How to Use a Tube Tester… and Tube Issues (in General)
by Bob Putnak, ©2010. All rights reserved.
Your tube tester is often the first test instrument that you reach for when diagnosing problems in tube gear. All tube testers are a compromise in functionality, compromise in accuracy, and none are perfect. No models test for all tube characteristics. Not even the tube manufacturers themselves had test equipment that could detect all bad tubes, or verify all good tubes, or test for all tube characteristics. A 100% accurate tube tester is not even possible because a tube can work excellent in one circuit and not work at all in another circuit.
Tube testers were designed as a tool for repairmen to detect “bad” tubes …tubes that could be causing a problem in the item that the technician was repairing. Tube testers do a very good job in this role. However, many people do not use a tube tester properly, do not interpret the results correctly, or have unreasonable expectations of them. Other people have developed poor workflow habits that need to be improved. While each tube tester has different capabilities and therefore different operating instructions, here are my thoughts regarding tubes, testing, and all tube testers in general.
[new update Nov 2013:]
I found an interesting article about the value of a tube tester to a service technician in the Feb-March 1957 issue of National Radio-TV News. I scanned the article, cleaned it up with photoshop, and make it available as a PDF [HERE].
In the article, the author Walter Swontek argues that a primary value in having a tube tester was as a tube-selling aid. It was hard to sell a tube to a cautious customer just based upon the word of the repairman, so the tube tester was “proof” that the tube was bad.
Swontek also correctly observes that a tube tester should be looked upon as a fact-indicating device — not as a judge. The results must be interpreted and judged, no different than you would with a voltmeter or other test equipment. This isn’t what a novice wants to hear, but this is reality.
“At the first contact with any sort of testing device, it is only human to expect a yes-or-no answer. In other words, the device is automatically expected to say ‘yes, the item is good’ or ‘no, the item is bad’. Actually, in real life, very few things are definitely good or clearly-and-undeniably bad.”
Swontek than nails it when he observes:
“What usually happens is that the novice technician becomes acquainted with a tube tester and learns how to manipulate the knobs. He puts the tube tester on a pedestal as a tin god or supreme judge of all tubes. His tube worries are over! All goes well for a while, and the novice is a rabid tester of tubes. His faith is implicit. He supports the tester ardently in discussions with more experienced technicians. The, one fatal day, the tube tester misses a bad tube. The bad tube is reinserted in the set and troubleshooting begins. All bad parts are replaced; everything is tested. The stubborn symptoms refuse to disappear. The technician becomes a nervous wreck. Finally, the help of a more experienced friend is sought. The friend swaps tubes and presto, the trouble is cleared. “But I checked that tube…,” howls the novice. His friend just laughs with that horrible, superior air. The novice’s faith in testers is completely shattered.”
Bulls-eye. Swontek could hardly have known how prescient his words would be. Everyday on chat forums, you find fanboys of this-or-that-model tube tester, discussing their tin god as the arbiter of tube testing excellence, but without themselves having any understanding of the technicalities of the test circuit, the strengths or weaknesses of their model (and the weaknesses are many…), nor any real knowledge of how tubes work. It is rare indeed for one to say, simply and concisely, that their tester does a good job at FINDING BAD TUBES.
Word up, folks — a tube tester is a diagnostic tool designed to FIND BAD TUBES — not “rate” good tubes.
Finally, Swontek also agrees that the Shorts test is of far more value than the emission or transconductance test.
Want another opinion about tube testing? Here is the article entitled “Fine Points on Tube Testing” from Test Equipment Annual 1958. A PDF of the article can be downloaded [ HERE ].
Table of Contents (summary of what is discussed below):
- Use a calibrated or professionally serviced tube tester.
- Do not test tubes with dirty or rusty pins.
- Tubes with multiple sections, each section must be individually tested.
- Configure test settings before inserting the tube.
- Adjust “Line control” both before AND after inserting tube.
- Filament test is always the first test made.
- Shorts Test(s) will be the second test made and must always be performed before any Emission or Gm test.
- Leakage test will be the third test made (if Leakage test is separate from Shorts test in your tube tester).
- The Shorts and Leakage test(s) are the most important tests you will make.
- For any tube, there is no one “TEST SCORE” that is “right”.
- Some tube types do NOT even have a “test score”.
- No tube tester will tell you whether an oscillator tube will work properly in-circuit.
- Now that you have some understanding how LIMITED the usefulness of a “test score” is, you can proceed to make your Emission/Gm test.
- Do NOT perform the Emission or Gm test until the tube has had sufficient time to warm up.
- Do NOT perform the Emission or Gm test for ANY LONGER TIME than necessary to get a reading.
- Interpreting Good/Weak emission and Gm test results.
- Repeat Shorts and Leakage tests again
- Know YOUR tube tester
- Noise / Microphonic tube issues
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