12AX7 Electro Harmonix, how we test them


12AX7 tube

So what makes a GOOD 12AX7? First, you need to set the tester for 250V plate, and then adjust the grid voltage until you have 1.2mA. This is called the auto bias methode. The average value must be 2.0Volt. Another test method is fixed bias, where you choose a fixed bias voltage of 2-0V and see what plate current does. Fixed bias is easier to do, but the value you find for transconductane is less useful, since transconductance must be tested an average plate current.

So for this critical tube, like 12AX7, we use autobias testing. From the historical datasheets the good/bad limits can be found, using +/-30% deviation of the plate current. We want to find the corresponding grid voltage deviations, which are the limiting values for a good tube. With a typical transconductane of the 12AX7 of 1.6mA/V that can be easily found with following calculations: Average plate current: 1.2mA. We take 30% of that, which is +/- 0.36mA deviation. To get 0.36mA you need a grid voltage change of (0.36mA)/(1.6mA/V)= 0.22Volt. So a good tube may have not more than 0.22Volt deviation of the avarage vakue of 2.0 Volt. Note, this is completely in line with the all historical parametric tube testers. What we talk about here, is the bias setting.


Example of a 12AX7 tested in Fixed Bias mode, with AMPLITREX AT1000 Tube tester
(This tube tested here, on the pictire, is what rejected tube looks like)

at 250V
1.2mA
1.7
1.8V
1.9V
2.0V
2.1V
2.2V
Grid voltage

For guitar

For guitar
HiFi
avarage
value
HiFi
HiFi

The above values are for ONE triode, but inside the bulb are TWO triodes of course. That gives the following table. When you see two "xx" in one colun, it means both triodes have that particualr value.

at 250V
1.2mA
1.7V
1.8V
1.9V
2.0V
2.1V
2.2V
What kind of tube?
Remark
 
Reject
xx
For Guitar (GT Series)
x
x
For Guitar (GT Series)
 
x
x
For Guitar (GT Series)
 
x
x
Reject
Such difference is not normal in one tube.
 
x
x
Reject
Such difference is not normal in one tube.
 
x
x
Reject
Such difference is not normal in one tube.
 
xx
For Hifi (GB Series)
x
x
For Hifi (GB Series)
 
xx
For Hifi (GB Series)
 
x
x
For Hifi (GB Series)
 
xx
For Hifi (GB Series)
 
x
x
For Hifi (GB Series)
 
x
x
For Hifi (GB Series)
 
xx
For Hifi (GB Series)

Transconductance: Lower than normal Lifetime However, is estimated from a loss of transconductance reduction (S or Gm) . Though theoretically this value may be 30% off as well, such a deviation is unaccepted for new tubes.

After incoming inspection, over the years, and over many shipments, there is an average result which is like this for 12AX7.