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In Reply to: RE: Well for one I would not use an emissions tester. . . posted by nick-seattle on June 15, 2012 at 21:15:20
Another interesting point, I used in my amp for a couple of days - it was measuring 1.1 millimhos where rejection is 1.2 - worked beautifully. Now I tested it on Hickok and it shows 2.5!!! Every time. Is it possible that it's burning in?????
What brand of tubes are these? It's important to know, because I probably have the explanation - if I know what tubes they are.
These are Ei 12ax7 tubes.
The Ei quality was pretty awful towards the end of production - but I don't know whether that is what you are seeing. You may have tubes with cathode poisoning, and they need to "wake up" (by getting rid of the contaminants on the cathode surface) before they operate normally.
I'd recommend you run them for a few hours before testing them. And remember - the ultimate test of a tube is whether it works properly in a circuit!!
Thank you for all your help. I will continue playing around with the tubes to see if I can detect a pattern :) So far although they show below minimum on Hickok I'm loving them in my amp! Very surprised by the quality of sound.
...Although it should be a gradual phenomenon, not intermittent. I've experienced this with the 12AX7LPS. Another possibility is that the tester is oscillating under certain conditions. This is a lot more likely with hi gm tubes like 6550 and almost impossible with something like a 12AX7. One check is to put your hand over the tube while under test. If it's oscillating, the reading will jump around a lot while you do this. Hickok got around this tendency by using ferrite beads on all socket leads on their later testers. Another point to keep in mind is that a lot of Hickok testers way overdrive hi gain tubes like the X7 while under test. Only the best versions use an appropriate 0.25 or 0.5VAC as the test signal.
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