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.
Test result for Gm.
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
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
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.
Subscribe or unsubscribe our Mailing List