Tubes Asylum

Questions about tubes and gear that glows. FAQ

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RE: This might be a dumb question but that's my specialty

Posted by Tom Bavis on January 20, 2017 at 08:12:13:
Tube tester readings don't always correlate with amount of usage or remaining life - a tube reading 80% COULD last longer than one reading 100%. And tube testers don't read amplification factor - they read cathode emission or transconductance, which is CURRENT amplification, not VOLTAGE. A 12AU7 (mu=20) will peg the meter if tested as a 12AX7 (mu=100) - it has twice the transconductance and higher cathode emission. Actual performance will depend on circuit - many weak 12AX7s will operated as well as new ones if the circuit is very low current - as most voltage amplifier stages are. The same weak 12AX7 may perform very poorly in a driver stage with more current.