How to Test Electron Tubes

© COPYRIGHT NOTICE All Rights Reserved How to Test Electron Tubes (Last updated: 10-May-2018 19:06 )
I get many requests from people who want the "best" testing for their tubes, and they have some ideas of what needs to be done. Like please select for the best gain, or hightesr transconductance. Or re-select the selected and pre-packed tubes, to get them "extra selected", because all I need to do is plug them into a tube tester, and read on some meter for "good...better...best". Like on the above picture. Such requests drop in by email, prior to ordering. Well, definitely there is a large need for doing the right thing, and buyers are absolutely right with that. I wouldn't want to have the crappy tubes in my own equipment eitger. So I would have to write long emails full of technical stuff, which is not appreciatred. Alternatively compress the answer in three lines, both of which doesn't seem right. So here is a univeral anwer, pick from this what you are interested in. It is devided in some parts: * What is a "must" and what is a "want"? * When is a tube a good tube? * * Working principles of some tube testers. * What is a "must" and what is a "want" ? I don't want to lecture on you here, but the situation is, you "want" something, and to achieve this, you "want" something from the tubes. What most people want is of course the best sound, and unless you are talking about electric guitar amplifiers, best sound can only be achieved by the best tubes. In many cases, the "want" is described directly as it is. What do we need? Warm sound, because nobody ikes cold sound. Bass must be tight. The highs must be clear but not to bright, and the mids must be well defined, and all the seller needs to do is use a (fine calibrated) Hickok tester, and pick those tubes from the lot. But look, if the key to good sound is that easy, tube manufacturers would have build them just like that from the beginning and all would have used Hickok testers for final inspection, and pick those where the needle moves to the right as far as possible. You already get it, things did not work this way. But why demand it today? I do not understand that. Better is to divide this into must and wants. So do not want things which you made up yourself, and probably makes it only more expensive and difficult. At the same time, do not compromise on basic and healthy "must have" specifications of the kind any engineer says they are important. Read the next part for that.... * When is a tube a good tube? There are three things hidden inside the bulb: Quality, Lifetime, and Reliability. Quality is the specifications. Like: "this tube has little hum". This part is like: Say what you will do. Reliability describes the chance that the quality is maintained during the Lifetime. This part is like: Do what you have said. People have sometimes the idea of exactly measuring electric data from tubes, in highest possible precision, to get more succes with their audio equipment. Such an aproch is not wrong, but we must say it belongs in the category "want to have". This can be interesting, when you have the time and the money, and the engineering knowledge to understand what tester you need to this, how to obtain one that is in instrument condition, how to use it, and how judge the data it gives. Like this, it can be very fun to do, and a great hobby. Lets put this approach at one end. F At the other end, we see an fully basic approach. Meaning you have a tube of unknown condition, some equipment you want to use, and a waste basket. The decision is throw the old tubes away, or still use them, and there is no interest in the "how and why" of tube testers. Like an Ebay seller, saying the tube tests "strong" on whatever tester he uses. This approach is all on the other end. Then, everybody is somewhere positioned in between these two extreme positions, let's say in the middle. In this article I try to explain why it is important to set the expectation right. So choose a kind of tube tester that it matches your personal needs and capabilities. Some of the possible working principles are sure not listed here, but these are the most occuring: Emission Testers. Let me begin with the approach of Max Funke, by the Funke Test Equipment factory, in Germany. He sais, tubes generally get bad by loss of emission, and from this result the useual trouble with parameters and bad performance. Sometimes there can be leakage or shorts as well. So suppose a tube passes a leakage test, that covers shorts as well. If a tube as a second step passes an emission test, and baisically shows basic functrion, then he says the tube should be in the same condition as when it left the factory. Meaning, we do not have to measure Gm precisely, and Ip, and all of this tube data. This provides of course the emission test ist done under realistic conditions, such as full dissipation, and leakage test is done carefully. Gm testers, without control of the working point. This apprroach we find with most Hickok testers. Test conditions, you may or may not be able to measure this from the tester, but's all very unclear, and no explanation anywhere as to how the result is produced. Just some faint stories about the great patent. Even calibration procedures are vague, and many other things too. Yet the idea is: When the tester itself of good, it produces a test result that is good, the tube is good, and that's it. So you can "so called" test the Gm of a 6SN7 tube, your're not really doing so. The reference is only to compare the test result with the "must be" result of a new tube. The fact that you are doing so at unknown plate current, at too low plate voltage, and probably too high heater voltage, all these things play no role for aH ickok. A new tubes reads above 2500, and a used tube is bad under 1700, and that's all. As long as this is done well, and the tester is in good condition this is a reliable test, but you can't use it for mathing or anything else. DC testers, with Gm test possibility, and control of the working point. As now Gm can be tested as well as the working point, such testers generally do not go very a far in voltage of current as this is not needed. The idea here is, to bias a tube for one of the working points that you can pick from the tube datasheet. In this working point, the grid voltage needed hould not be too far off, as this would give bias problems. Then, in this working point, Gm should be close to the datasheet value. If below, this is either wear out or tolerance. So logically tolerance has nothing to do with wear out. This can be kept apart nicely with such testers. DC testers, without Gm test possibility, but with control of the working point. This apprroach we find with the Metrix 61 Series, Neuberger 370, 37 may other testers too. They cannot measure Gm directly, but the idea is, same with Emission testers, if the tube can do a full power test by the data book, and passes a leakage test, there cannot be much wrong with it. Such testers are useually very heavy, and expensive too. As users know what this is, and not too many were made. Gm testers, with user control of the working point. In this category, keep apart the AC testers from the DC testers. An AC tester simulates the DC test, though latest generation testers can do so really nice, it stays a simulation, and precision is limited. Such testers are AVO Mk2, Mk3, M4. Later model Taylor tester, the polisch Elpo, some of the Metrix, and the Central 752. Sure I forgot many here, this list is not intended to be complete. Patented Testers. These have all one thing in common: They don't test the tubes in a straight forward, state of the art way, because no patent office will grant you a patent for that. You can only register your model, and that makes copying harder. Just, to get a patent, it's mainly two requirements: 1) you have to achive something what was not achieved before. 2) It must be achieved in a none-obvious way. That last requirment, you must look at it. This is the hardest, but without this, there can be no patent. For instance, when you can build a battery that lasts 2x longer than any battery that excisted before, but there is no new technology inside, there is noting to patent inside. So a patented tube testers is always not using normal, obvious and excisting technology. Instead of that something "patented", and not the way any good engineer would prefer, if he was free to choose. So results, are useually not in clear metrics, but on a "good...bad" scale without any explanation of what that is and how it's done. If theit patents were "the best" you may wonder why not everybody uses them today for free, as a patent is valid for 25 years only. So a patent on something that doesn't need a patent, is something with a funny smell always. Today, when looking at a 1950's tube tester, and it's full of "patent pending" numbers, it's like saying you: This tester is a tool, and not an instrument.
© COPYRIGHT NOTICE All Rights Reserved

How to Test Electron Tubes
(Last updated: 10-May-2018 19:06 )

I get many requests from people who want the "best" testing for their tubes, and they have some ideas of what needs to be done. Like please select for the best gain, or hightesr transconductance. Or re-select the selected and pre-packed tubes, to get them "extra selected", because all I need to do is plug them into a tube tester, and read on some meter for "good...better...best". Like on the above picture. Such requests drop in by email, prior to ordering.

Well, definitely there is a large need for doing the right thing, and buyers are absolutely right with that. I wouldn't want to have the crappy tubes in my own equipment eitger. So I would have to write long emails full of technical stuff, which is not appreciatred. Alternatively compress the answer in three lines, both of which doesn't seem right.

So here is a univeral anwer, pick from this what you are interested in. It is devided in some parts:

* What is a "must" and what is a "want"?

* When is a tube a good tube? *

* Working principles of some tube testers.

* What is a "must" and what is a "want" ?

I don't want to lecture on you here, but the situation is, you "want" something, and to achieve this, you "want" something from the tubes. What most people want is of course the best sound, and unless you are talking about electric guitar amplifiers, best sound can only be achieved by the best tubes. In many cases, the "want" is described directly as it is. What do we need? Warm sound, because nobody ikes cold sound. Bass must be tight. The highs must be clear but not to bright, and the mids must be well defined, and all the seller needs to do is use a (fine calibrated) Hickok tester, and pick those tubes from the lot. But look, if the key to good sound is that easy, tube manufacturers would have build them just like that from the beginning and all would have used Hickok testers for final inspection, and pick those where the needle moves to the right as far as possible. You already get it, things did not work this way. But why demand it today? I do not understand that. Better is to divide this into must and wants. So do not want things which you made up yourself, and probably makes it only more expensive and difficult. At the same time, do not compromise on basic and healthy "must have" specifications of the kind any engineer says they are important. Read the next part for that....

* When is a tube a good tube?

There are three things hidden inside the bulb: Quality, Lifetime, and Reliability.

Quality is the specifications. Like: "this tube has little hum". This part is like: Say what you will do.

Reliability describes the chance that the quality is maintained during the Lifetime. This part is like: Do what you have said.

People have sometimes the idea of exactly measuring electric data from tubes, in highest possible precision, to get more succes with their audio equipment. Such an aproch is not wrong, but we must say it belongs in the category "want to have". This can be interesting, when you have the time and the money, and the engineering knowledge to understand what tester you need to this, how to obtain one that is in instrument condition, how to use it, and how judge the data it gives. Like this, it can be very fun to do, and a great hobby. Lets put this approach at one end. F

At the other end, we see an fully basic approach. Meaning you have a tube of unknown condition, some equipment you want to use, and a waste basket. The decision is throw the old tubes away, or still use them, and there is no interest in the "how and why" of tube testers. Like an Ebay seller, saying the tube tests "strong" on whatever tester he uses. This approach is all on the other end.

Then, everybody is somewhere positioned in between these two extreme positions, let's say in the middle. In this article I try to explain why it is important to set the expectation right. So choose a kind of tube tester that it matches your personal needs and capabilities.

Some of the possible working principles are sure not listed here, but these are the most occuring:

Emission Testers. Let me begin with the approach of Max Funke, by the Funke Test Equipment factory, in Germany. He sais, tubes generally get bad by loss of emission, and from this result the useual trouble with parameters and bad performance. Sometimes there can be leakage or shorts as well. So suppose a tube passes a leakage test, that covers shorts as well. If a tube as a second step passes an emission test, and baisically shows basic functrion, then he says the tube should be in the same condition as when it left the factory. Meaning, we do not have to measure Gm precisely, and Ip, and all of this tube data. This provides of course the emission test ist done under realistic conditions, such as full dissipation, and leakage test is done carefully.

Gm testers, without control of the working point. This apprroach we find with most Hickok testers. Test conditions, you may or may not be able to measure this from the tester, but's all very unclear, and no explanation anywhere as to how the result is produced. Just some faint stories about the great patent. Even calibration procedures are vague, and many other things too. Yet the idea is: When the tester itself of good, it produces a test result that is good, the tube is good, and that's it. So you can "so called" test the Gm of a 6SN7 tube, your're not really doing so. The reference is only to compare the test result with the "must be" result of a new tube. The fact that you are doing so at unknown plate current, at too low plate voltage, and probably too high heater voltage, all these things play no role for aH ickok. A new tubes reads above 2500, and a used tube is bad under 1700, and that's all. As long as this is done well, and the tester is in good condition this is a reliable test, but you can't use it for mathing or anything else.

DC testers, with Gm test possibility, and control of the working point. As now Gm can be tested as well as the working point, such testers generally do not go very a far in voltage of current as this is not needed. The idea here is, to bias a tube for one of the working points that you can pick from the tube datasheet. In this working point, the grid voltage needed hould not be too far off, as this would give bias problems. Then, in this working point, Gm should be close to the datasheet value. If below, this is either wear out or tolerance. So logically tolerance has nothing to do with wear out. This can be kept apart nicely with such testers.

DC testers, without Gm test possibility, but with control of the working point. This apprroach we find with the Metrix 61 Series, Neuberger 370, 37 may other testers too. They cannot measure Gm directly, but the idea is, same with Emission testers, if the tube can do a full power test by the data book, and passes a leakage test, there cannot be much wrong with it. Such testers are useually very heavy, and expensive too. As users know what this is, and not too many were made.

Gm testers, with user control of the working point. In this category, keep apart the AC testers from the DC testers. An AC tester simulates the DC test, though latest generation testers can do so really nice, it stays a simulation, and precision is limited. Such testers are AVO Mk2, Mk3, M4. Later model Taylor tester, the polisch Elpo, some of the Metrix, and the Central 752. Sure I forgot many here, this list is not intended to be complete.

Patented Testers. These have all one thing in common: They don't test the tubes in a straight forward, state of the art way, because no patent office will grant you a patent for that. You can only register your model, and that makes copying harder. Just, to get a patent, it's mainly two requirements: 1) you have to achive something what was not achieved before. 2) It must be achieved in a none-obvious way. That last requirment, you must look at it. This is the hardest, but without this, there can be no patent. For instance, when you can build a battery that lasts 2x longer than any battery that excisted before, but there is no new technology inside, there is noting to patent inside. So a patented tube testers is always not using normal, obvious and excisting technology. Instead of that something "patented", and not the way any good engineer would prefer, if he was free to choose. So results, are useually not in clear metrics, but on a "good...bad" scale without any explanation of what that is and how it's done. If theit patents were "the best" you may wonder why not everybody uses them today for free, as a patent is valid for 25 years only. So a patent on something that doesn't need a patent, is something with a funny smell always. Today, when looking at a 1950's tube tester, and it's full of "patent pending" numbers, it's like saying you: This tester is a tool, and not an instrument.