Test Instruments: The Tube Tester
August 1960 Popular Electronics
a fool, many years ago I donated a perfectly fine vacuum tube tester
that had been given to me by an über-engineer/ham I worked
with during the time (nearly 30 years ago) I was restoring my first
vintage tube radio. Bad move. It was a really nice tester: a B&K
Model 650 Dyna-Quik Dynamic Mutual Conductance Tube & Transistor
Tester. It was sold shortly after I had also given away as a wedding
gift the Crosley floor console radio that I restored. Another bad
move. Now, many moons later, I am working to restore yet another
Crosley tube radio and I sure
wish I had held on to it. Similar tube testers are routinely selling
on eBay for $100-$200. At some point I'll buy another tube tester,
maybe even another B&K Model 650 just for nostalgia's sake.
August 1960 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.
See all articles from
Test Instruments: The Tube Tester
the inside story on the devices designed to gauge the vacuum tube's
PART 1 - Checking for Shorts and for Noise
By G. H. Harrison
The vacuum tube - delicate heart
of most electronic equipment - is understandably subject to many
ills. Its elements can become shorted together, disconnected from
their pins, or loose on their mountings. Its filament or heater
can burn out, just like a light bulb. Its cathode, intended to supply
a steady stream of electrons to be shaped and molded by the tube's
other elements, can partially "dry up" and refuse to part with enough
electrons. Or its grid or some other element can begin acting like
a cathode and start spurting out an electron stream of its own.
Then, too, the tube itself can become gassy or noisy. Or
it can just get "tired out" - no specific trouble may show up, but
the tube simply doesn't have the "oomph" to do its job properly
Some of these troubles can be tracked down without a tube tester.
A simple filament continuity tester, such as the EICO Model 612
shown in Fig. 1, will quickly reveal open filaments. Alternatively,
an open filament or inter-element short circuit can be located with
an ohmmeter. Tubes suspected of other troubles can be yanked out
and replaced with new ones to see if this makes any difference -
if, that is, you happen to have a spare of the right type on hand
or don't mind buying one.
Fig. 1. A continuity checker, such as the EICO Model 612,
is among the simplest types of tube testers. Helpful in
tracking down tube troubles, it has a pilot lamp which will
light if tube filaments or heaters are okay.
But these methods have their shortcomings.
A continuity tester or ohmmeter provides only the crudest type of
test. And the replacement method is subject to error because other
circuit elements, in addition to a defective tube, might be faulty.
Thus, plugging in a new tube in such cases might make no difference,
and the troubleshooter might conclude that the original tube was
For these reasons, service technicians and electronic
experimenters look to their tube testers for quick, accurate information
about the condition of- the tubes they use. Tube testers are equipped
to probe every aspect of a tube's "state of health." Most testers,
in addition to giving some general indication of tube quality, also
check for shorts, loose elements, and other possible sources of
Types of Testers. Tube testers are divided into
two general types, depending on the method used to test overall
quality. Some, called emission testers, have the plate and all grids
tied together. A positive voltage is applied to the plate and grids,
and the current in the cathode circuit is measured. In other words,
such testers show just how many electrons the cathode is capable
of emitting under given conditions of plate voltage.
since the primary purpose of a tube is to amplify (except for diodes
and other special-purpose tubes which are always given simple emission
tests), the most accurate and revealing test is to see how efficiently
a tube operates as an amplifier. This is called mutual-conductance
testing. Mutual conductance is simply a measure of the effect small
variations in grid voltage have on plate current. To put it another
way, mutual-conductance testers measure how well amplifier tubes
work under actual operating conditions. Since they are more complex
than emission testers, they naturally cost more.
through the normal tube-testing procedure to see what you should
and should not do when using a tube tester. We'll digress from time
to time in order to examine some tube-tester circuitry in detail.
Testing for Shorts. First, turn on the instrument and adjust
the line-voltage calibration control. Most testers provide this
adjustment-it simply insures consistent indications by cancelling
out normal line-voltage variations. Next, locate the tube type
to be checked on the tester's tube chart, and set all of the dials
and levers as specified for that particular tube. Make sure that
they are all accurately set, and that you haven't confused two .
tubes on the chart with similar designations-a 6J5 for a 6J6, for
instance. Now plug in the tube. Wait about 30 seconds for the tube
to warm up, then test for shorts according to directions in the
tester's instruction manual.
Figure 2 shows how the EMC
Model 211 tube-tester checks for shorts. Each element in the tube
under test is connected to the center arm of a s.p.d.t. switch as
shown in this simplified diagram. Next, each switch is thrown to
the test position, one at a time, then returned to "normal." When
one switch is in the test position and the others "normal," all
elements except the one being tested are hooked to one side of a
circuit containing a power source and a neon bulb. The isolated
element is hooked to the other side of the same circuit. If a short
exists between the isolated element and any other tube element,
the circuit is completed and the bulb flashes on.
In testing for shorts with the EMC 211, as with most other testers,
disregard momentary flashes of the neon bulb when you throw one
of the switches. These flashes are caused by the discharge of inter-electrode
and stray circuit capacitances. It's also a good idea to tap the
tube under test gently with your finger throughout the test; this
will reveal any loose elements which might short out under vibration.
Fig. 2. Inter-element shorts can be quickly and easily located
with tube testers such as the EMC Model 211. For simplicity,
this diagram of a portion of the 211 's circuitry shows
only one of the unit's tube sockets.
testing for shorts, make sure that the indicator bulb does not glow
even faintly, except for flashes when you throw the switches. A
very weak glow, if .continuous, can indicate a high-resistance leakage
path, even though no direct short exists. Most tube testers are
not equipped to make sensitive leakage tests - such tests were not
usually necessary until FM and TV came along. Some AM radios and
amplifiers operate unimpaired with a leaky tube, but sensitive FM
and TV circuits generally react adversely to even the slightest
leakage. For this reason, many manufacturers are now turning out
highly sensitive leakage testers, both as separate units and as
part of regular testers. More about this next month.
"shorts" will show up across the filament or heater terminals, and
in cases where single elements are connected internally to more
than one pin. A "short" indication here, of course, is perfectly
normal, and the tube-tester chart will indicate where these normal
"shorts" should appear.
If shorts other than normal ones
show up during testing, the tube should be discarded. A shorted
tube can, under certain conditions, damage a tube tester if the
tube is tested for emission or mutual conductance. For this reason,
tubes should always be tested for shorts first, and thrown out immediately
Testing for Noise. Many testers provide a circuit
for testing noisy or potentially noisy tubes, and this is a logical
test to make next. Loose tube elements frequently cause noise. These
elements tend to vibrate, changing inter-electrode spacing and hence
capacitance and other circuit constants.
Figure 3(A) shows
a simplified diagram of the noise-testing circuit of the Superior
Model TW-11 tube tester. Using a switching circuit similar to that
used for short testing, one element at a time is hooked through
a pair of magnetic headphones (crystal phones won't do here) to
one side of a transformer. The other tube elements - all shorted
together - are hooked to the other side. Figure 3 (B) shows a still
further simplified diagram with the grid under test, and the switching
circuits eliminated for clarity. Tap the tube lightly with your
finger during the test; if there are any loose elements, they will
vibrate and cause a ringing or "pinging" in the headphones.
At this point, we are ready to test emission or mutual conductance
- depending on your tester- and run checks for open elements and
gas. Next month, we'll examine these functions in detail, and look
over the field of "quick" testers, cathode-ray testers, transistor
testers, and other special-purpose instruments.
Fig. 3. Simplified schematic (A) of noise-testing circuit
in Superior Model TW-11 tube tester (see photo to the left).
Further simplification of this circuit (B) shows the tube
under test for grid noise; the remaining elements are connected
Posted November 26, 2013
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