How to Use a Tube Tester… and Tube Issues (in General)
by Bob Putnak, ©2010. All rights reserved. Last update: Jan 2014.
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 real expertise regarding the technicalities of its 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
♦ Use a calibrated or professionally serviced tube tester. I always find it interesting when people put faith in the ‘test scores’ or ‘test results’, or whether the tube does (or does not) have Shorts or Leakage, when they know that they are using an “as is” tube tester or tube tester of “unknown” accuracy. They seem to feel that just because the meter provides a reading then apparently “it works”. It is rare to acquire a tube tester that does not have a problem. In fact, I cannot remember the last time that I purchased a tube tester that did not have significant issues preventing it from working to the best of its ability. Apparently some people actually believe that a tube tester — which is 40 to 70 years old — magically works properly some 40 to 70 years after it was manufactured just because it gives a reading on the meter. I say “magically” because I am not aware of too many 40 to 70 year old devices that work properly without being serviced!
Tube testers vary widely in their capabilities and overall accuracy, but almost any model (except Quik-Check “drugstore” types) can be a useful service instrument if it is working to the best of its capability. But even the “best” tube tester of unknown condition cannot — by definition — be relied upon. If your equipment requires service or calibration, we service most makes and models.
♦ Do not test tubes with dirty or rusty pins.
Doing so will only wear out your test sockets faster, and the poor pin continuity will yield unreliable test results. Tube pins must be cleaned before testing. Lazy won’t get it done.
♦ Tubes with multiple sections, each section must be individually tested.
There is a reason why many tubes, such as 6SN7, have two listings on the setup chart. This is an example of a multi-section tube. EVERY section of a multi-section must be tested separately for Shorts, Leakage, and Emission/Gm. Each test is explained below.
♦ Power-on the tube tester and configure test settings before inserting the tube.
Failure to do so can risk tube damage from incorrectly high filament voltage and/or applying voltage where it does not belong.
♦ Adjust “Line control” both before AND after inserting tube.
If your tube tester has a “Line” adjustment to compensate for filament current draw, it should be adjusted both before and after inserting the tube. Failing to readjust the Line control after inserting the tube is a common mistake and will apply wrong voltages to the electrodes. That said, this initial LINE adjustment has nothing to do with transformer sag DURING the load test (whether Gm or emission). That is a SEPARATE issue, discussed below, and is a huge limitation of most tube tester, including most Hickok’s.
♦ Filament test is always the first test made.
This may be obvious (and overlooked) because you can see the filament glow in a glass envelope tube, but it is not obvious that the filament is good in a metal envelope tube. If your tester has a Line control adjustment, as discussed above, the need to readjust the Line demonstrates that the filament is good. For testers without a Line control or tubes with very low filament current draw, there will be a lever to move that will light the “Shorts” lamp to indicate filament continuity.
♦ Shorts Test(s) will be the second test made and must always be performed before any Emission or Gm test.
Since most people do not think much about the filament, this is the first real test you will perform, and it MUST be done BEFORE any Emission/Gm test, otherwise you could immediately burn-out the tube tester meter. Shorted tubes should be discarded immediately.
What is a Short? It is not necessarily “zero ohms”. There is a lot of overlap between “Short” and “Leakage”. In fact, in many tube testers, “Shorts” and “Leakage” are tested simultaneously. There is no universally accepted definition to distinguish “Short” from “Leakage”.  A “Short” is often defined as a resistance of 300,000 ohms or less between tube electrodes, whereas an objectionable resistance above 300,000 ohms between electrodes is defined as “Leakage”. In tube testers that use a lamp to detect shorts and leakage, the brighter the lamp glows, the “more shorted” the tube is, as opposed to “leakage”.
The Shorts Test is the most important test that you can make. Shorted tubes will NOT perform correctly in-circuit. Shorted tubes are often the cause of circuit defects. Shorted tubes CAN test OK for Emission/Gm depending on how they are shorted, which would give the novice user a false impression of the tube condition as cause serious damage.
Shorted tubes can easily ruin transformers and other circuit components. Ironically, many novice users only test for Emission/Gm and ignore the Shorts and Leakage tests, when in fact if they were only going to test for ONE thing, they should test for Shorts. Many tubes tested by amateurs on auction sites are often not properly tested for shorts or leakage.
♦ Leakage test will be the third test made (if Leakage test is separate from Shorts test in your tube tester).
This is another test that many novice users do not make or do not put much emphasis on, but grid leakage or heather-cathode leakage is the source of many tube defects that can cause any number of circuit problems, including hums and objectionable noise, and this defect will NOT show up in any Emission/Gm test. I feel that everyone serious about tubes and tube equipment should own one tester that has a sensitive leakage test. This is one area that Hickok really does not do a very good job at all, and leakage tube defects will be much better detected with a Sencore or B&K and a few other brands/models.
♦ The Shorts and Leakage test(s) were the most important tests you will make.
Yes, I did say that, and we still have not tested for emission/Gm and therefore still have not determined a “test scores” aka “readings”.
So why does everyone worry about “the test score”? I really feel that it is a societal conditioning from everyday life. You get “test scores” in school, “test scores” for your blood pressure, “test scores” when you use a voltmeter and ohmmeter. Therefore, people feel that a tube should have a simple easy-to-understand “test score.” That does not exist.
It is always a good idea to repeat the Shorts and Leakage tests a second time — after the Emission/Gm test. Sometimes shorts and leakage are a heat issue. Repeating the shorts and leakage tests immediately after the Emission/Gm test allows you to evaluate the tube at a higher internal temperature.
♦ For any tube, there is no one “TEST SCORE” that is “right”.
There is no single universal test score that anyone can give you that will tell you “how good” the tube is, nor how good the tube “will sound” in circuit. This statement should NOT be interpreted that the test score does not matter. Every tester must be functioning to the best of its capability. This fact is a given. The statement simply means that different tube tester models WILL give different test scores and different test results, and that does NOT mean one is wrong. There are many reasons for different test results:
- Emission test methods vary significantly among emission tube testers, therefore the numerical “test score” varies among testers.
- There is no such thing as one “standard” Gm value for an amp tube. Gm test scores vary based upon signal voltage, DC bias, plate load, and plate/screen voltage. Different testers (even among same manufacturer) can use different voltages, therefore results are different.
Since there is NO SUCH THING as one standard mutual conductance value for an amp tube, you should NOT expect your tester to read same (or even similar) to the nominal Gm value listed in a tube characteristic book.
To emphasize this important point, I will offer you this quote from “Latest Instruments for Servicing Radio-Television”, ©1953, Coyne Electrical and Radio School, pp 270-271:
“If mutual conductance readings are to match those given on tube manufacturers’ data, all of the applied voltages as well as the plate circuit load must be exactly as listed in the same ratings…. The fact that there is no one standard mutual conductance or transconductance is evident from Fig 2.”
- No tester will test for “all” tube characteristics, because such a tester would be impossible. Even the tube manufacturers did not have such a device! Therefore, different test methods can provide completely different test results because they are evaluating different tube characteristics.
- The tube will be used at higher voltages in your amp or other equipment. Its Gm at higher voltages may not correlate to the Gm tested using the voltages of your tube tester.
- While some people worship Gm scores, Gm is only *1* characteristic of a tube and is useful only to measure the gain characteristic of the tube under the EXACT voltages used in the tube tester configuration. This Gm result does not tell you anything else.
♦ Some tube types do NOT even have a “test score”!
This fact really separates a knowledgeable source from a poseur.
Example: All the time I see people selling 2050 tubes on eBay and they give “test scores” for their #2050 tubes. A 2050 tube is a thyratron, and a thyratron is basically an electronic SWITCH. A switch does NOT have a test score in any practical way. A switch is either ON or OFF. Giving a test score in this instance just reveals the seller’s lack of knowledge about his/her tube.
Routinely I see people with say that the “minimum score is 25″ or the “minimum score is 40″ when testing a thyratron with a Hickok, and then they tell you that “the tube tests 70″ or some other “high score”.
Well, that tells me a lot. It tells me that (1) the seller does not understand his product at all, (2) the seller cannot read, because the Hickok thyratron chart note reads “Tube Strikes at #” – it does not say “minimum score”, and (3) his tester is certainly NOT calibrated, because that chart NUMBER on the Hickok notation is where the tube should strike. It is NOT a “minimum test score”. “Strike” is where the tube will conduct (think of a switch that turns ON). In other words, at a certain bias point the tube will conduct and further dialing the bias less negative will not, in any way, change the meter reading. When the bias is made more negative from the point where it began to conduct, the tube will “cut-off” and will crease to conduct (ie, no reading on your meter). That’s it. Nothing else to say about it.
It is even more funny when the person says that his tester was calibrated. Obviously not correctly! There is a reason why the Hickok roll chart notation says “strikes at about 25″ (using Hickok 6000 as an example) — because that is where a good new #2050 tube SHOULD strike — at or near 25 in that example! If it strikes at 70, there is a problem somewhere!
The same discussion could be made using any brand of tube tester in this example. Obviously people give “test scores” for thyratron tubes using a variety of tube tester brands. There is NO test score regardless of what tube tester or test method that you are using.
There are many other examples in this “some tube types do NOT have a test score” category. Voltage Regulator tubes are another example. You really cannot even test a voltage regulator tube properly on most tube testers, even though they are listed on the chart of some tube testers. A VR tube is meant to drop a specific voltage, within a narrow voltage range, across its entire operating range. The tube either correctly drops this voltage (for example, 0A2 drops nominal 150v +/- 2v from 5ma to 30ma), or it doesn’t. Unless your tube tester actually reads this voltage drop and can be read across the operating range of the VR tube, your test is worthless.
Similarly, diodes and rectifiers do not have mutual conductance test scores because they do not have ANY mutual conductance. So if someone selling tubes provides a Gm “test score” for his or her rectifier or diode tube, proceed with caution. (A “Gm test score” in this rectifier/diode example should not be confused with a Good-Bad scale-based test score).
♦ No tube tester will tell you whether an oscillator tube will work properly in-circuit.
That is a fact. The only test for an oscillator tube is in the exact circuit that the tube will be used in. And just because that tube may not work in a particular radio, it may work good in a different radio. That is quite common with oscillator tubes, yet confusing to many users.
♦ Now that you have some understanding how LIMITED the usefulness of a “test score” is, you can proceed to make your Emission/Gm test.
Depending on your tester, you will either be testing for Emission or Gm (Mutual Conductance, or sometimes called Transconductance). Either test method does a good job at evaluating whether the tube is likely to work or not. This is really all that a tube tester was ever designed to do. It is a test instrument designed to determine whether a tube is likely to work properly in-circuit. This is all that the test instrument was designed to do — to provide useful analysis to a repairman. It was NOT designed to tell you which tubes will “sound better” or which tubes will fulfill some other esoteric desire you may have.
IMPORTANT Notes for performing the Emission and/or Gm Test:
- Do NOT perform the test until the tube has had sufficient time to warm up. Many people insert the tube and immediately push the test button and watch the needle move upscale. This is poor workflow. The tube should be at full operating temperature before engaging the test. If the meter needle does not immediately go upscale and stop, but instead continues to travel upscale, you have not waited sufficient time for the tube to reach operating temperature. Small tubes will reach operating temperature usually within 30 seconds. In general, the larger the tube, the longer it will take to reach operating temperature. (Filament-based rectifier tubes are an exception and heat quickly). It is not uncommon for power tubes, such as a 6336 or 6550 as a few popular examples, to take a couple minutes or longer to reach operating temperature in a tube tester.
- Do NOT perform the Emission or Gm test for ANY LONGER than necessary to get a stable reading. Any longer and you risk DAMAGING the tube with some tube testers. With emission tester, this damage is cathode depletion. With Hickok testers, the signal voltage is too large for many small tubes causing you to exceed the tube’s max plate dissipation ratings. In either case, the old-timers called this “testing the tube to death” or “death by tube tester”. This issue is not necessarily a defect in the tube tester (although it does reveal a serious limitation in the circuit design); the issue is improper usage by the operator.
- If your tester has a LIFE TEST function, perform the life test at this time in accordance with the instructions for your tester. This is often a switch that is engaged during the Emission or Gm test. In other testers, you reduce the filament/heater control one setting. Life Test is especially helpful in finding questionable rectifier and power output tubes.
As I have explained, the Emission/Gm test is the last test you will make, and often it will be the least useful — even though most people incorrectly put the most emphasis on it. In summary (and assuming that a test score is even RELEVANT to your tube type!), a higher test score does NOT mean that tube will have LESS NOISE or BETTER SOUND or anything else that most people ASSUME that the “test score” means. Really! A tube testing “100” is no more likely to “sound better” or “work better” than a tube testing “80”, or vice-versa. Technicians know this, but the buying public does not.
Interpreting the Tube Test Result
We already know to immediately discard shorted tubes and tubes with leakage. Ok, so how do you interpret the test results from your calibrated tube tester?
EMISSION TESTER interpretation:
1) A weak or questionable emission test result means that the cathode is supplying less electrons than a “bogey” tube of the same tube type, as determined by the manufacturer of your tube tester. A bogey tube is a tube that is considered to be best representative of an average working tube of that particular tube type. A manufacture selects a “bogey tube” after testing many many tubes of the same type.
In general, the middle of the “Green-Good” scale reflects the test result from the bogey tube, and the large Green range of the meter reflects the fact that tubes with Emission anywhere within that range were found to operate good in most circuits. In my experience, if a tube tests low emission, I have always found low-emission to be a reliable indicator of a tube that will not work satisfactory in most circuits, and therefore a reliable indicator of a bad tube.
2) A “good” emission test result is certainly a very good sign, but not foolproof. You can be reasonably sure that the tube is good, but there will be the occasional exception. There are rare instances where the tube may still test good emission but have a different defect. This situation is uncommon. Weak Emission (or weak life test) + Shorts + Leakage will find the overwhelming majority of tube defects.
Gm Test interpretation:
Note: The meter in Hickok testers can be read in two completely different ways. You can setup the machine to read GOOD/BAD, or you can setup the machine to read numerical Gm scores (which will have no relationship whatsoever with the Red-Green colors). These different setup options are based upon different settings for the “English” or “Shunt” control when reading the Good/Bad scale, as opposed to when you want to read the NUMERICAL Gm score.
This is a VERY common mistake that many users make. They setup the tube using the number on the chart for the “English” or “Shunt” control, and they think they can also read the numerical test score. That is wrong, so re-read your manual.
Furthermore, some Hickok testers give numerical Gm guidelines based upon a NEW bogey tube, and some Hickok testers give MINIMUM Gm scores. Read your manual or setup chart to determine which type of Gm scores that your tester provides as its baseline reference, assuming you have a need for the exact Gm score. The exact Gm score is seldom useful to any technician.
And to further confuse the topic, Gm test scores are NOT consistent among different Hickok models. Many models use different signal voltages which therefore provide a totally different Gm score. So any Hickok Gm test score is only relevant to that particular model of tube tester.
After explaining those mistakes, misconceptions, and differences — how might you interpret the Gm results?
1) A weak or questionable Gm test result means that the tube’s gain characteristic (which also accounts for emission) is below that of a bogey tube.
In contrast to my opinion above regarding “weak emission”, a tube that tests below-average Gm will not always be as “bad” as your tube tester is telling you, and it can often work acceptable. For example, a below-average-Gm #45 triode will often work fine as an output tube in an antique radio. Therefore, before discarding an expensive or rare tube simply because it tests below-average for Gm, you may want to try it in-circuit.
Furthermore, most Hickok’s have a considerable amount of transformer sag DURING the Gm test of power output triodes/pentodes, in which this sag drops ALL of the applied voltages below reference. These popular “common” Hickok’s (just about any Hickok that is affordable, including models 530, 532, 533, 533A, 534, 536, 580, 600, 600A, 605, 605A, 6000, 6000A, 6005, 750, 752, 752A, 799, 800, 800A) have NO WAY TO RE-COMPENSATE for this transformer sag that occurs during the Gm test. (Do NOT confuse this transformer sag DURING the Gm test vs the generic Line compensation that is done BEFORE engaging the Gm test.) This sag can be VERY material. For example, a 2A3 filament voltage (2.5 nominal) can easily sag to below 2.0v during the Gm test; I have measured it as low as 1.8 during the load test on some machines. That is a 20%+ reduction and VERY material to the output capability of the 2A3, and quite honestly, is a garbage test. Similarly, I have measured the filament voltage of a 6L6 to sag to as low as 5.7 vac during its load test on many of those models, again… a very material 10% reduction. Your Gm score WILL be materially lower in those examples vs having that same tube tested on a more professional tester, such as a Western Electric KS-15560, Hickok 539 series (A,B,C), or even a TV-7 (which, while you cannot re-compensate the sag on a TV-7, the transformer has much lower overall sag so the issue is less significant). The issue is particularly bad for the 2.5 filament power triodes, but all power output tubes are affected. Of course, not just the filament voltage sags. ALL of the applied voltages sag. Premium “top-of-the-line” testers such as the WE KS-15560 and 539 series, the Line can be re-compensated during Gm testing, and the bias can be read exactly AND re-compensated as needed. This brings all of the applied voltages back into spec DURING the Gm test. This is a SIGNIFICANT difference in test quality vs all other testers.
The same “sag DURING test” issue is present in almost all emission testers, too, with one notable exception being that the Simpson 1000 has provision to re-compensate the Line during its load test (making the Simpson 1000 a superior instrument vs the Eico 666, even though the machines are otherwise the same circuit design). Thus, the same thoughts apply here to emission testers, but the issue is less important because you are dealing with a “basic” test to begin with. So what’s one more issue on top of the already mountain of compromises these test instruments have….
2) Conversely, I have found that a “good” Gm test result is a reliable indicator that the tube will work good in most circuits (assuming, of course, that you have already tested for, and rejected, tubes with Leakage — using a tube tester with sensitive leakage detection.) The same caveats apply for oscillator tubes, though.
Repeat Shorts and Leakage tests again
It is always a good idea to repeat the Shorts and Leakage tests after the Emission/Gm test. Sometimes shorts and leakage are a heat issue. Repeating the shorts and leakage tests after the Emission/Gm test allows you to evaluate the tube at a higher internal temperature.
Once you have performed this entire process for all tube sections, your testing is complete.
Know YOUR tube tester
This is a good axiom for all test equipment that you own, regardless of whether it is a tube tester, oscilloscope, audio generator, etc. It is far more important to develop long-term experience and familiarity with YOUR tube tester — knowing its idiosyncrasies, strengths, and limitations — than worrying about which model you should buy or use. This view is shared by others. A technician with years of experience using a basic tube tester will be far more effective at interpreting its results than a novice using a $1,000 tube tester.
Experience matters. In the real world, a technician wants to know whether a tube will work in the circuit. He or she does NOT care what “number test score” is obtained — except to the extent that his many years of experience with HIS tube tester + having tested many of THAT TUBE TYPE over the years + knowing that the tube is being used in equipment that he has serviced many times before …all of that experience tells the technician that a tube that tests above a certain number on HIS tester has consistently worked well for him REGARDLESS of what the “score” was (either low or high). There is NO substitute for this experience. It is no different than the obvious fact that an experienced technician using a $10 multimeter will be a more effective technician than a novice user, even if the novice user is using the most expensive meter that money can buy.
Therefore, use a properly serviced and calibrated tube tester, and get to know YOUR tester. If you have not already purchased a tester, buy one that has a panel layout that appeals to you because “good usability” is very important and is often best determined by your first impressions. If your first impression is that something about the layout is not “logical” to you, then do not expect that you will grow to like it just because someone else likes that model.
Noise / Microphonic tube issues
Tube testers do not test for these problems in the manner in which you think, except to the extent that a tube tester with sensitive leakage detection will find far more “noisy” tubes than a tube tester with only “fair” leakage sensitivity, to the extent that the noise is due to this problem. Loose elements or other defects will not be found.
What about the “noise test” in a tube tester? It is true that some testers do have noise test jacks that connect to either headphones or to the antenna/ground terminals of an AM radio. There is nothing wrong with performing that noise test. However, this test is not going to tell you whether the tube will be silent in your SET (single ended triode) amp or in anything else. Ultimately, the “most satisfactory way of testing a valve which is suspected of being microphonic is in an actual chassis or amplifier similar to one in which it is intended to be used.” (RCA Radiotron Designer’s Handbook, Third Edition, p.245). Since amps have a wide variety of circuit designs and tube placement, this test is most effective in YOUR AMP or one that is almost identical. Hence, most people advertising tubes as tested for Noise or Microphonics is probably NOT testing them in any meaningful way for you, and may be outright puffery that the “noise test” was ever performed at all.
True “noise testing” (other than using the tube in the exact equipment you plan to use it…) requires a highly sensitive mechanical-based or vibration-based test apparatus, and such devices were ONLY created and used by professional engineering and testing labs run by very large entities back in the old days– such as the US military, Westinghouse labs, etc.
Furthermore, “Noise” and “Microphonics” are often buzzwords for other issues. Certainly there is a small percentage of tubes that have a problem, but far less than most people think. In properly designed circuits, (which has NOTHING to do with how much you paid for your amp…), objectionable “tube noise” is seldom found. In other words, it is a rare tube that will be noisy when used in most circuits.
Thus, if you find yourself rejecting more than one or two tubes because of “noise”, then you probably have equipment with aggressive circuit design or other design issues (such as transformer placement, shielding, etc). For example, some amps push a tube to its maximum design ratings. That is asinine. No electronic component is truly expected to operate successful at its maximum design rating. In fact, the opposite is true. “De-rating” is a standard practice in electronics. For example, if a circuit calls for a half-watt resistor, you install a one-watt (or bigger) resistor. So, it is true that aggressively designed amps will certainly cause many normal tubes to exhibit “noise.” Blaming this on the tube is incorrect. The blame should be placed on questionable circuit design, and just because you paid “top dollar” for your expensive amp or preamp does not mean that you bought quality. It only means that you paid a lot. Accordingly, if you own equipment that is susceptible to “noisy tubes”, then place the blame where it belongs — on your equipment, NOT on the tubes. Expect that your cost-of-ownership will be much higher because you will buy many tubes that you find noisy but that most everyone else would not have any problems with them in their equipment. You have to pay to play.
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Hopefully this article will allow you have a better tube-testing workflow and provide a better understanding of how to interpret the tube tester results. The information is not intended to be all-encompassing because that would be impossible to do a single article. Entire books have been written on the subject. I feel that I have discussed what most users need to know. I occasionally update this article as I think of something useful to add to the discussion.
Bob Putnak, ©2010. All rights reserved.
1 Shorts vs Leakage demarcation varies from model-to-model. Some examples (and page number from service manual as reference, where available):
Sencore TC162: 300,000 ohms or less, (page 3)
Sencore TC154: 300,000 ohms or less, (page 3)
Sencore TC28: 200,000 ohms or less, (page 4)
Sencore TC136: 180,000 ohms or less, (page 6)
Sencore TC142: 180,000 ohms or less, (page 6)
Sencore MU140 and MU150: 180,000 ohms or less, (page 14)
Jackson 648: 250,000 ohms or less
B&K 700/707: 1,000,000 ohms or less, (page 6 of B&K 707 manual)
Eico 667: 100,000 ohms or less, (page 6).
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2 “If the operator realizes the shortcomings of his instrument, he can allow for them and thereby get good service from the tester. A knowledge of the shortcomings of the various types of tube testers and a perusal of the instructional manual for your tester should help you efficiently use your tester and get a good return for the money you have invested in it.” Fine Points on Tube Testing, William E. Burke, Test Equipment Annual 1958, page 70. [Back to post]