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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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In Reply to: Tube strength and readings posted by Maddnotez@gmail.com on September 7, 2014 at 16:59:56:
There are tube testers and then there are really good tube testers. Typical tube testers are not really typical; due to their vintage, original cost, type and manufacturer. There are generally two types; the generic "emission tester" and then the favored and much more reliable "transconductance or "mutual conductance" type.
Emission testers simply connect the internal elements of the tube under test together to set the tube up as a diode. In this way, they marginally prove the tube can function. If we try the "life test" by lowering the tester's filament voltage one voltage setting, (say from 6.3V to 5V filament or from 12.6V fil. to 10V fil), we can get a better idea of the tube's age and abilities. However, we cannot conclude how the tube will perform under "real world" receiver or amplifier actual "in use" conditions. This is where the transconductance tester excels.
Transconductance testers were high priced when originally sold; compared to the low-cost emission testers. These mutual conductance testers actually set the tube up as they were intended; to conduct or pass current according to their proper voltage relationships within their internal elements. To make it easier to understand, realize that when certain tube elements are energized properly, they conduct properly; that is: they pass signal accordingly. Proper voltages on the right tube elements will show proper current flow; measured in micromhos.
The best micromho testers were, and are still very costly; even the very vintage units. Consider the Hickok testers; worthy of research and acquiring BTW...With any Hickok transconducatnce tester that is measured indicating micromhos, their specific test will yield results with a 12AX7 that are fairly common; within the Hickok tester family. Minimum acceptable for ECC83/12AX7 is either 800 or 875 micromhos (depending on which Hickok tester used)which represents a 65% still usable tube. While new 12AX7s typically test at 1250 micromhos with the Hickok tester circuit, many 12AX7s might measure even higher in micromhos. A 12AX7 that reads 1150 micromhos is still very strong. One that reads 800 or even 700 might still be very usable. The lowering filament "life test" is even more reliable when viewed with a transconductance tester.
While some emission testers have very sensitive "shorts" and "leakage" tests, they still cannot indicate the real usability of a tube like the venerable transconductance tube testers...
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Follow Ups
- Tube strength and readings are very tester dependent.... - Interstage Tranny 18:36:29 09/07/14 (0)
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