Author Topic: Valve and tube testers  (Read 2630 times)

Offline stevenvalve

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Valve and tube testers
« on: October 19, 2015, 02:37:56 PM »
I have sold Valves, Tubes, around the world for many years, always tested on my mint Hickok 539C, Hickok 6000 and Taylor 45C valve testers. I often wondered why some valve testers give strange readings particularly the modern designed Amplitrex AT1000 and some vintage AVO testers including the mark/4 so I did some research. Be carful when buying valves tested on these machines, From personal experience i am suspect about them. Here is some info

The Amplitrex AT1000
The problem is rather the Amplitrex software is not 100%. Please let me explain. The hardware of the Amplitrex AT1000 is capable to test Emission, but the software is not. What this tester is doing, measured plate current is related to the average plate current as in the data sheet. So if the average is 60mA by the data sheet., and your tube is 50mA, Amplitrex says emission is 80%. Sorry, but this is plain nonsense. Emission is tested completely different, but it is beyond this explanation here. Fact is, what is measured by Amplitrex is deviation of the average current, and since the present it in "percent", you can compare is quickly to the tolerance. So what is the tolerance of Plate current for new tubes? For this we refer to the Funke Tube tester Manual of 1959, saying it is +/- 30% for European tubes and +/- 40% for American tubes. Meaning, that any tube within these values is regarded good. Any tube, new out of the box, may have this tolerance. So you get plain wrong information by the Amplitrex, telling you so called "emission" is 90% whereas what this means is -10% deviation from average plate current, and emission in reality can be at 130% for the same tubes. We can not explain in 10 lines here how to test emission, and it is sad enough we have to say this about the Amplitrex. By the AT1000, also Gm is tested the wrong way, if done so in the FIXED bias mode. Of course you will measure something, but what were you looking for? Probably you are trying to compare the measured Gm, with the data sheet value. Well, for that you must do so under the same conditions. That is at that is always average (=100%) plate current. Not at a random plate current which results by coincidence from the fixed bias mode. So to stay with the numeric example we already had here, if the tube has 60mA in the data sheet., you CAN test it in fixed bias mode, which may be 50mA for this (individual) tube. If you do so, you are getting a Gm value which can not be used to judge how far Gm is off specs. You can only do so at 60mA.

Furthermore there is a serious issue with the grid voltage interpreted wrong by the software, but this gets beyond the scope of the FAQ here. Conclusion: This is very good tester, but it has it's issues too. We recommend to use this tester in the AUTOBIAS MODE only, and then indeed results should be identical to the test results as we write on the tube boxes.
« Last Edit: October 19, 2015, 02:44:18 PM by stevenvalve »

Offline stevenvalve

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Re: Valve and tube testers
« Reply #1 on: October 19, 2015, 02:42:21 PM »
AVO testers have a lot of trouble testing 2A3s 300Bs etc, I know because I used to own one, more info


AVO Mk4 Tube tester
The AVO Mk2, Mk3, Mk4 are very nice for small tubes like 12AT7 etc, but not suited for triodes with low plate impedance and high plate current. AVO admits this when you look in their data tables for the 6080 tube. Also you will not find the 300B in most of their data books. That should tell enough. You can work around this, when you have a pair of new EML 300B (or 300B.mesh). The factory test values of our tubes are very accurate and guaranteed to reproduce within 5% on any good tester. With the AVO you need to set the plate voltage as on the EML tube box, and initially use -60V grid. Then change the grid voltage until you have plate current as indicated on the EML tube box. With this grid voltage you should measure the transconductance. The value should be close, to what you see on the EML box now. Leave the tester as is, and plug in the other EML tube. If both tubes show the correct transconductance this way, they are ok and your AVO also. Any differences are due to the AVO measuring with AC voltages instead of DC, and also many AVO need a new calibration, and by default are nor accurate on the 100mA setting, while using tubes of impedance below 1000 Ohms. This is a shortcoming of the AVO, which the factory has always tried to hide. Even the 2A3 measurement results, they publish faulty in the data book, so to hide this shortcoming. At jacmusic.com under tube testers, you will find a correction formula to use. Like this you can measure with the AVO the normal way, enter the result in this formula, and the result is the REAL plate current as it should have been. If you use the formula, results will be almost correct, but do mind the EML method id DC heated, so you need also test at a corrected grid voltage, of half the heater voltage. When also apply this correction, AVO results must be 100% correct

Offline stevenvalve

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Re: Valve and tube testers
« Reply #2 on: January 20, 2016, 09:23:26 PM »
More info on the Amplitrex 1000 problems

http://jacmusic.com/Tube-testers/Amplitrex/index.htm

Offline zenelectro

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Re: Valve and tube testers
« Reply #3 on: January 25, 2016, 02:48:38 PM »
More info on the Amplitrex 1000 problems

http://jacmusic.com/Tube-testers/Amplitrex/index.htm

It's worth bearing in mind this guys is developing his own tube tester and will, I assume, compete with the Amplitrex. 

Most of those problems should relatively easy fixes for Amplitrex - so they should be made aware of them.


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