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The text below is my feeble attempt at writing a white paper on this tester,
but nothing can compare to the Navy’s original specifications. Reading this one
cannot help but to be impressed with the tester that resulted from these
specifications, and also the designers that implemented it. Click here for the 273 page
pdf, and here for
the supplement. Below is just an oversimplified summary.
The Cardmatic tube tester is an ingenious design. Various
power supplies, a gm bridge circuit for measuring transconductance,
a decade resistor box, and an accurate metering circuit are all connected
together at the card reader so that anything can be connected to
anything. Replace the card reader contacts with relays and you have an
ideal platform for computer control.
The tester is capable of at least 17 different
test configurations, ranging from simple triode transconductance
tests to high voltage rectifier tests to voltage regulator tests.
We'll start with the shorts test. One of the
functions of the power supply is to supply a regulated positive and negative
150VDC. Among other things, these voltages are used for the shorts
tests. They are applied to the tube elements in the opposite polarity
from normal. This means no worry of cathode stripping, and any sort of
grid emission will cause a short lamp to flicker. If there is enough
leakage between any elements to allow enough current to flow to light a neon
lamp, then the tube is defective. The sensitivity of the shorts test is
adjustable and is part of the calibration procedure.
At the same time, the meter is placed in series
with the heater and cathode to measure heater/cathode leakage. The meter
can be programmed to reject tubes with as little as 10 uA
of leakage current or as much as 170 uA. It can
also be disabled, which is done in the case of directly heated filamentary
tubes.
For the quality test, the tester can be configured
a number of ways depending on the tube to be tested. Here are some
examples:
Transconductance Test:
· Full scale meter
reading can be programmed from 500 to 128,000 uMhos
in 100 uMho steps.
· The tube can be in
grid bias or self bias mode.
· Negative grid bias, if
used, can be set to -.1VDC to -99.9VDC in .1 volt steps.
· In self bias mode,
the cathode bias resistor can be anywhere from 0 ohms to 70K ohms in 10 ohm
steps.
· In self bias mode,
the cathode bias resistor can be bypassed by a 1000uf capacitor.
· A regulated signal
voltage of .222VAC is applied to the grid.
· The plate voltage is
regulated and can be from 10VDC to 260VDC in 10 volt steps.
Plate Current Test:
· Full scale meter
reading can be programmed from 100 uA to 500 mA in
100uA steps.
· The tube can be in
grid bias or self bias mode, or can be a diode.
· Negative grid bias, if
used, can be set to -.1VDC to -99.9VDC in .1 volt steps.
· The plate circuit
resistance can be set anywhere from 0 ohms to 70K ohms in 10 ohm steps.
· The plate voltage is
regulated and can be set from 10VDC to 260VDC in 10 volt steps, or an
unregulated 360VDC, or up to 1200V P-P AC.
Knee Test for Sweep Tubes
· High current test
where the tube is having to charge/discharge a large capacitance as its
load. Most testers lack this test.
These are just a few of the more common tests, and
we've already hit quite a few different combinations of test
configurations. Since most of the more obscure test configurations are
used only for tubes that are already programmed into the database we will not
go into details here.
During the gas test, the tester simply measures
grid current. If there are gas ions present in
the tube, they will be attracted to the negative grid and grid current will
flow, causing the meter to deflect. If it deflects above the green area
for gas, then more than 3 uA of grid current is
flowing and the tube should be rejected.
This tester also has:
· A filament power
supply that is programmable from .1 volts to 119.9 volts in .1 volt increments.
· An auxiliary B+
supply that is regulated and continually adjustable from 30 to 300 volts (used
mostly for voltage regulator tube tests).
· A bias off supply
that is used to keep unused tube sections biased off during tests.
· A test button that
allows us to test two identical sections within a tube with one test.
· Protection circuits that
automatically shut the tester off in case of power supply overload or grossly
off scale meter deflections.
All in all, one nice tester!
04-14-2016