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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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There is no sub-forum for "tube testers", so I just post it at "Tubes" here. Hopefully, giving some more attention to this software error, will help to get things moving, which is beneficial to all of us Amplitrex AT1000 tube tester owners.
A major issue is the grid voltage of DHT is represented wrong by the tester. This issue is only for Directly heated tubes (DHT) and for indirectly heated tube it doesn't matter. Reason is simple: The tester's hardware by definition connects one end of the heater to ground, the other to the heater DC voltage. However, by definition of tube datasheets, the grid voltage of DHT is defined to the CENTER of the filament. This definition can not be changed. If you would do so, you get test results deviating from the real values the tube have in the manufacturer datasheet.
Let's describe the problem by using the 6B4G tube. (This is a 2A3 version with 6.3 Volt heater). So curves and operating points are the same as with 2A3. That is: At 250V Plate, and -45V grid, a bogey tube draws 60mA. You can look this up in the 2A3 or 6B4 datasheet. (Bogey tube is a word for a tube that has exactly the data sheet values).
So this means if I have such a tube here, and I put it in a circuit with 250V, -45V, it will draw exactly 60mA. When I put that tube on a professional vintage tester, like Metrix U61B, Neuberger RPG375, or any other tester of this kind, that data will reproduce. So these testers read 60mA and nothing else. Besides we need to understand, this 60mA is not a result of a test method, it is an Inner Value of that tube. This tube is 60mA, at 250V /-45V and nothing else. Any tube tester that tells you another value than 60mA is making a mistake. For instance of you test the tube with 10% too low heater voltage, you sure will not get 60mA any more, but perhaps 50mA. It would be wrong calling this a tolerance problem of the tube.
The problem with the AT1000 is the grid voltage. With the 6B4 tube, that we take as example here, the heater CENTER is at +3,15 Volts. (So that is half the heater voltage). Then the grid gets supplied by the tester with -45V. So the resulting voltage between grid and the heater center is -48.15 Volts, and not -45 Volts. So we are -3.15 Volts off specs. Now with the transconductance of 5.5mA/V this would roughly result in 3.15 x 5.5 = 17.3mA less plate current. So instead of 60mA you get 42,7mA only, and the tester would say this tube is at 71% only. Though in fact this is a 60mA tube and it is at 100%.
This problem gets less with tubes that have lower heater voltage. So when doing the same exercise with a 2A3 bogey tube, you would get 53mA instead of 60mA. So I think nobody ever noticed this!
To my personal understanding this is an error with the tester's software, and should not be called an alternative test method. Interesting, this test method DID excist with some of the very old tubes from the days before we had a mains voltage, and tube equipment ran only on batteries. Many of such very early receiver tubes were defined with the heater grounded on one end, because.... that's the way you were supposed to use them! ut I must say these were extremely rare tubes already by then, and all later DHT tubes were defined for AC heating, which means the heater center being the reference for the grid voltage.
The sofware correction for the AT1000 is very easy to do. Just subtract in software half the filament voltage from the grid voltage. That is only for DHT tubes of course. So if the user want to apply -45V to the grid of a 6B4G, and the tester lifts up the heater center 3.15V, the tester must output -41.9V to the grid, the tube will read 60mA as it should.
I understand from Chris Terraneau (Amplitrex company owner) this is what he has is mind as a wprk around for the problem. Only my objection is with the way it is done. When I understand it correct, the tube test data table is adapted simply for the higher grid voltage. The first problem is the way this is displayed on the test results. So the tester is going to come up with a tube that tests 60mA at 250V -41.9V. What is right is the 60mA. What is wrong is the -41.9V. This is in reality -45V for the 6B4G. It may appear not so bad in the stand alone mode, since users are likely focussed on the plate current adn transconductance they get, and these values are displayed correct.
However the TEST CONDITIONS are now displayed with an error in the grid voltage. This gets visible more clearer when you create a printed test result. If anyone tries to verify those test results with another tester (which has no such error) he would set his tester for those conditions. If he uses -41.9V, he finds 77,3mA instead of 60mA as on the test report, and I get a complaint. So 77,3mA vs 60mA is really an unacceptable error.
Another issue is when somebody sets up new tubes with the editor program, and also the tube curves are plotted with all grid voltages wrong. This doesn't get solved this way.
So all in all, the proposed solution is very unsatisfactory. To my understanding the only realy good way it to correct the grid voltage in software. That would remain the tube data tables unchanged too.
To have no misunderstandings, I LOVE the AT1000 for it's realiable hardware. It is definitely worth it' price, and much recommended. Also the Email support with technical questions is just excellent. The weak point from my view is the software, which doesn't get out of this product what could be in it. I send several proposals to Chris Terraneau before which would greatly improve the User's value of the tester, but so far only one was realised. (This was the a loud clack noise with Audio output when using a headphone).
Generally I would say, we need not always a "work around" for a problem. There comes a time where the software needs a general update, and all known issues get covered.
Jac
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Topic - Amplitrex Tube tester Error. - EML 12:16:59 04/28/12 (1)
- RE: Amplitrex Tube tester Error. - coolhand 04:13:04 05/3/12 (0)
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