Technical Bulletin 08

The only right way to TEST Electron tubes

This page is about how to test Emission LabsĀ® tubes the right way. There are many ways to test a tube, some are good, and some simply not. Add to this, that many new production testers suffer some lack of understanding about how tubes have to be tested in the first place.

The following list is definitely not complete! Please inquire if you have a tube tester you believe that fits in here, we can add it.

Here are the categories of tube testers for as far as I know, which can be used to test EML tubes.

  1. Really good tube vintage testers for testing EML tubes, are the Neuberger 370,375, Sofia by Audiomatica, AVO VCM163 with adapter socket for UX4 tubes.
  2. Some really good tube vintage testers, though these can not verify the EML at high voltage are: for the smaller EML tubes are: Kalibr L1-3, L3-3, Hickok cardmatic, Metrix U61B.
  3. New made testers which can give misleading results: e-tracer and utracer. So yes, they can be used, but they have not the right test method for high power tubes.
  4. New made testers with compromises: AT1000.
  5. New made tube testers which we think are good, I know only one: Helmut Waigel's 'Roetest'. Though this is a kit, it is at this moment the finest product on the market and nothing comes near. (Apart from the SOFIA tester, but this one is vintage, and extremely hard to find)

Of course users become confused with all of this, but the greatest confusion to my opinion results when people who do not know, ask other people that also do now know. Better is to read what the tube manufacturers write, for instance completely read such a datasheet like TELEFUNKEN C3g, and trying to understand WHY they write the things they write there, and why they say how to test this tube, and you will soon see, the auto bias circuit in this datasheet is not some recommendation for how to use this tube, but it represents the only correct way to test this tube. Any "other" way is nice for yourself only, and specially when that way is more simple, that was is not as relevant the original advice by Siemens. At Emissionlabs we go the same way. We explain here in this technical bulletin how to test our tubes, and frankly any other method is not in accordance with that.

Contents

  1. Short instructions for testing of EML triodes
  2. What causes misunderstandings
  3. Fixed current Testing vs. Variable grid voltage.
Short instructions for testing

EML Tubes must always be tested at the VOLTAGE and at the CURRENT as we say. This can be done with a classical auto bias circuit, but it may also be done with a tester, or some other set up with variable grid voltage, such that the SPECIFIED CURRENT is set exactly. At this CURRENT, all other parameters have to be tested. Most of all, Transconductance and grid leakage, and for who is interested, Gain and Plate impedance can be tested during the same test.

Furthermore, testing should only be done after thermal balance. So, directly after adjusting the anode current to the EML specified value, it can be seen it begins to change slowly. This may take some 5...15 minutes, and only when the tube is fully warm, including the socket part, the test results become significant. This means the anode current has to be re adjusted until it becomes stabile.

Here is the same instruction, listed in steps.

End of Life

If even after longer waiting for final stability, the INITIAL parameters appear to have changed, we have the question about the life time prediction. What counts for this, is the CHANGE of those parameters, compared to initial values, which are written on the box.

We see it over and over again, that "specialists" which build tube testers, know nothing better to do, than comparing random tubes with data values as published, while testing the tubes with fixed grid voltage. THIS IS WRITTEN NOWHERE IN LITERATURE, AND IN NO MANUFACTURER's DATA SHEET. This is ignoring the fact, that data sheets represent only average data. And even so, and no point is talk about fixed grid voltage. Because as you will see, the plate current is also fixed, and a fixed plate current can only be achieved by variable grid voltage.

This for instance is from the RCA 2A3 data sheet:

Anode Voltage 250V, Grid Voltage -45V, Plate current 60mA. Where does it say: Grid voltage must be "fixed" and plate current is variable? It does not so say anywhere! You may as well say plate current must be fixed, and grid voltage is variable! Please look up the original data sheet by RCA! At EML we can not speak for RCA, but we should not "read" things from their data sheet, which RCA has no written.

The use condition of a tube depends mainly on the change versus the initial factory values. Ignoring this data is really not very intelligent to do, because it is the best data to use! But also it seems to suggest that any tube which is not "average" has seen a certain "problem". This may indeed be so. And it may as well not be so. This is why this method is so wrong. It tells you nothing. Any 100mA tube which appears to have only 70mA can be totally unused, and such a tube will have normal lifetime as any other. Or even so, an unused tube may appear to have 140mA, suggesting it has 140% lifetime or whatever this 140% is supposed to mean.

Whatever this number means, 140% percent is silly. A tube can not be newer than new. This variation (like 70mA....140mA) is only caused by natural factory tolerance of the anode distance. The smaller this distance, the higher the plate current, but the smaller transconduction becomes. It should be clear this does not change the cathode quality, and for that reason it does not change the lifetime expectation. If it was that easy, we would at EML build all tubes with too small anode distance, and they would all be 140% of new. But please take not, this silly mistake is totally in line with most new made tube testers. That is why we invest so much time here, explaining how the right way of testing is.

So what DOES determine the life time expectation?

Wear out of a tube takes place in four phases.

These phases are as follows:
  1. The required grid voltage becomes less negative, this indicates simply some use. Wear out and use is not the same. Use hours may be significant, and yet wear out can be low. Use hours can recognized by this change in required grid voltage. Though a small loss of transconductance goes along with this, mainly the grid voltage changes, and this can begin to take place after some hundreds of hours already.
  2. In this phase, the loss of transconductance begins to accelerate, but no loss of plate impedance yet.
  3. In the third phase, plate impedance begins to rise. This is a beginning wear out, though the tube will still work, and this phase can take quite long as well.
  4. Gain of the tube is the multiplication of plate impedance and transconductance. For a very long time, the plate impedance will marginally go up, while transconductance will marginally go down in the same rate. This keeps the gain constant. The lifetime comes near the final phase however, when also gain begins to reduce. Not only the natural function of the tube (Gain!) begins to get lost, simply problems will accelerate now very fast, sound problems will already appear.
Some important informations:
What causes misunderstandings
Fixed current Testing vs. Variable grid voltage.

Our target is to offer tubes that are well matched, in a way which can be verified by others. At EML we begin with factory testing, which is for emission, for quality and for data limits. Then match the tubes on the AT1000. We are not using the AT1000 because we think this is the best tester. It is not a good tester at all. Only it is the most widely used tester. And though AT1000 has many shortcomings, it can actually be used for matching, if you know how.

AC Heating vs DC heating.

Tubes can be tested either way. Just test result is anther for DIRECTLY heated tubes. That is because half the DC voltage effectively influences the grid to hearer voltage. So any directly heated tube like for instance 300V will not give the same test result, DC heated or AC heated. (If DC heated, the tube will draw less current at the same grid voltage)

EML Test method:
Test result for Gm (2). Small expert talk

This is an ever lasting subject. There are a few methods, which differ by precision of the result. We begin with the best method.

  1. Transconductance is a dynamic parameter, so to say an AC signal parameter. For best accuracy, it is s measured by applying a distortion free sine wave signal to the tube, with an oscillator, and then monitor the output signal of the tube, while using a band pass filter. Testers like the Russian L1 or L3 testers are doing this, and some other high class vintage testers. The reason why this is the best method, is any distortion elements from the output signal gets filtered out, as these come in the form of higher harmonics. Also any hum and noise is filtered out. So hum, noise and distortion make the output signal artificially too large, any make the output signal (and so Gm) look larger than it is.
  2. An oscillator method which uses no filtering, is better than non at all, but it is not ideal.
  3. A lower class method is just use a DC change on the Grid 1, but this doesn't filter out any of the above mentioned signals, and some testers suffer a drop of the plate voltage in such a case too. So in case of any difference with the first method, it is clear where it comes from.
  4. By software, it is possible to derive Gm by calculating it via the Barkhausen formula, but that may have resemblance with method 3. However when with clever algorithms it may be possible to make it look more like method 1.
How Gm depends on the working point ( a lot)

1602 Electron trube by RCA

Keep in mind, Gm depends heavily on the plate current, as you can impressively see from the curve on the left. This is from the 1602 which is nothing but a better 10Y by RCA.

As you can see here, Gm varies from 520 to 2100 depending on the plate current. So a huge difference of 1:4 So it should be clear when somebody says he 'has measured Gm of 1600', this by itself has nothing to say. Also results from tube testers, saying 'this is the Gm' but not saying the working point are meaningless too. If this Gm of 1600 was measured at a plate current of 60mA, the tube is bad, and when measured at 10mA, a Gm of 1600 is very high. . From this it becomes evident that saying 'this tube has Gm of 1600' is a useless information, as long as we do not know the plate current. The Gm value can only be compared with the data sheet at the SAME operating point where the data sheet specifies it. So as long as you aware of these potential error sources, it is ok. It would be totally wrong however to measure Gm at any random current, like done with fixed grid voltage testing, and then compare this with the data sheet value for a bogey tube.

Why fixed grid voltage biasing is useless for tube testing.

Here is a numeric example. The tube in this example is the RCA 10Y (1602 special). The data sheet specifies Gm of 1330 at 10mA, which should happen with an average tube at a grid voltage of -23.5V and 10mA plate current. Of course no two tubes will have average values. In order to compare Gm of an unknown tube with the data sheet value of 1330, you need to set this tube for 10mA, by adapting the grid voltage to whatever value is needed.

It would be wrong, to set the grid voltage to simply -23,5V because there is no 'must be' for this value. In fact, plate current is so variable, that minimum and maximum values are not even NOT specified that way. So it becomes totally silly to measure plate current still, and compare it with the average value of a bogey tube. Any 100% strong, factory new tube, at -23.5V will draw anything from 7 to 14mA. So it would be wrong, to let the tube draw whatever current from 7...14mA, measure Gm at that plate current (whatever it is), and compare this with Gm plate current at 10mA.

Seeing this from anther perspective, you can test a tube at any place on the Gm curve you like, but not compare such a measurement with another one, which is NOT on this curve. Logically, the only way to synchronize this, is by testing any 10Y at 10mA, and compare this with the curve at the left at 10mA. Or, alternatively do the whole process at 20mA or any other current you like.

Having understood the above, it should be clear, a Gm measurement should be made always at the data sheet specified DC current, and never at a specified grid voltage.

Instructions of how to repeat the tests with the AT1000

This is written here is some detail, because we use the AT1000 at the factory for the data on the tube boxes. We do not do so, because it is such a good tester. The reason is, it is commonly used, and indeed it is one of the very few testers that can test under full anode power. AT1000 from before a certain date, have errors in the internal data tables for all directly heated tubes. Please communicate with Amplitrex directly, how to repair this. (We are not a service address for them) However when you set the tester to AUTO BIAS, which is required to repeat the factory test, this problem plays no role anyway. However you will soon see, sometimes (not always) tubes that test 'strong' on Auto Bias, may test 'weak' on Fixed Bias, or vice versa, so you can already see one of the methods is not right. Use only: AUTO BIAS.

AT1000 Stand alone mode or computer controlled mode.

Make good note, the stand alone mode has generally lower precision, because the tubes warm up only the heater, and not the anode, before a measurement.

Stand Alone mode

Computer controlled mode

For higher precision, the computer controlled mode should must be used, which allows heating up of the entire tube until thermal stability occurs, which is the only really good way to test a tube. For new tubes, the difference between stand alone more and computer controlled mode will be small. Used tubes however, benefit more from thermal stability. So, a quick test in the stand alone mode, will make used tubes look less good as they are.

Refer to the manual, so you know how to do the following:
Important Note for non-USA users of AT1000
Using digital curve tracers

Devices like 'utrace' and 'e-tracer' works amazingly nice, but they have a disadvantage: They test the tube in pulse mode, and have no way to warm the tube up properly. If you have an analog tester, so one that can set a voltage and current for as long as you want, you will see test data of a tube changes +5...+15% after letting the tube run long enough. (which means no change any more over 5 minutes). A tube should only be tested fully warm, including the socket. Anode distance is a function of the temperature, caused by thermal expansion of the metal. Moreover, anode heat radiates also back inside the tube, causing mechanical shape changes of the grids, and additional heat up of the cathode. Particularly older tubes will benefit from this. Moreover grid emission gets only visible when the tube is running close to maximum dissipation. So small grid leakage measured this way, means probably you have massive grid leakage in reality Whereas any medium grid leakage in reality, will probably result in 'no grid leakage at all' , if tested in pulse mode only.

Though we have to say here, Emissionlabs tubes are not expected to develop grid emission anyway. So all in all e-tracer and utrace seem useful testers here.

Due to lower anode temperature, a pulse mode tester may give not quite the same result, like at full power. A difference in the range of 5% is possible for new tubes, and 10% is possible for very low emission tubes. So even when the software says the tube is biased at for instance 700 Volt, 100mA, the heat development is just a few Watt, and not 70 Watt. Since the only good method to test a tube, is by definition at thermal balance, so this difference of 5...10% is a system related error. As long as you take this into account, you are safe. We much recommended the e-tracer and u-trace

Using full power analog testers, DC heated

This should repeat EML test data precisely. We have tested this with the Russian L3-3 and it works nicely.

Using full power analog testers, AC heated

This should repeat EML test data precisely, as long a you correct the grid voltage for half the heater voltage. We have tested this with the Metrix U61 and it works nicely. Unfortunately it limits at 250V DC. (and a little bit outside if you try with an external voltmeter) Yet it can do not many large tubes.

Using the Sofia curve tracer

This tester has a possibility to heat the tube under full power. After this, a 10 curves chart is made in just a few seconds. This should repeat EML test data precisely, as long a you correct the grid voltage for half the heater voltage. It can deliver unusual high RMS power to a tube. Unfortunately such a tester is not made any more.

Using the Russian L3-3

This is the top class of the vintage testers. Measurements have reference quality. Unfortunately it is limited to +300V, but when using an external voltmeter, and not too full DC current, it can be used above 400Volt, provided you have the right test cards for this. We have test cards for EML 5Z3, 5U4G, 80, 81, 274A, 274B, AZ4, 2A3, 2A3-Mesh, PX4-mesh, AD1, 300B, 300B-Mesh, 20A, 20B and 20B-V4.

Using the Roetest

This is the Roll Royce amongst the tube testers. You do have to set the tester for DC heated test results, in order to find the same results as we do.