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
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 are hereby
acknowledged. See all articles from Popular
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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 any longer.
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 okay.
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 trouble.
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.
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.
Let's run 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
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.
Incidentally, "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"
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 if shorted.
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.
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 in parallel.
Posted November 26, 2013