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Been testing some 27's for use in the SE 27 - 6A5G amp I've posted about on the DIY board. I have a couple of ST mesh plates that test new or near new, depending on the tester. Results on my Eico 666 were 111 and 110. The Hickok 600A has them at 1050 and 940 (1000 is new). No shorts or inter-element leakage. But they both fail the life test on the Hickok.
I'm never surprised if a weak tube fails the life test but it seems odd that a tube that tests so strong would fail the life test. Anyone have any insights?
I've used these before in a previous version of the amp. That was before I had the Hickok. They sounded great so I'm not concerned about using them again now.
I guess I'm wondering what to expect. Does anyone have any experience with how long a tube that fails the life test will actually last? Is the transconductance likely to drop quickly? How important and/or accurate is the test?
Follow Ups:
Personally, I've found "life" tests on a tube tester most relevant for power tubes and borderline meaningless for voltage amps and such like the 27.
Most Hickok testers check for "life" by lowering filament voltage by ~10% or so and noting what happens to either gm or emission depending on tester. I don't bother with this test for common signal tubes. Also, for power tubes I've found that lowering filament voltage by one or two notches (5 or 4.3V for a 6.3V filament) is more discriminating than Hickok's test. A new tube easily meets min gm while a well used tube generally will not. But you need to know how your tester will react under these conditions in that weak testing tubes may be entirely usable depending on app. And as always, YMMV.
Especially for power tubes because most testers just do not put enough DC across the tube to get to get a real indication. I measure B+ across a KT88 from plate to cathode in the Hickok 539c and it was 150vdc. When I measured it across the Heathkit TT-1,it was 275vdc.While neither is near the operating voltage of a typical KT88,that extra 125vdc does help to give a more accurate picture of the tube IMHO.I do like some tests on the 539c such as the noise test with an AM radio however,I did set up add a couple input and output jacks where I can inject and monitor the signal of dual triodes on the TT-1.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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