Tube Identification

[Wayniac]

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.

 

[zelgall]

Measure the resistance on the heater pins. The ax types will all be close and you have a single au with markings. The one that measures the same as the au is the other au.

 

[Wayniac]

Thanks for the quick reply. I didn't realize filament resistance was a variable within a given series of tubes (12**7).

Do you have any opinion on the possible manufacturer of these tubes?

 

[Parafeed813]

If you have good eyes you can peek through the hole in the anode and spot the grid.
The grid of the 12au7/ECC82 has wider spacing than that of the ax/83.
Heater resistance should be the same as both types draw .3A @6,3V.

 

[DF96]

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.

 

[Wayniac]

Thank you gentlemen, for the replies. I was able to use Parafeed's method and discern the difference. I used a bright light and a magnifying glass. The difference is easily noted once you know what to look for!

 

[kszd]

they have different anode current set milliammeter in the anode

12au7 current of about 5-6 mA
12AX7 current of about 1.0-1.7 Ma