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Re: Tube tester rating ambiguity

Posted by Chris Haedt (A) on July 7, 2003 at 03:58:54

In Reply to: Tube tester rating ambiguity posted by anthony v on July 6, 2003 at 22:02:40:

The typical mutual conductance tube tester sets a reject point for a tube at 35-40% below what an "average" new tube would test. Some testers may use tougher standards for voltage amplifiers (like a 12AX7), than current amplifiers(like a 6L6). The smaller Hickoks, show what an average tube should test on the roll chart, rather than the reject point. Adding to this confusion, is the fact that a test on one model tester will yield a number that cannot be directly compared with the mutual conductance figure from another model. A test of a 6L6 on a Hickok 600 may be 4900, but the same tube tested on a Hickok 539C may be 6000. This type of variation will exist even when both testers are properly calibrated. As you can imagine, all the old testers in use today are NOT properly calibrated. So--this adds to the problem.

The only way to make good comparisons is to know what a tube tests in comparison to the reject point of the tester. Sounds like you are being given this information in the auctions you mention. The remaining useful life is very hard to judge. This is because all tubes did not start out the same. You used an example of a tube testing "42". This tube could have started out life with a reading of "45" and have lots of life left. OR the tube might have started out at "60" and have thousands of hours on it. Tubes varied alot from brand to brand, but most tube circuits were pretty forgiving and this did not matter. In other words the "average" tube was just that. Some new tubes tested much higher and some lower.

Tubes suffer many defects, but usually "wear out" due to loss of emisson from the cathode. The time to wear out varies from tube to tube, and by the circuit the tube is placed in. Most tube testers attempt to predict remaining life by doing a test that reduces the voltage applied to the filament. This lowers the temperature of the cathode. This temperature reduction will have very little effect on the reading a new tube gives, but a worn tube's reading will drop off significantly. Cathode emission is greatly influenced by temperature, and that is why the test works.

I hope this information helps. There really is no easy answer to your question. There is an inherent lack of precision in this whole area, which charms some folks, and drives others crazy. But--if you think about it--it's the variation in tubes that is the core topic of this Forum. If all tubes were exactly the same, there would not be much to talk about.

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