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Tube Identification


2010-10-12 5:42 pm
I have 6 tubes which appear identical. Two of the tubes are 12AU7. The remaining 4 are 12AX7. The painted-on markings are almost gone. I can only identify 1 12AU7 by the markings. I have a basic emissions tester and all tubes read between 59-65 on the meter.

What is the best way to identify the other 12AU7?

These tubes came with a pair of McIntosh MC30s which I restored.

The tubes are branded "The Fisher" but I'm not sure of the manufacturer.


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Wayniac said:
I didn't realize filament resistance was a variable within a given series of tubes (12**7).
It isn't. All have 12.6V/150mA or 6.3V/300mA heater so indistinguishable.

The only way to tell is to measure a characteristic which is different for the different types, such as transconductance or amplification factor. As the 12AX7 and 12AU7 are so different you may be able to tell simply by grounding the grid, connecting the cathode to ground through a 1k resistor and applying around 100V to the anode; measure the voltage across the resistor. If it is around 1V then it is a 12AX7; if significantly higher then it is a 12AU7.