We have done so much, for so long, with so little,
we are now qualified to do anything with nothing
Before you read the rest.... When you are here, you probably are interested in tube testers in some kind of a way. Same as me! I remember I threw away a shoe box full of used ECC81, many Mullard and Philips, using a defective tester (in 1988) and they tested all in the '?' range. I didn't have much knowledge about tube testers. I thought an old tube tester is like an old multi meter. Not very accurate, but when it works it is probably ok. Later I learned, the old tester was indicating too low value for all tubes, and perhaps these tubes were all fine. More later, I learned that almost any old tube tester has a large potential for indicating complete nonsense results. It is amazing how completely wrong they can read. You may not notice, and use it 20 years like that, and feel great about it. That is because you have no reference.
With an old multi meter, the situation is easier. You can measure the mains, or test a battery with it, and you have a fair indication if it is good. But with a tube tester, what will most people do? They stick new tubes in it, and regard this a good test of the tester. Also myself I learned the hard way, that such test results may be very inaccurate, because new tubes, usually test "good" in any tester. Just when it comes to rejecting tubes (and that's what you have it for!) the results are unreliable still, as long as you have no reference tubes, with known values.
When it comes to buying tube tester on Ebay, also for me, there is often no alternative for this. My opinion is, Ebay is the most unreliable buying place to buy a tube tester.
Over the years, this page has grown much larger as expected. Please note this: This website is a commercial one, meaning I live from it. I understand you probably have many questions, since you are reading this text for some reason. Still I need to take care of orders first, so I can not answer emails asking for free technical support. Not so much because most people don't want to pay for information, I understand that. It is just I don't HAVE the time to take care of this for you. So for any questions about a broken tester you bought on Ebay, or somewhere else, please discuss it with the seller, or post it on this tube tester forum. Sure initial response is slow there, but you may get good advice after some time. In any case your question doesn't get lost there, and is helpful to all of us.
Have fun reading!
READ THIS TEXT FROM 1958:
The point in question now is whether the emission or transconductance test is the best.
This is determined in part by the circuit in which the tube is to be used, in case of an RF or IF amplifier tubes, the stage gain is proportional to the transconductance; for this the Gm test is the most important.
In case if an output or power rectifier tube, it must be capable of supplying large amounts of current; here the emission test is the most important.
From: Fine Points of Tube Testing. by William F. Burke, Consulting Editor, which appeared in the 1958 Test Equipment Manual.
TUBE TESTER FORUMFor questions of how to repair a broken tube tester, we advise to use the Tube Testers Forum
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I receive this question many times be email. People have understood from this article, Ebay is not a good place to buy a tube tester, but they want buy a tester anyway. So the idea is often to buy a new made tester, and I am asked for advice. The question is then often like this: I have decided for an E-tracer or an Amplitrex, is it a good tester? To answer this, you need to define your 'must' and what is your 'want'.
Roughly tube testers go into three categories:
Parametric testers, Curve tracers, and Quality Testers, and the better ones are a combination of those. The Sofia tester, which is to my opinion the best tester ever made, can do all three.
It is very important top set your 'must' and 'want' accordingly. Are you looking for the details and accuracy of a parametric tester? Do you need to replace bad quality tubes only? Or prevent buying such? Or are you designing an amplifier, and you need tube charts? This is more or less the direction your choice has to follow. Probably you want the 'best', but keep in mind then, the best for you is quality tester, if your interest is tube quality. Same for the other categories. A Funke W19 will find a leaky between grid2 and grid3 right away. Whereas a pure curve tracer is not made for that. On the other hand, even the best Quality tester, like the Funke W19, is not helpful for amplifier design. And no, a parametric tester or a Curve tracer is no replacement for a Quality Tester or vice versa. So this is the most important message for beginners: Look again at those three categories above here, and make your choice from there.
The big 'must' and 'want' category. Not always the most 'accurate' test data is better. It is so interesting to have a curve tracer, but when you can read 'weak' or 'strong', from the tube curves, you can do more than I can. The thing is, when tube curves look optically bad, it is far too late. Much before that, some better ways to measure would have helped you more. Tube curves are for constructing the working point, during amplifier design. Well and also for people who like technical stuff.
These are for people who know a lot about tubes, because parameters by itself only make sense in relation to each other, and not everybody truly understands that. So people often end up by declaring the average working point a 'must'. Knowing this is unreasonable, they allow 10% and call that reasonable. Some others say 20% or 30%, or they say 30% yes, but that is a used tube. This may perhaps be so. Or perhaps not. Just look at this silly situation. They have an ultra precise precise parametric tester, and then need to estimate if the results are good or bad, and for that pick limits just randomly. They would have been far better off, with a Quality Tester, which however makes no mistakes in separating the good from the bad tubes.
Parametric testers are mainly used for tube matching, BUT that will only work when you know exactly how this is done yourself. The testers itself will not touch the subject. Some exceptions exist, lie the good old Sofia. Do not think you will get meaningful results quickly, from learning by doing. You will get results yes, but are they correct? There will be make mistakes in the beginning, such as being an over-critical matching professor, and not understand what burn in does to the tubes and to the matching. I realize people ignore this text probably, but it's the truth.
And why you can not replace such by a curve tracer
In simple words, a curve trace will tell you the curves, and a quality tester will tell you the quality. Judging deriving quality from the curves, is only possible when the tube is excellent, or very bad. Anyone can do that. But be more precise for tubes in between, has always been the domain of the quality testers. The difficulty is when the curves are not average. Then, what? Good quality curves don't have to be average. In fact only 10% of new tubes have such average (bogey) values. Specially with used tubes, a curve tracer is the #1 source of mistakes. It has always been like this, that for quality testing, you need to use the tube in auto bias, and warm up the anode until working temperature. There never was any other way, apart from using the tube in diode mode, which is also very useful to find out quality. So any testing with fixed grid voltage is fun to do, and suggest precision if digitally presented, but conclusions about quality are in fact with elementary errors, and it becomes totally unserious. So when a normally good quality tester says the tubes are good, and a curve tracer says the tubes are bad, that is because you can't use it for that. All curve tracers I know of, use fixed grid voltage. I can show you many places in old literature, saying fixed grid voltage testing is essentially a wrong method, and I do not think this has changed recently.
A good word about quality testers. Some people think a quality tester is for amateurs only. Well, I will use the word amateur later in this text :)
Specially when people understand how tube curves work, if armed with a curve tracer, they become bad listeners. They say, they don't need a stupid old quality tester any more. (I think they never owned a good one, or they would not say that). The opposite is true! and the following text is about this subject.
Myself I had the same opinion at first. I was focussing on fixed grid voltage testing, until I found out, my Funke W19 and Hickok testers sometimes qualified a tube as 'strong', which by my fixed grid voltage method were supposed to be 'weak'. I didn't understand it. I was using my own self made limits, which I thought I were so well done. After a lot of comparing, my final conclusion was, the Funke W19 and Hickok testers (I have a few) are the ones that do it right, and myself I was doing it wrong. If such fine machines from the past, made by real experts, say something else than I 'thought' myself, then it is pretty arrogant to say these machines are wrong. Well, every day new dreamers will wake up, but myself I don't dream about this any more. Another approach is the following:
Let me ask you: Do you think a digital tester will give a more accurate Gm reading than a Hickok (quality) tester. You will say, yes of course, the digital device was MADE for this. (I agree with this!).
So here comes another question: What do you think makes a tube fail in equipment in the first place? If a test result says low emission, or low plate current? (at fixed grid voltage). Now if you say low emission and low plate current is the same problem, you make a fundamental mistake. What is right (with this mistake), of course a bad tube will have low emission and low plate current (at fixed grid voltage) at the same time. What is wrong (with this mistake), is that a new tube may show significantly lower than average plate current (at fixed grid voltage), but of course will test very strong for emission. This is very difficult to understand, and you people make it harder for themself, by BEGIN thinking it should be always the same.
The fact, that it is not the same, you can it already see here: You can change Plate current with the grid voltage, but emission you can not change. Please think of this carefully. (Well you can however change emission by reducing the heater voltage). Why this is so different, is evident. If a worn out tube fails to work in a circuit, you can never force it to work by changing the bias. That is because emission is low. You can however force it to work with 25% higher heater voltage. Here you see again, plate current and emission are something totally different. In simple words: Plate current results from emission, and not the other way around. Of course that is clear to everybody, but it should be just as clear then, it is something totally different indeed. So in case you find find out what emission the tube has, the whole shebang with curves or parameters you don't need, result is still more reliable. Provided of course, you use a real quality tester, and none of the many fake products that were made for decades.
So a tube with good emission, is good tube by definition, when there are no shorts or leakage. It is as simple as that. Of course that doesn't give you any parameters, but that was not the question when you test for emission.
Sometimes customers with a curve tracer, send back NOS tubes, because they say they are BAD. The strange thing is, the curve tracer didn't say so at all. Because there is no way for that. So when I ask the buyer, what test limits he uses, and from what source he has that information, the answer is: 'ehhhh...well... that was is my own method, but I used it with good result always'
Then I say: No, no, not always, because I test them on a Hickok or a Funke W19, and they are GOOD. So indeed, we have the situation, people buy a curve tracer and think they know it better than a two generations of Hickok engineers who made those superb testers for 50 years.
hen I ask curve tracer users in detail how they do that magic, they simply expect all tubes to be average +/-10..15%, (and here it comes...) tested with fixed grid voltage. But that is not how it works at all. This was never written so in any data sheet or test manual. That is a clear case of monkey see, monkey do. You can show me a tube manufacturer's information which declares this a correct way, and I will revise this text. When you search had enough, you will find even RCA and Telefunken saying testing with fixed grid voltage is not allowed to do. Please don't tell me they were all wrong.
So where are we with quality testers?
The aim of a quality tester, is only to find out if the tube is good or bad, and nothing else. Some, like the Funke, tell you exactly how the tube was tested, and what the good-abd limits are in terms of mA. Some others are so small size, that alone by that you can see they're useless, and there is no way to verify the test. So options to sell a piece of junk were many. When done right however, it will separate the very nicely separate the good from the bad. And even Funke will do so PRECISELY and add a number to it. Just to give you an example: Funke W19 tests tubes 12AX7 not at 1.2mA because that is the useual point in amplifiers. They are not interested in that. They test the tube at 3.2mA, and at far too low working voltage. It is obvious that worn out tubes can have problems with that. Then, if such a tube fails the test, Funke is still not interested if it works fine at 1.2mA. They call it a reject. And indeed, remaining lifetime of that tube will be low. Even so, their "?" range means not: "We don't know, you can find out for yourself". They just say such tubes may work im some applications, but not in all. So if the 12AX7 we talked about would give only 2mA it's not necesarily bad. Perhaps it works at 1.2mA indeed. That's what the "?" means to say. Just trust in such a tube, and quality is lower. They say that, and I know they are right. So you can feel happy to own such a tube, and perhaps it tests fine on a curve trace, but when we talk about purchasing a Telefunken ECC803S which costs like 400 Euro for a good used one, I do not purchase it. As a rule of thumb, you will see that low emission tubes will also 'just pass" at an auto bias test. Which comes down to the same, so such test results say, it will work now in an amplifier, but not extremely long. This is what an emission test is intended for, and there is no "better" method.
In his 'blue book', Max Funke writes: The principle with an emission test is, when the tube obviously seems strong, it can be assumed that all data is good as well, because the only thing which makes tube change, is lower emission, shorts, or leakage. If emission is strong, and no conductive failures are present, then grid functioning needs to be proven only as such, and actual values can be assumed correct with highest probability.
We have to mention, you can easily fool the users with a miniature emission tester which looks fancy. Such were sold like sliced bread, and even today on Ebay they give buyers the suggestion of a good buy. Just everybody sells them again after a while, so sales will never stop. Unlike a fine Funke W19, Hickok TV7 or I-177, AVO, very many were made, but few people will sell it.
A good quality tester is easy to recognize: They are expensive, they are heavy, they have many sockets, and often have seen a lot of use.
Some testers can do two things at the same time, like combine a parametric test with a quality test. Like AVO Mk1 and higher are famous for that, but also L3-3 can do that, with specially made cards. Quality testing and parametric testing, in most testers however is not combined, because it requires too much circuitry, and also because there is no market need. So who wants to know quality, normally wants to know only that.
It seems impossible to put all testers systematically nicely in categories. But I will try to sort them a little bit.
Such are quality testers. These can work both with AC or DC anode voltage. With those, the tube under test is used to supply current to a load resistor. The tubes are normally used in diode mode, because the interest is just to find out the emission, and eliminate behavior of the grids. Why is that done? Why not look at what the grids are doing? Well, look at this EL34 tube as an example. It reacts extremely strong to mechanical tolerance on the grid. This can easily make thes tube draw 30% more current as average, new out of the box, with a unused, 100% good cathode. So plate current of a new tube can be 100mA +/- 30%. This is not because of weak or strong cathode, but because of grid distance. So a new, fine tube can be from 70...130mA at fixed grid voltage. Let's take two new tubes out of their boxes. One has 70mA, the other has 130mA at fixed grid voltage, but both will have 100mA in auto bias.
On a parametric tester the 130mA tube tests very strong. So to say at 130%. Now, after 5000 hours, life of an EL34 comes to an end. The cathode has only at appr 80% emission, and the plate current of initially 130% has dropped to probably 100mA. Something similar happened to
Now, let's look at the same tubes, but this time on a good quality load tester. As it was new, and tested as diode, this eliminates the function of it's three grids. So now we are finally only looking at the cathode quality. This diode tests at 100% when the tube was new. It should be clear, that this tube after 5000 hours, when emission has gone down. Let's say it dropped 20%. This is indeed the same for both tubes, and a good emission tester will come with that result. This reflects the condition a lot better.
To have the aha! effect, do the following, which is really interesting. Take an weak EL34, which you wanted to dispose of away anyway. Test it on a parametric tester, and it will do only 40mA instead of 100mA. So it is bad. Now, use the tube as a hammer, and knock the sides on something which is hard, but will not break the glass. Like a piece of wood. Re test it with fixed grid voltage, and voila! the current is now 55mA instead of 40mA. Please do this indeed, you will be much surprized! So has the emission improved? Of course not! So how is this to be explained?
In case you own also a load tester, you should also test the tube on this one, before you do the destructive knocking on it. So by the simple logic, that this knocking can of course not improve the emission, there should ne no difference. The test result before and after knocking will be the same indeed. These are only words here, but when you really tried this out, it will open your eyes for this subject. So you can't fool a load tester with this, but you can easily fool a parametric tester this way.
The quality of the load tester itself
DC types are the best, these can more accurate predict the behavior of the 'diode' and give more reliable results. This is so, because diodes by nature have low impedance, meaning they react very strong on voltage changes. So things like random mains voltage variations, or voltage drop of the power supply, due to load, or voltage drop of the whole transformer with tubes of very high heater current, these are things with quite an influence in the test result. Yet, very good AC types exist indeed, with separate heater transformers, like the Neuberger, Metrix, or AVO, but unfortunately it is so easy to build a piece of junk too. They can be build almost without electronics at all. So in a minimum version, inside you'll see only wires, a miniature transformer and one resistor only. And of course very few sockets. Such are cheapest to make and easiest to use. . Most radio and TV repair men had them. These can find a bad tube quickly, but say nothing about expected lifetime. Like a bad dentist, and soon after his work, you have new trouble. Such tube testers do pick out bad tubes often, but let some of the dirty ones pass. If your requirements do not go any further, this kind of tester for you. Actually some of those CAN be really good, but these are very few only, and most of them are toys. Specially the often seen Lafayette, and clones called like 'TC-02' etc, are useless.
Click here to read about the accuracy of a Funke W19 load tester.
New made testers
I am a bit reluctant here. People are often impressed by the illusion of digital precision. But really there is limited meaning in a number like 60.25mA and the tube is tested with cold anode, and current is down 10% because of that. So you read 60.25mA with 10% error on it. Or like AT1000 is doing, presenting the transconductance of a 6SN7 with four digits, but stepping like this: 2500, 2600, 2700. That is fake resolution, so a 2550 tube may as well read 2500 or 2600. Any 60 years old AVO will do better. I am not against digital testers, but I am against fooling the user with fake precision.
So the choice comes down to what is your 'must'.
The mostly asked question is: How good is a used tube, and what's the lifetime left in it? It is so weird, but all new made tubes testers avoid this answer, but give you all kind of data which may lead to this answer, but there is no guidance for that. Moreover, and I need to say this very clearly, parametric results are not ideal to find out remaining tube life. This is rather the domain of the Quality Testers, which are made for this purpose specially.
Vintage testers seldom present parametric information to the operator. Yet in the end they come up with the answer to your question. They present on a good/bad scale the condition of the tube. After some experience with that tester, you will understand what '85%' means to you, for this particular tester and this particular tube. A few rare Quality Testers will can also present parametric information, along with the quality percentage.
Good quality testers are: Most of the Hickoks, AVO Mk2, Mk3, Mk4. Funke W18, W19. (very incomplete list! ). The useless ones, are no real help to you. So the 150...300 Euro these go for, is a waste of money.
Value: When this is an issue to you, a vintage tester always keeps it's value, and new made products not so much. With vintage tester's prices, the most expensive ones are the easiest to sell, because everybody wants them. It's like with vintage cars. There are many rusty cars for sale everywhere. But when you want a specific model in rust free fine condition, that will have a price tag. Yet, this car will hold it's value, and stays easier to sell than a piece of junk. With vintage tube testers, it is just the same.
Why use a vintage tube tester?
This repeats some of the above issue, in more detail. Vintage testers offer functions, which new made digital testers do not have. Digital testers suggest precision. However that is very probably not the case, and not what the average user needs. Most people in reality only need to know: Is the tube still good, and how long will it still last? Modern testers like the AT1000 give no answer to that at all. Even so the AT1000 is a very confusing tester, and makes many mistakes. So fine, you know the plate current is for instance 81.5%, but 81.5 of what? Wel.... of 100% of course. But was that the question you had? Will you reject this tube, because 81.5 is not 100? You and the digital tube tester make a bad team, if neither of you don't know when a tube is bad or not, and at least the AT1000 does not know it. Put the same tube on a Funke W19, and it says. '20'. Good starts at '12'. Look! Now that is clear. Put the same tube on a Hickok and it will come very far into green. Wow.... and you were so disappointed about only '81.5% of 100%' as the AT1000 told you. Such experiences I made many. Or even the AT1000 will say the tube is at 100%, so you are happy. You put in an amplifier and after 10 hours the speakers say: 'CRAX&%&%$'. Well, you better should have passed it through the Hickok or AVO leakage test, that would have spotted that leaky electrode, which the AT1000 can not test for this. Or, you dig up some radio tubes from the 1940's and the new made tester -OF COURSE- doesn't have the sockets for it. Or the settings are missing, which you can -OF COURSE- make yourself, but is that what you wanted?
This is what I always say: You must see of a vintage tester as a concentrated amount of knowledge and experience, put into that machine, by those old tube specialists, and you're now buying this experience and comfort, along with the old tester. The lack of experience with makers of new tube testers, such as AT1000 is painful. So if Hickok found out exactly this or that setting to reject a 6SN7, you can never replace that by a simply plate current measurement. I have learned to trust a test result from a vintage tester which is a good condition, more than what digital testers tell me. I have a fully restored Hickok 750. Though the size a large, I love it. When this tester says: 'Bad tube', well than that's what it is. At the moment I see many people returning to those good old analog tube testers, and I can understand it.
Another compare is with the uTrace or E-tracer curve tracer. Same situation here. In contrary to the AT1000, the software of uTrace and E-tracer is constructed with a lot of pride, and regularly updated. These little testers can do so much. However, also here, you have to judge for yourself if you like the test results or not. The question is, are you really capable of that. There is no help for the user, saying if the tube is good or not. So I see often people inventing self made rules, for when a tube must be replaced. Such rules are mostly wrong. Of course they have no other option, but that doesn't make the situation any better. That is because the digital testers don't help hem saying if the tubes must be replaced or not. To my opinion, such people are better off using a really fine, vintage tube tester.
So I prefer to test a tube with a professional tester, from the period when these tubes were in daily use. These were made just to say how good the tubes still are, and for me that is often the main question. If the tube is still 'fine' by such a tube tester, then transconductance etc suits normal applications. For very old tubes like the 45, or old rectifiers, I prefer an old (but good) tester like for instance the Hickok I-177. It gives me both 'Gm', and 'Quality'. Whereas the later model TV7, gives only 'Quality'. Very high gain tubes like ECC83 should not be tested on an I-177. Though official settings were created later, the input signal to this tube is too high. Such high gain tubes didn't exist at the I-177 was designed. Or, the AVO Mk1...Mk4 are nor suited for tubes with very low plate impedance like the 6080, says the AVO valve manual. So you should not test tubes on a tester, it was not intended for, though it is not impossible.
Order Nr: 624-014-55
See price list (appr. line 639)
What you receive:
Guide to tube tester repair and calibration.
45 Pages color printed, in binder.
The use of calibration tubes.
You can say at every AC Tester, you simply need such a tube for calibration. There is no other way as to take a known tube, and set the tester's reading for that value. From Hickok you could even buy them, but that is long ago. Even so if you would find one of those on a flee market, it will not have it's value any more after 50 years. At Hickok they used the RCA Steel tube 6L6 for this. At AVO they recommend smaller tubes, such as ECC82 or 6SN7. Also for margin: 0;r any DC tester they are a safe ways to verify the tester with a range of tubes, and check at several operating points. Also important it to check the leakage function of your tube tester, you need the leakage test tool. First the tester must be able to detect leakage in a reliable and predictable way. Then, if this function is verified, it also proves the deck with the sockets is not leaky, since any problem here would false any test result.
Where are you now?
Most tube collectors have a tube tester, that was not serviced for 50 years. Those testers that have serviceable parts inside, are likely to be off calibration in a terrible way, it shouldn't come as a surprise. If the tester was stored in a basement or a garage for the last 30 years, the components usually suffer from this very much. Capacitors have gone leaky and carbon resistors have internal cracks, and higher resistance resulting from this. This condition is where most tube tester are in, specially the Ebay ones. However most things inside the tester can be repaired professionally as long as the basic material is good. With this I mean the meter, the transformer, and wiring. With an unknown condition tester comes first VERIFICATION, which means you check a tubes with known values, and the tester should reproduce this within it's specified tolerance. Then, normally you find larger deviation than that, and you can do a calibration attempt. This means you calibrate the tester with a known tube. After this, some other types of known tubes should also indicate correctly, which is not necessarily the case. If you calibrate the tester for a 6L6, next you need to try if it works well with a know 6SN7, and then 6922. So this is what the calibration tubes are for.
Suppose you find out then, your tester has some deviation even after calibration, you can still decide if you want to keep it, or repair it.
The the tube calibration set we sell, has following contents:
Customer feedback from Sweden, for the calibrated tube set (Dec 2009):
* * Scroll down to read about tube testers! * *
TUBE TESTERS - FOR SALEI am selling off some testers from my collection to free some space. Once these are gone, I will have no others for sale. Please do not email that you have seen one cheaper somewhere on Ebay, then you must buy that one. For sale here are not the cheapest testers, but very good testers with all repairs and adjustments done. Actually with Ebay prices going crazy, without logical reason, it can also be that you have to pay more at Ebay for a tester sold 'as is' as for a good one here. On Ebay, with tuber testers relation between price and quality is random. Price is determined by the highest bidder getting into this 'I must have it' irrational mode, and by the optical appearance. Not by the electrical condition of the tester. For instance I have been looking over 12 years to buy a SOFIA. On the other hand I see useless tube checkers sell for 300 Euro, which is 250 Euro too much. (like this one here)
I am working on a GRUNDING TUBATEST Mark2 / latest version with the large, integrated socket adapter - Price: 400 Euro, completely restored and calibrated.
Haven't we all read this before on Ebay:
These tubes all test "GOOD" on my TC2 Tester.
Each category of tubes is tested different.
The problem comes, when you want to know the quality of a used tube. There was never a general method published for this by any tube manufacturer. In the 'Blue book', which belongs to the Funke testers, is a piece of text about this, and it is the only reliable information about this that I know. This was always left up to manufacturers of tube testers. So... it means: Many tube testers, many methods.
Some ways to test a tube
1) Plug in a brand new tube in your amplifier, and see if the function improves.
This result can't be denied! The method was used by many service technicians. The tube manufacturers had special cases for that for service man, with a collection of new tubes, one of each. If you have no other way, take this way! It is a very good one.
2) Measuring the DC plate current in an AUTO BIAS circuit is a good thing because it is realistic, and it's hard for the tube. This is no job for a low cost, or also not for a 'patented' tester. There is nothing to patent on a normal, straight forward DC tester, and this is the ONLY 100% safe way for power tubes. So once the tester has patents in it ... Well may be they are good, but I would not trust an unknown brand.
For instance the ECC801S / 6201, from the TELEFUNKEN DATA SHEET:
It is supposed to give 10mA at 250Volt plate, and Rk = 200 Ohms. It is 'good' when not higher than 14mA and not lower than 7mA. This is from the original Telefunken Data sheet (can be found at the tech-corner of this web site). The end-of-life is below 6mA, or when the original value it had when it was new, has lost 40%.
It should be obvious you can NOT repeat this data without Bias resistor! Well you can but variation for a good tube will be significantly larger than +40% / -30%. How much larger, nobody can say, and it is not interesting, because fixed grid voltage testing is a wrong method to determine quality, anyway. The RCA data sheet of 6922 is clear about that. Though characterized at 15mA, guarantee ends above 5mA with fixed grid voltage.
Now this TELEFUNKEN data sheet is a very clear specification :) So of course THIS is what you like to verify.
The GE Data sheet of the KT66 simply says below what current the tube is bad.
Unfortunately these are the good exceptions, and most tube manufacturers avoided that question like monkey plague. When measuring real DC plate current, you need very precise and heavy electronics, these tube testers are expensive, and may need calibration or periodically verification of the tester. In the instruments world the definition about calibration and verification is a reversed one.
Please take good note of this. It means when specified NOTHING, the instruments needs yearly calibration. If it says it needs a calibration every five years, then this counts instead. So no information = Yearly calibration. This the way the world of instruments is defined. So if your tube tester is from 1965, nothing is written about calibration in the manual, and it was never checked ever since, FORGET the results. It missed the calibration every year ever since 1966. Verification is a process which must be specifically ALLOWED by the instruction manual. So when not mentioned, it is no good way to test this tester. Verification means you compare the result to a know tube, and when ok, the tester is ok. VERY FEW tube testers are so good they can be checked that way. For instance Funke W19 can be checked that way.
When is a tube good or bad?
DC current testing. Generally it can be said a tube is bad when the original plate current is below 70% of the initial value for European tubes, and below 60% for USA tubes. However we should only use such numbers if there is no specific instruction in the data sheet. If there is a test circuit in the data sheet (this is so with many long life tubes, or other High Quality tubes) this if course replaces any other test method.
In lack of manufacturers test data, take not 60% from the Data sheet 100% value at fixed grid voltage, because that should not be used for anything in a serious way. Take 60% of the initial value the tube had when it was new. That initial value may have been lower than the 100% value or higher. Hard to say what the initial value was after many years. The best is always to mark it on the tube. Otherwise you have a dirty method here. To give an example: a tube has for instance a 100% Value of 100mA, by the Data sheet Suppose the initial value of YOUR tube was 70mA, when it was new. A European tube may drop down to 70% of that, so 49mA and is still good. But.... suppose the initial value was 140mA, then 70% of that is 98mA. Below that the tube is worn out! So you see how important the initial value is. Always mark it on a new tube when you know the value. Has the initial value got lost, you have to guess it.
3) Measure Transconductance. This is also good, because you can construct good tube testers, with little electronics, and get useful results from that. These often need no or little calibration, or just verification. So verification requires a calibrated tube, or another tester of the same kind to compare the results. However.... Testers that measure transconductance with little electronics are not never accurate with ALL tubes, and that's per definition. So the Hickoks are typically calibrated with a 6L6, and you can fully trust the readings with any kind of similar tube, like KT66 or KT88. However they are not capable of testing tubes like 2A3 or 300B under real life conditions. These are 5000 micromho tubes, but on the Hickoks they read only 3000 micromho. So these test results can not be used to verify against the data sheet, but the severe deviation is however integrated in the roll chart, and like this it works well. So the roll charts just take another bias voltage which will make a good tube indicates good, and a bad tube indicates bad on the red-green scale. How about precision with that? VERY good, I would say, you can trust a good Hickok always.
What is not so good, with 'Transconductance only' testers, is the unknown DC conditions for the tube under test. I have many Hickoks, and I find that a 'bad" tube on by those testers is definitely bad. These tester makes no mistakes with that. But how it came to that conclusion is unknown. If you do not want to know why a tube is bad, but you are only interested if it needs a replacement, a Hickok is THE tester. The ideal way to measure transconductance is with an AC (audio) Test signal, and adjustable DC and original Data sheet settings to go with it. Very very few testers work like this. Almost every tester has it's own way to avoid this. Reason is, as said before, expensive electronics.
Some very good testers however, are the higher class Hickok testers. These perform a real transconductance measurement, with a (known) Grid bias voltage, put a known AC signal on the grid, and from that measure the real AC plate current. Keep in mind, ALL Hickoks work with AC wave shapes, that represent equivalent DC voltages, and this limits the precision, no matter what you try or do. A better method for this, is not using the mains voltage a signal, but a higher frequency from an oscillator. Why is that? You may say you can also measure transconductance at 50 or 60Hz also? The answer is: No you can not measure that precisely at the mains frequency. Reason is, if the tube is humming (so it has a defect) the hum signal will be a part of the plate signal. So such the tube will read better than it is. The Russian L3-3 uses a 1400 Hz sine wave at the input of the tube, and a 1400 Hz selective Voltmeter. This is in fact a very band pass filter. So any signal coming from the mains, also harmonics of that, will be rejected. Also distortion and white noise of the tube is not added to the result, since only 1400 Hz is measured, within a few Hz only. The second harmonics is 2800 Hz, and filtered out. This method is the one and only true measurement of transconductance.
I found almost no tester that can accurately measure the transconductance of law impedance power tubes like 2A3 or 300B. The testers I have, so far the only one that produces an EXACT and fully correct reading is the Russian L3-3. All other analog testers produce values that are quite imprecise. Also the great and highly valued AVO Mk4 doesn't come near. Wisely they left out the 300B of the AVO manual. Also most Hickok roll chart avoid mentioning the 300B. I found out the reason for that, is the too low plate resistance of those tubes. There is a hint about that in the AVO manual, but this hint is written in such a way that you will not understand the real meaning of it. What they mean at AVO is: Forget the results for all tubes with low Rp. The Hickoks also can't measure such tubes accurately if you ask me. Those Hickok testers that can't do it, also don't have the 300B in the manual. If you do test it, by simply put it in 2A3 and set the filament to 5V, you get wrong results. So it will read 3000 micro mho for a tube that has 5500. Also for your 2A3. But... who knows the real Gm of a 2A3 before testing it ...? So nobody finds out, and nobody will complain. The digital tube testers AT1000 ( we have three) is good, but the L3-3 is better. Just not as convenient, and the AT1000 software is a bit crude. The Sofia software is extremely delicate, like you can point with the mouse at any position of the a curve, and you get the three main parameters in one window, for that particular point. (Gm, Rp and gain). Well for me, the Sofia and the L3-3 beat any tube tester I have worked with.
Actually I didn't have a good place for this item, so I have put it here. There is great misunderstanding about panel meter protection. I guess because we all think this is so easy to do. I mean just two diodes, and a capacitor, and you're done. But... are you really? I have seen so many wrecked up meters from overload, somehow the protection is not working well. We have to see there are some kinds overload. The first is too much steady current. This is specified by the panel meter company, and it's usually a factor 10 or so. The other is how much mechanical energy can the poor coil can absorb when the needle it is hammered violently in the corner. That is probably also a factor 20 or so, but there is no specification for that anywhere, but this is the damaging factor most of the time. The damage can be a bend needle and coil deformation. A bend needle is a problem for multi scale meters, as you can not zero this away for both scales. So when it is at zero for the outside scale, the meter is not at zero for the inside scale. The larger Hickoks have five or more scales, and the AVO's have two. Another problem with a bend needle, is the balancing weights of the needle are now at the wrong position. That is a big problem when the meter is not horizontal, and can causes an errors of 10% at some positions of the scale. The damage is also deformation of the coil, when the needles hat hit the corner violently. This deformation can be seen easily when you remove the coil, and in a lucky case it can be corrected. If not, you end up with meter unlinearity.
So how to protect the coil...? In the AVO-8 multi meter I saw a system with a mechanical sensor, sensing the needle comes out of it's range, and via a pre-loaded spring and a clever mechanism, it switches a contact that interrupts the meter coil. So you have to press 'reset' again, also you pre-load the spring by this. Though very many AVO-8 have a deformed coil, that system seems to work not very good.
Another method is a capacitor over the meter. This works quite good, but the meter gets slow from it, and you loose a typical advantage of course an analog meter has. So add just a small cap is a good idea. By itself not a problem, when the meter has little friction. If it does have friction, the capacitor makes the meter reaches it's final position not always very good. The last millimeter it needs to move, it gets sort of stuck in the friction, whereas without capacitor that doesn't seem to happen so quickly. So a capacitor is a good thing to protect a good quality meter. Now, I see most of the time electrolytic capacitors, but these tend to develop a very high series resistance. This is not from drying up, as the sulfuric acid inside attracts water anyway. The failure cause is, the 100milivolt over the meter coil is not enough to maintain the capacitor's formatting, and they just end up with leakage, and too high internal resistance at the same time. Some Hickoks I have seen with a foil capacitor over the meter, and that's a lot better. In case you want to use a foil cap, they could be something like 5...10uF and today their size is really reasonable. If you do not want to change the vintage look of the inside electronics, a nice solution is also a so called Hermetic Sealed Wet Tantalum Capacitor. These are hardly made any more today, but any used or NOS is fine. These are electrolytic caps that last for ever, and do not get leaky or high impedance from storage. (The green picture here, are such caps)
Use of protection diodes. Of course you want low forward voltage diodes, and you probably think Germanium diodes are superior. Well most of the time they are not. Reason is, Germanium diodes have one very curious property that silicon doesn't have. Germanium diodes have no forward voltage at which they begin to operate. If you put a Silicon diode on a curve tracer, it begins to conduct somewhere around 0.6 Volts, we all know that. Below that, let's say at 300mV, the diode is just 'not there'. It is a capacitor, and that's all. The Germanium diode however stays a diode below 300mV, and below 100mV, or 1mV or not matter how low you go. Always the Germanium diode is better in forward direction as in reverse direction. It's a pretty bad diode at 1mV, so with lots of series resistance, but.... it's still there. This is why a crystal receiver can rectify signals of just a few mV. Not with high Effiency, but it can. You can never do that with a silicon diode. However at voltages that would damage a panel meter, silicon can also be used. Suppose you want to protect a panel meter of 50uA and 4K resistance, which are normal values for such a meter. That would be 200mV full scale. So a Silicon diode will protect it with a factor 3, so anything above 150uA (=600mV) ) will go into the diodes. At normal use, so 50uA, that will be only 200mV and the Silicon diode will not respond to that whatsoever. Whereas a Germanium diode will respond to 200mV with some current. (Both forward, and reverse current). Also a Germanium diode will respond to 100mV and to 10mV. This is the reason, you see silicon diodes protecting very sensitive meters, and not Germanium. Now, why do they use power diodes for that, like these really big 4 Amps diodes in AVO tube testers, or like the 1N5401 in the Unigor multi meters. Why not these little tiny ones like 1N4148, that are made for low current? Well... suppose you connect one 1N4148 to a voltage of 600mV and it pulls 1mA. Just suppose. What happens if you put 10 in parallel? They will draw 10mA. And 1000 in parallel will draw 1Ampere from 600mV. Now larger size diodes are nothing but a larger chip. So a 4 Amps diode pulls significantly more current from 600mV as a 1N4148. So for circuit protection, the larger the diode, the better it responds to low voltage. So a capacitor and a pair of anti parallel Silicon Power diodes is the best protection network for very sensitive meters.
Have you already been cheated with a tube tester on Ebay? Welcome to the club!
Ebay is not a good place to buy a tube tester. When prices are rocketing, there is too much interest to buy whatever is for sale. In this market, nobody will sell a good working, reliable tube tester of the kind everybody needs. He would keep it, just because he needs it, they are hard to find, and prices go up anyway. That is why they are so hard to find in good condition. What is for sales is the whole category of testers that are of little practical use, or seem impossible to calibrate, or have repairs coming up, have fungus inside, or other reasons, to dump them on Ebay. So don't believe you're going to be the lucky guy, paying only a low price for a fine tester. Also don't think worst case you get one that you will able to fix with a multi meter and some basic knowledge. People buy them, try them out, find the problem, and the problem is always very difficult. I have lost of negative experience on Ebay. You will hardly ever find a tester that you can repair elegantly. I thought so myself too, but that was a long time ago. Typical problems are always hard. Like partially shorted transformers, bad panel meters, none repairable switches for Funke and Neuberger. All kind of things that were impossible fix by the previous owner makes the thing land on Ebay. The seller always tries to appear the untechnical Mr. nice guy, so he wants to put you in the 'lucky' position. Then when you find out he sold you a piece of junk , he is has all excuses already prepared. The Ebay text was formulated with great care, and in the you end up as the 'unlucky' guy. Reality is often, the seller has been trying like mad to get it fixed, and gave up on it. Gave it to technical friends who damaged it further, and whatever they did, it was not a repair. With the crazy prices on Ebay, he just sells it, and buys another one for the same money. WIth a few tries he can be lucky. So he can't go wrong. And nooo... I am not paranoia. You have to read those Ebay adds with a sharp eye for detail. Just look at the above picture. What animals do you see there? A cat and a mouse? It's a cat and a hamster.
Here are some observations I made
When the tester is sold as 'defective' they bring half of a 'very good' one. When they are advertised as 'from my uncle who passed away', that is regarded ideal, because there is common belief that dead uncles leave always good working tube testers to untechnical nephews. What you are buying, in reality is: Untested, no guarantee and no returns. Most of the time, you find feedback of the seller, related to tubes. So the seller is active with d electronics, but when his uncle leaves him a tube tester, he sells it untested. Look at yourself, would YOU sell a tube tester untested? Wouldn't it be better to check if it works, and get a higher price? I see this so often, and in the end the people come to me, asking for repair advice. Really in the end you are always better off, buying working instruments, a not buying defective items for half the price.
Forget testers that were found in a garage
You can fix the rust with contact spray, and remove the fungus with a brush. Or, the seller says it was 'found on the attic' and you think that is good, because attics are dry. Well, you will may find out the reason why somebody put this tester asleep decades ago. For instance the transformer has a defect. Then today, first you have to repair the rusty witches and bad contacts, and after that you will come to the same conclusion as somebody 50 ears ago. Such things happened to me several times!
Don't take a tester with a cut off mains cord
This means, perhaps the seller cut if off, as an excuse for selling a broken product. Or it means long ago, the mains cord was worth more than the tester. You can quickly see that on the cut of the cable. It is clean, or was it done 50 years ago? That looks quite different. In both cases not a good sign.
Or, when you buy a 110V item, with the USA connector still on it, while living in a 220V region, the seller claims he has no way to test it. he really found it somewhere, would you he not buy a auto transformer on Ebay for 10$? Probably yes. And then what will he do of the tester appears is broken? Sell it untested.
Look at the deck screws.
Also don't take a tester with the deck plate screws missing, or it has shiny new screws. It only means, long ago someone thought it wasn't worth putting it back together, and the screws were gone
Damaged case, or just a very much used case.
This is a very reliable hint: If case looks treated like junk, then that is how it was. The question is only, what makes you think he was wrong?
Or, a tested looks extremely used. Specially around the leakage switch. Though this doesn't look nice, but it may be a good sign. Obviously it was used if for decades by somebody, and he had reason to keep on using, and using it.
Take only a tester that is guaranteed to be OK
and ideally it was used by the seller itself. It won't be cheap, and still the seller knows better than you why he sells it, but you get at least a basically working tester. Sorry for the lecturing, but i know this from the many requests from people who bought their broken testers on Ebay.
SO WHERE DO I GET MY TUBE TESTERS FROM?
I get them from people I know, and for the rest of it, I am not buying from strangers unless cheap. Though sometimes I had no other choice than Ebay, knowing the big risk. I try to make the proposal to try it out, and when not as described, I can send it back. But usually the story comes like '...no no... we sell it as is, and we know nothing about it...'
On the other hand, when people proposed to really make an honest deal, usually that's what I got. For instance I received an AVT CT163 from South Africa without pre payment, and option to return. Actually he told ne later, he was afraid to ship it to an unknown buyer, and get Paypal trouble afterwards. So shipping it to me, was a better option for him, because he definitely knew the tester was good, and he trusted me coming to the same conclusion. Needless to say, this worked out good for both parties. We stayed email friends afterwards, and he bought a pair of vintage fog lamps from me, for his Mercedes car collection.
The following link shows information about a very clever seller from Hungary, who fooled me badly. He is still on Ebay, playing innocent, and privately selling equipment from his university, under changing names. His adverts still look the same, and shipment city is Budapest. Beware of this.
Some popular vintage testers.
Mk2, Mk3, Mk4, VCM163
TV7, I-177, 750, 539C
Click here for analog Testers Part1. (Better use the menu at this page top)