I get a lot of questions from people looking at my website. Most questions are about the posted projects or requests for recommendations for test equipment. However a topic which comes up regularly is about vacuum tube testers. The questions range from simple a “Should I buy one?” to complex requests for information on types, operation, “accuracy” and the ever present question “Are they worth it?”. Unfortunately, the topic is actually much more complex that most people realize.

Before we delve into the topic of tube testers too deeply there is one specific situation I would like to address. This is the case of someone buying surplus or bulk vacuum tubes and reselling them individually. This person is not a builder or experimenter, per se, but is instead running a business. In this situation you need a good quality transconductance type tube tester, in good repair, which is fully calibrated. And every tube you sell should go through that tester. In this situation you need to both know exactly what you are selling, have the means to represent tube condition to your customers, and have the ability to retest return items. This builds credibility with your customers and allows you to run your business in a forthright manner.

However, for those of us not involved in such a business, Things get a little murky. Let’s start with a little personal background.

My first experience with vacuum tube testers was about 45 years ago at a local drug store in the town where I grew up. It was one of those big floor standing units with backlit signage above it and a big storage cabinet below where all the replacement tubes were kept. The knobs, switches, and all those different sockets fascinated me. Soon I was looking for old tubes everywhere I went just so I could plug them into that machine and run the tests.

By the time I was about 14 years old, I had become the “go to” person in my family for fixing appliances and such. If something quit working, it was my job, to open it up, take it apart, find the problem, and fix it. Looking back now, I think I got that job not only because I was a fairly capable “Mr. Fixit” but because I was always eager to see how things worked. I couldn’t wait to explore their construction and operation and learn the secrets of each machine. And I loved to troubleshoot the television and the kitchen radio (an old American 5) most of all.

So when the TV started to misbehave I would sit down, look at all the symptoms, and try to identify which portion of the set was having problems. Then I’d remove the back and consult the vacuum tube layout diagram pasted inside the back cover. Usually this resulted me pulling a couple of suspect tubes and riding my bike across town to the tube tester. And this is where things get murky.

Sometimes the tester would clearly call out a tube as bad; the red needle moving to the “Replace” side of the scale. I’d replace that tube and all would be well. But sometimes it would tell me that more than one tube was bad. Even at my young age I suspected that such a result was highly unlikely. Usually I’d replace one of the tubes (either the “more bad” one or simply one at random). Sometimes this worked and sometime it didn’t. As time went on, my trust in the magical tube tester began to wane.

Then it happened that one of our neighbors got a new television and I was offered their old set on which to experiment. This set worked quite well; both good picture and sound. So the first thing I did (probably because I was young and foolish) was take every tube out of that set, carefully organize them in a box, and test every one of them in the tube tester. The tester told me that at least five of those tubes we “bad” and needed replacement. How could this be? So I returned to my basement, put all the tubes back in their sockets, and turned on the television. It worked fine. Obviously the folks designing tube testers and selling tubes had a different definition of what was “good” or “bad” than I did. This was a valuable lesson to learn.

In a few years, sometime when I was in high school, the tube tester went away. I still dabbled with tube circuits but I managed without a tube tester. It always bothered me that it seemed that tubes could be clearly marked as “Bad” by the tester but work fine in complex circuits. Then came Engineer school and my introduction to solid state circuitry. Soon all things tube related faded from my mind.

Now we fast forward to today. I never really lost my skepticism concerning tube testers, in spite of my buying an old emission style tester around 2010. I used that old tester for a short while, but I was never happy with the results. It always seemed that there was only a weak correlation between what it was telling me and how the tubes performed in circuits. In the last decade I’ve examined the schematics for dozens of different tube testers and tried out a few others attempting to divine some answers. But it’s only left me with more questions.

As some may know, most of the tube testers we’re likely to encounter today work on a few different principles. The first is an emission tester. This is what was the tube tester of my youth. This type of tester simply measures the emission current from the cathode. The thought is that the most common failure mechanism in most vacuum tubes is failure in the cathode coatings with extended use. Low emissions would mean a bad tube.

The second type is a transconductance tester. This type of tester supplies an alternating signal to the grid and measures changes in plate current. In theory, this should be a much better test. Unfortunately in practicality, it’s not as much better than the emission tester. This is because the transconductance measured by the tester is usually a large signal transconductance at only one bias point. This value may or may not correlate to the small signal transconductance in any particular circuit.

The third (and most expensive) type is the curve tracer. This is usually a laboratory piece of equipment that provides a set of plate curves for the tube under test. One might think that this is a great test but in reality, it’s probably the worst of all. This is because even though it provides a full set of plate characteristics, they are only for one tube. Since published plate curves are for a statistically valid sampling of a type, and there is no documentation of the variance between tubes, there is no way to compare the single set of characteristics to the published data.

In pondering the idiosyncrasies of tube testers I stumbled upon a short writeup in the RCA “Receiving Tube Manual”, RC-30, titled “Tube-Tester Limitations”. Instead of telling you what it said, I’ll simply include it in its entirety.

Tube-Tester Limitations

A tube tester can only indicate the difference between a tube characteristic and those which are standard for that type. Because the operating conditions imposed upon a tube may vary within wide limits, it is impossible for a tube tester to evaluate tubes for all applications.

Commercially available tube checkers vary widely in purpose, performance, and significance of results. They range from relatively inexpensive portable units to costly laboratory-quality instruments. Design trade-offs are made by tube checker manufacturers to provide a product simple to operate, capable of testing a wide variety of tube types, and in some cases, low in cost. Accuracy of readings, completeness of testing, and even proper testing conditions for certain tube types are sometimes sacrificed in these trade-offs. Recognition of the individual tester limitations are absolutely necessary before valid judgments on tube quality before valid judgments on tube quality can be made from test results.

Tube checkers generally make two types of evaluations: tests for inter-element shorts (leakage) and an electrical test of quality that is either an ac cathode-emission test or a more complex large-signal transconductance test.

The shorts or leakage tests are often more sensitive than those of the tube manufacturer and also, in some cases more stringent than circuit application requirements. Leakage sensitivity of 100 megohms between elements is available in some tube checkers. Some can be adjusted by the user to even higher sensitivities. Many tube checkers tie several elements together to test many parallel paths in a single test position. As a result, multiple paths having individual inter-element leakage resistances which are acceptable result in parallel combinations which cause the tube to read as defective.

Quality-test interpretations must be tempered by knowledge of the character of the quality test. Large-signal transconductance (gm) often does not correlate with small-signal transconductance, or the control limits for applications that require this characteristic. Cathode emission, as read on many tube checkers, is a function of both the emitting capability of the cathode and the mechanical spacing of the tube’s internal parts. While high cathode emitting capability is generally desirable for all tubes, a high emission reading obtained by close mechanical spacing of parts can result in a false indication of good quality. In addition, high or low indications in a tube checker are often caused by compromise test conditions rather than the quality of the tube being tested.

The set-up instruction and charts furnished by the tester manufacturer establish the conditions and limits which the tester manufacturer considers adequate for the tube types evaluated. These conditions and limits are usually established independently of the tube manufacturer and without consideration of application requirements.

The tube tester cannot be looked upon as a final authority in determining whether or not a tube is satisfactory. An actual operating test in the application will give the best possible indication of a tube’s worth.

In light of all of this, I generally advise against the purchase of a dedicated tube tester for most people working with vacuum tubes. Most project builders utilize a restricted variety of tubes which are easily checked in a simple amplifier configuration. Looking at the manner in which a 12AU7 performs in a 4S preamp (or other simple circuit) will give the builder a much better idea of how the tube performs than any tube tester regardless of type. Bulk checking tubes with which you may be unfamiliar may be served with a tester, but it really doesn’t impart much useful information about the tubes’ specific performance potential.

In general, I fully agree with the final sentence from the RCA manual above: “An actual operating test in the application will give the best possible indication of a tube’s worth.” So when it comes to tube testers I recommend saving your money for other things. In general, you can buy a lot of tubes for the price most sellers get for a simple tube tester.