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TESTING EML TUBES
Last updated: 14-Jan-2018 22:33



A Burn in table for 10 tubes at EML factory

Introduction

This page is about how to test Emission Labs tubes the right way. There are many ways to test a tube, some are meaningful, and some can be misleading or just wrong. Add to this, the large variety of tube testers. Historic testers are mostly unserviced since 60 years, but some are professionally taken care of. New made testers can be good, but some suffer lack of understanding how tubes have to be tested correct. So we find yourself in a bazaar of different testers, old, new, analog, digital, full power, or impulse testers only, and hardware or software can have errors. Users become confused with all of this. With this little overview we try to shed some light on this, but of course this can not replace profound knowledge. In all cases, a tube tester is a combination of hardware, a database, involves a test method, and must compare the results with the manufacturer data. Each of these elements is a challenge by itself.

Methods of testing.

Our target is to offer tubes that are well matched, in a way which can be verified by others. At EML we have our own way of testing in the factory which is for ourself. Then for shipment, we match the tubes on the Amplitrex AT1000. We are not doing so, not because we think this is the best tester, but because it is the most widely used tester, and so users can verify the results.

We had to choose for a method which most users can reproduce. This was DC heating, DC current testing only after thermal stability, and testing transconductance with a tone signal. These things the AT1000 can do.

Overall methods for all testers.

  1. Use Anode voltage and Current as on the tube boxes
  2. Use Auto bias testing when your tester can do so, otherwise, set tester by hand to specified current.
  3. Warm up the anode under those conditions for 3...5 minutes.
  4. Measure grid voltage needed, and Transconductance resulting from this.

Test result for Gm.

  1. To judge remaining tube life, transconductane can only be compared to the value on the boxes.
  2. Deviation of +/- 5...10% is related to burn in and tolerance of the equipment.
  3. Above 70% the tube should be working good in all amplifiers.
  4. From 60...70 % the tube will work in most amplifiers.
  5. Below 60% consider an exchange.
  6. Below 40% the tube should be exchanged always.

Transconductance (Gm) testing. (expert talk)

This is an ever lasting subject. Transconductance is a dynamic parameter, and ideally is measured by apply a distortion free sine wave signal to the tube, with an oscillator, and then monitor the output signal of the tube. Any non wanted elements from the output signal have to be filtered out. These are mains hum, white noise, and distortion. These three factors are not "output signal" as such, or otherwise would make the transconductance look higher as it is. Doing it like this, is the only right way. However this requires a lot of electronics, like a narrow bandwidth filter, and most testers try to avoid this. However this does result in some error, and no two such tube testers will give the same transconduction result for that reason. It has to be emphasized here, that deriving the transconductance from a differential measurement may seem ideal, but this is not so. This will not remove the distortion from the result. It has to be said, when the signal swing is larger, the distortion error becomes larger, which problem applies for a differential measurement as well. Yet in the end, a Gm measurement will be possible within 3..5% error, and an individual calibration option for the tester (can be in hardware or software) can reduce the error.

Keep in mind, Gm depends heavily on the plate current. On the left is the Gm characteristic of the 10Y tube by RCA. As you can see here, Gm varies from 520 to 2100 depending on the plate current. So it should be clear when somebody says he has measured Gm of 1600, this by itself has nothing to say. If he measured 1600 at a plate current of 60mA, the tube is bad, and when measured this is 10mA, this is in fact too 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 be compared with the data sheet value at any plate current one chooses, as long as you aware you are doing so. So it would be wrong to measure Gm at any random current, and then compare this with the data sheet, which specified this at the characteristic current. So random and characteristic is just not the same thing.

Here is a numeric example. The tube on the left is RCA 10Y. The datasheet 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 datasheet 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. It so variable, not even minimum and maximum values are mentioned. So a random, 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 a specified DC current, and never at a specified grid voltage.

Instructions of how to repeat the tests with the AT1000

Stand Alone mode

All 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 with wrong data tables will play no role.

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.

AT1000 in Stand Alone mode

  1. Refer to the manual, so you know how to do this.
  2. Inside the tester is a tube data table. If the tube is missing, you have to add it first, which is not difficult.
  3. Configure the tester in general for AUTO BIAS. This is most important, not just for EML tubes. In FIXED BIAS mode, Gm results will be INVALID totally.
  4. Do the testing, and verify if the tester used indeed the same Anode Voltage and Anode current as we have on the tube boxes.
  5. At the end of the test, the result will be: The Grid voltage (-Ug) and transconductance (Gm)

For higher precision, the computer controlled mode should must be used, which allows anode and socket heat up 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 t in the stand alone mode, can l make used tubes look less good as they are.

AT1000 in Computer controlled mode

  1. Refer to the manual, so you know how to do this.
  2. If a tube is missing, add it with the tube editor program.
  3. Alternatively use set up files This allows changes easier.
  4. Edit the tube data, until it is the same as on the tube box.
  5. Choose "Auto Bias" before testing.
  6. Under tests, choose only "Noise Test". Make sure, no multiple tests are selected.
  7. Start the test, and observe the anode current in the AT1000 display ALL OF THE TIME
  8. This will make the tube very hot. Anode rises slowly above it's initial value during warm up.
  9. Stop the test when Anode current rises above 110%
  10. If the test had to be stopped, restart the test right after. This will set anode current to 100% again.
  11. Again stop the test when Anode current rises above 110%, etc.
  12. You can use a head phone during that time, and check noise.
  13. After a total of 5 minutes the tube is thermally stabile.
  14. Stop the noise test, and do the "Tube test" immediately after.
  15. Now -Ug and Gm will be tested under the specified anode current, this should correspond to the tube box.

Using digital curve tracers

Devices like "utrace" and "etrace" 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 for 5...10 minutes. A tube should only be tested fully warm, including the glass stem. 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 beneft from this. Moreover grid emission exceeds limits when the tube is running close to maximum dissipation. At 75% of maximum dissipation, grid emission (may be called grid leakage) may reduce by 50% or lower. Though we have to say here, Emissionlabs tubes are not expected to develop grid emission anyway. So all in all etrace and utrace seem good 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 acount, you ae safe. We much recommended the etracer and utracer.


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 ouside 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 unuseual 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 of the new made tube testers. You do have to set the tester for DC heated test results, in order to find the same results as we do.


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