In a nutshell, the name says it all.  Tube testers are devices designed to test vacuum tubes, in an effort to determine if they are "good" or "bad".  Between good and bad of course, are all points in-between, including several other tests that may evaluate the quality and viability of a given tube.  While the simple terms "good" and "bad" were sufficient in the days of yore for the TV or radio owner to use a drug store tube tester, these simple terms may, or may not, suffice for you today.

Tube testers can test for the following parameters, although not all testers do all these tests.  This is simply a listing of most tests that can be performed, and different testers do more or less than others.  These tests are shorts, leakage, gas, microphonics,  emission, transconductance, and life expectancy.  Some testers add a "noise test" whereby you can actually listen to the shorts test function and others.  There may be others as well.

Tube testers can be loosely grouped into three basic types.  Within each group, are many variations.  The basic types are the Emission tester, the Transconductance*  tester, and the Laboratory Analyzer.  A sub type of Lab Analyzer is the Curve Tracer, which will be discussed with the other analyzers.

What's in a name?  Tube testers are referred to as Emission, Dynamic, Dynamic Emission, Transconductance (Gm), Dynamic Transconductance, Mutual Conductance, Mutual Transconductance, Dynamic Mutual Conductance, and others. It is important to cut through the marketing hype of old, to determine which type of tester you are looking for, or at.  Some of these terms meant different things to different manufacturers.  It is likely that a lot of these terms evolved to get around patent and copyright issues for purely marketing purposes.  Any tester using the words Transconductance, Gm, or Mutual, are "probably"  transconductance testers.  Others may be, but in general, without including one of those terms, the tester is likely to be an emission tester.  The word 'Dynamic', to paraphrase a note in Alan Douglas' book, claimed to simulate a mutual conductance test.  Alan's testing of that type of tester showed little difference, if any, from a basic emission tester.  The term "dynamic', when used for a Gm tube tester, generally means some form of AC is used instead of DC as the bias voltage for various elements.

Before putting any tube through a test for emission or Gm, it is very important to first test them for shorts.  If this step is omitted, and a tube turns out to be shorted, it can damage or destroy your tube tester.  If you are considering a tube tester that does not have this feature (virtually all of them do) .... consider a different one.  A shorted tube can not only destroy a tester, it can also damage or destroy anything it is plugged into.  Shorted tubes need to be discarded without further ceremony.

The Emission tester is the most basic type of practical tube tester.  An emission tester attempts to check the cathode emission of the tube under test.  This is a static test, meaning that the tube is tested at one fixed operating point, and can not attempt to simulate the way a tube would be expected to work in an actual circuit. They did their testing by essentially connecting all the grids to the plate, and testing the tube as a diode.  There are variations on this scheme, but all emission testers share this basic format.  The meter, when inserted in series with the plate or cathode, would measure the plate current in mA.  Whether in the plate or cathode circuit, the reading would be the same, since Ohm's Law applies.  (the current in a series circuit is the same at all points in the circuit) more or less.

Emission testers are limited in the amount of information they can give you about a tube, but are generally sufficient for a GO/NO GO decision, and basic health indication, for tube's condition.  Keep firmly in mind the disclaimer at the very beginning of this paper.  

Quoted from the RCA TUBE MANUAL:  "...the emission test is subject to limitations because it tests the tube under static conditions and does not take into account the actual operation of the tube".  This is one of the reasons that a tube may test good, but will not work in all circuits.  (Thanks to Tone Lizard for the reminder)

By their very nature, the Gm testers provide a test of a tube under conditions much closer to the way a tube actually operates.  To do this, they are, of necessity, much more complex.  This makes them much more difficult to restore and calibrate, but once done, they provide a considerably better view of the health of any given tube. 

The Gm tester includes the basic shorts, leakage, and gas tests that the Emission testers provide as well, since a shorted tube can destroy or damage your tube tester.  See the end of the first section of this paper, what is a tube tester...discard shorted tubes without further testing..... this cannot be overstated.

Even though a Gm tester will provide a more accurate snapshot of a tube's overall health, it still needs to be kept in mind that a tube testing good, may not work in all circuits.  Likewise, a tube testing bad might be perfectly ok in some circuits, but fail in others.  The device in which the tube is being used is the final word on whether a tube is good or bad.

The final basic group of tube testers are those known as Laboratory testers.  While they certainly deserve honorable mention here, they will probably not be the ones selected by someone wanting to just test tubes.

They were used for designing tubes, evaluating prototypes, precision testing of tubes on an assembly line, or much more in depth testing of tubes for those that needed a high degree of precision.  Some were developed for the military, others for the FAA.  Others for factories, or by factories, or just for general analysis and development.

Basically, they provide real time, front panel access to each tube element, and allow the user to adjust each voltage, adjust or read each element's current, and basically allow the operator to vary all the operating parameters of a tube.  These could then be read on the multiple meters on the front panel usually without any, or at least without too much, switching.  For just three examples of this, see the first few photos on this website tube tester pages (add link). 

Most of these featured regulated power supplies and precision adjustments.  Where meters were not provided, for the most part provision was added, via binding posts, to use an external meter to monitor a parameter.

While some consider the more advanced Gm testers to be lab grade, I do not consider them in this category.

A splinter group of lab grade tester is the CURVE TRACER.  This provides, in real time, in graphical fashion, the operating curve of a tube at several different grid voltages.  They are the preeminent choice for MATCHING tubes, but are more expensive, complex, and require a good deal of technical knowledge to operate properly.

You've now got a basic understanding of WHAT the different testers are, and do.  And more importantly, some of what they do not do.  So, which one is the correct one for you?

lots of stuff to follow.........