If available, an oscilloscope
can be used to monitor the output waveform of the amplifier by connecting
it across the load resistor. At about the 5-watt level, a goodlooking
sine wave will be seen on the scope. Pushing the amplifier to the
12-watt level (by boosting the input signal voltage), the scope
will begin to show "clipping" as in Fig. 12(A) Boost the input voltage
even more, and a waveform resembling Fig. 12(B) results.
At any rate, just barely visible clipping, all other factors
being equal, usually corresponds to about 1% harmonic distortion.
This test is used by a number of manufacturers to rate the power
output of their amplifiers.
Now that we've determined how
to get the twelve watts out of the amplifier, let's use our VTVM
to trace the signal from the input through the jungle of resistors
and capacitors which make up an amp's "works."
By Stage. If we want to start our tracing at the high gain
input of the amplifier, we will have to feed in a signal in the
5-10 millivolt range. If your audio generator lacks an attenuator
switch, the circuit shown in Fig. 13 will take a one-volt signal
and knock it down to approximately 10 millivolts. Use your VTVM
to set the A-B voltage and trust to the attenuator for the correct
C-D output voltage.
Set up the amplifier's controls as before,
but set the selector switch at the "mag. phono" position and plug
the lead from the attenuator setup into the mag. phono jack Turn
up the audio generator's output, using your VTVM to monitor the
output wattage across the 16-ohm load resistor.
the VTVM hits 14 volts (about 12.5 watts), transfer the VTVM leads
across A-B on the attenuator. On the Eico, if everything is going
well, the VTVM should indicate anywhere from 0.4 volts to slightly
less than 1 volt. An exact reading can't be quoted because there
are just too many variables for on-the-nose results to be expected.
What have we achieved with the preceding test? Well, with
one quick measurement, we've ascertained that the complete amplifier's
sensitivity and power output are both up to snuff.
want to make individual stage gain measurements, use a slight variation
in technique. Keeping the same input signal at mag. phone, as used
for the overall wattage check, take a measurement at the grid of
V2A. With your VTVM set for the lowest a.c. scale, expect about
0.5 volt. Remember, we applied about .005 volt (5 mv.) to the mag.
phono input; now, at the output of V1, it's paying off to the tune
of 0.5 volt. A little calculation will reveal that we've achieved
a gain of 100 on our investment.
Stage gain measurements
in the remainder of the circuit are as simple - but don't expect
the tube manual to be a reliable guide to the gains to be expected.
Remember, negative feedback is used throughout the better hi-fi
amplifiers and cuts down gain-per-stage considerably. For example,
in the tone control stage, with 0.5 volt from the preamp applied
to the input grid of V2A, less than 1.5 volts of signal will be
found at the plate of V2B - a gain of somewhat less than 3 for both
At V3, we come to the direct-coupled voltage amplifier/phase
inverter stage. About 1.4 signal volts applied at pin 7 of V3 will
get you 8 signal volts at its plate a gain of less than 6.
A Balanced Diet. When the 8 volts of signal
are fed to the grid of the split-load phase inverter (V3B), signal
voltages are developed across both the plate and cathode resistors.
The voltages (about 7.75) should be exactly equal and opposite (180°
out of phase).
The push-pull signal developed here feeds
the output tubes their balanced diet. Unequal signals at the plate
and cathode (which will cause "upset" in the output stage in the
form of distortion) are usually due to mismatch in the plate and
cathode load resistors. Optimally, they should measure within 1%
of each other.
This has been a fast guided tour following
a signal through a typical amplifier. Of course, there are many
more specialized measurements the VTVM can make in amplifiers as
well as other equipment. Anywhere that sensitivity and wide frequency
response are important, you'll find the VTVM ready and able to do
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