101 Tube tester
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
101 Tube tester
Ok, I've read through all the blather about meters, signal
generators, and oscilloscopes, and my question was about tube testers.
When are you going to talk about them?
Ok, fair enough. Roughly, tube testers can be divided up into a few
categories. (Your FAQ editor is flying somewhat blind here. I don't
own or use a tube tester of any type, and it has probably been forty
years since I tried to use one. Dan Schoo has kindly furnished
material on tube testers, which is included after this general
discussion).
1. Emissions testers. These are the type that used to be seen
in drug stores, and the Heathkit "Tube Checker" type of box. They are
fairly simple, have a heater/filament supply, a "good/?/bad" meter.
They generally operate by connecting the tube as a triode and seeing how
much current will pass through. Short circuit testing is by applying a
voltage across element pairs and seeing if enough current will pass to
light a neon bulb.
2. Transconductance testers. The best known of these are the
Hickock testers, both the civilian and the military models. As with the
emissions testers, they are provided with a heater/filament supply and a
shorts test arrangement. However, they are provided with separate DC
supplies and controls for setting the tube elements at one or more
operating points. Readout, as with the emissions tester, is on a meter
which indicates plate current. Most of these have two sets of settings
for the meter readout. In one mode, the meter reads the current as DC
transconductance. In the other, the meter reads on a "Good/?/Bad"
scale. The controls for setting the operating point parameters and
meter sensitivity maybe either potentiometers and switches, or sets of
contacts operated by a punched card. The type that uses potentiometers
and switches is generally used with a tabular listing (a roll chart in
the machine) of switch settings and readings. I am not sure how the
pots and switches are calibrated, and how easily one can reset the
operating point parameters, or read current values on the meter, for
taking a series of operating points to plot on a graph. I haven't seen
any discussion indicating that anyone is using them other than with the
tabular chart values for specific tubes.
3. Tektronix 570 Vacuum Tube Curve Tracer. This is a specialty
oscilloscope that can trace out families of operating curves on a CRT
X-Y display. The box is provided with a heater/filament supply, two
adjustable "fixed" voltages (i.e., they remain static during tracing), a
step voltage, and a sweep voltage. The display monitors current
(vertical) vs. voltage (horizontal), and may be switched to different
suppiies. Connections to the tube are via a patch panel at the front of
the unit, which uses jumpers to go to an adapter plate. Two of these
are provided, to allow side-by-side comparison of two tubes, and there
is a switch to select which set of jumpers is active. While the
switches are marked "plate," "screen," "grid," etc., the patch panels
allow connection of any of the voltages to any of the elements. This
device is not really a "tester," because it has no built-in settable
criteria or indications for "pass" or fail. The box can be set up to
provide a dynamic display of any set of curves, including such things as
suppressor and screen grid transconductance. These were low-volume
products, and the 570 was discontinued in the mid-1960's. So far as I
know, principal use of them was to match pairs of tubes for various
characteristics. Prices on the used market have been bid ought of sight
by the golden-ear tube audio crowd, and the last I heard, the going
price was well over $1000.
All of the above devices are DC tests only. While the Tek 570 provides
a dynamic display, it does so at very low audio frequencies. The
emissions type tester is clearly a rudimentary "pass fail" device whose
strength is in determining whether the tube will work at all. The
transconductance tester is somewhat more sophisticated in setting
parameters at an operating point, which may or may not represent the
actual application conditions that the circuit imposes on the tube.
Some of the major issues in selecting a tube tester involve the
configuration of sockets and socket adapters, and availability of test
data for specific tubes. Also, the condition of the tube sockets has to
be considered. The useful life of a tube tester was relatively short,
because of socket wear. While the sockets could have been replaced,
introduction of new tube types and socket configurations continued
steadily into the mid-1960's, and replacement rather than repair of a
worn unit was justified to support newer configurations. One must keep
in mind that the vast majority of tube testers of both the
emissions and transconductance type were sold and positioned in a
prominent place in a point-of-sale retail business. They were uncommon
in engineering and manufacturing operations.
What faults can a tube tester find? Obviously, it will cull out tubes
that are totally non-functional, such as those with open heaters. Tubes
that light and conduct, but indicate either "?" or the high end of the
"bad" range on a tube tester may function perfectly well in the actual
circuit. The real problem comes when a tube tester indicates "good" but
the tube won't operate properly in the application circuit. Hard faults
are fairly easy to find in-circuit. The only really valid test for tube
condition is in a test under actual operating conditions, and the device
under repair provides those conditions at the application socket. The
easiest and quickest test at the socket, beyond doing some simple meter
and scope checks, is to plug a known good tube into the socket and see
if that solves the problem. Even in a shop where sales policy required
100% testing of tubes in a tester, a significant percentage of tube
replacements were for faults found in in-circuit testing involving tubes
that tested "Good" on the tester.
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