July 1959 Popular Electronics
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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.
Test Instruments Part 6
THE VACUUM-TUBE VOLTMETER
Signal Tracing in a Hi-Fi
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.
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
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
. 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.
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 stereo amplifiers.
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
. 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 Aux. input.
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.
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
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." 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
When 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.
If you 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.
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 triodes.
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
. 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).
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 its part.