The difference between a simple
emission checker (like a Heath TC-1 or 2) and a mutual
conductance* checker is very important. An emission checker tests for
shorts & cathode (or filament) emission only. The meter reading gives a good-bad
reading by virtually tying all the elements of a vacuum tube
together (except the cathode & heater) and testing the tube for
cathode emission by subjecting it to test as a diode (or
rectifier). An
emission checker will basically tell you whether a tube is
usable or not and give you an indication how strong is the
cathode emission.
A mutual conductance checker will apply voltage to each element
of the tube, supply bias and a signal to the control grid and subject
the tube to a test much like it may be used in an actual circuit,
measuring plate current and indicating such in micromhos. If you
want to pair up tubes in "matched pairs", this is done
by comparing the "Gm"** reading and finding a pair with equal (or
within 10%) reading. You cannot do this with an emission checker.
This is not to say that an emission tester is of
no value, even the best tube testers can mislead and test a
tube that may not function in a circuit as a good tube.
* Gm.....the symbol for mutual
conductance
** mutual conductance....the measure of
alternating plate current (measured in micromhos) in relation to
the control grid signal voltage (simple explanation, it's really
more complicated).
More info:
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Calibration verification test tube
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