Amplitrex Tested Tubes

When we advertise tube from our website, with the words Amplitrex tested, it means tube tube data is taken with the Amplitrex digital tube tester, and a computer print of the data is provided with the tube. The Amplitrex is at this moment the best new made tube tester you can buy.

I have this saying, that I repeat all of the time: A "like new" testing used tube is better than a "like used" testing new tube. For instance the factory-new Fullmusic tubes, come out of the box with values similar like fairly used tubes. Similar effects can be observed with too long stored NOS tubes.

If a tube tests good, then expected Lifetime is of course what we all want to know. However there is no way to measure this in hours. We can only derive it more or less meaningfully from other parameters, and it requires an experienced technician. Remaining lifetime can be derived from the transconductance, but it must be said transconductance is not the same as lifetime, and the relation is not a perfect one.

Another method is to use each tube in the diode mode, which is more directly looking at the cathode quality only. With the right kind of tube tester, and teh right interpretation of what you measure, this is a very good way to estimate tube quality. Such testers like FUNKE W19 try to draw relatively high current at relatively low voltage, which is hard for used tubes. This method requires also a good database that you have to make yourself, since nobody can help you with limit values for "bad" "?" and "good". For this reason the Amplitrex works by the transconductance method, since it is more practical and can use data sheet values. Transconductance is difficult to measure with lower technology testers, because it is a so called dynamic parameter, and for best results requires a calibrated tone generator of 1000...2000 Hz. So you get no confusion with 50 Hz hum signal, or harmonics components of this. So precise transconductance measurement requires a lot of hardware. The good thing though, it is relatively independant of various settings of the tube, which makes it a reliable number to say something about, and convenient because you can't do much wrong. All good quality old fashioned tube testers with a red-green scale worked that way. Transconductance must be close to, or above the factory value. (in the Spec column). When it's above, is generally very good. When it is below, is also fine as long as the DC plate current of the tube is high enough. Anything above 80% of the factory value is considered a "new" tube.

What do we see in this test report?

In the table "Spec" you see the settings used for the test. These are always the standard settings, by the original manufacturers, unless there is a very important reason to take other values. With the Amplitrex you can load the manufacturer data from the database. Now the ECC82 is a double triode, so two triodes in one bottle. So there are two tables, Result1 and Result2. We expect two main things: First, the test result should be close to the "must be" value from the "Spec" column. Second, the Columns 1 and 2 should be as equal as possible, making this a so called balanced tube.

Auto Bias vs Fixed Bias. Of course the tester can do both. Each has is own advantages. In auto bias, the tube is set by the normal plate current, by simply finding the corresponding grid voltage for that. Since transconductance is depending a lot on the plate current, comparing the measured value by a data sheet value makes only sense if measured under the same conditions. This is the nice thing of auto bias. So if a tube is specified at lets' say 10mA, and 5mA/V the tube is forced to biased at 10mA, and THEN transconductance is measured. So auto bias. gives the most reliable transconductance measurement, that is why we use it. Fixed Bias has another purpose. This is the older European test method, a very hard one, to measure the plate current with the fixed grid voltage. This tells in some case something about the remaining lifetime, but not always Only when the tube is below 60...70% of normal, a lower lifetime may be expected. Smaller deviations than that are tolerance, and can be found with factory new tubes as well as with used ones. So be careful saying tubes that are below the average value have less lifetime. If a used tubes is above average, you would not say (based only on this) it has more lifetime than a new one. By the same reasoning you can not say (based only on this) a used tube that is below average is having less lifetime than a new one.

DC Plate current European tubes must be within +/-30% to the "Spec"" value, and if so the tubes are good. USA tubes can be +/-40%

Tube gain is a quite constant parameter, and when gain is too low, there is something not so good with the tube.

Grid leakage can offset a tube in auto bias circuits, so must be low.

H/K leakage means heater to cathode leakage, this is a very important number since this kind of leakage causes tube hum if AC heated.

OTHER parameters are for the specialists.

In the end, we match the tube by the test reports, and those that we can not match get put back to the unmatched tubes. Bad ones get sorted out.

PERCENTAGES

Amplitrex AT1000