Mutual Conductance vs. Emission Test




There many types of tube testers, what I will discuss here are the three common types made for the consumer electronics industry during the late 1920's - 1970's.

The difference between a simple emission checker (like a Heath TC-1, 2 & others) 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. Both types will do a good job testing for shorts and leakage.

  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 good even though it may not function well in some circuits.

* Gm.....the symbol for mutual conductance
**  mutual conductance....the measure of alternating plate current (measured in micromhos) in relation to the control grid AC signal voltage (simple explanation, it's really more complicated).

There is a third type of test is the "Dynamic"  or "Transconductance", Precision calls theirs; "Electronamic". This type test is somewhat similar to a Mutual Conductance tester but uses AC voltages for the test instead of DC and does not give a direct Gm reading in Micromhos. The test provides the "Merit" of the tube under test by giving a percentage reading of the expected value. IE, the read-out will show the percentage of a predetermined value for an average tube of the type under test. The Mutual conductance is best but the Percentage Value test is better than a straight emersion test. The Dynamic or Transconductance tester is far better than an emission tester and can be used to match tubes, whereas the emission tester cannot.

Summing up, if you want a tester for occasional use for testing the tubes in your own equipment, you may no want to invest in the more expensive mutual conductance testers. On the other hand, if you have lots of tubes and want to sell them on eBay, serious buyer are not interested in an "emission" test. The mutual conductance value (as read on a properly calibrated mutual conductance tester) supplied along with the tubes you are offering are a must. A Dynamic tester would be a good second choice for the budget minded. Of course, if you want the best test, and have a fat wallet, there's the; Amplitrex.

C.E. Clutter


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