2 Tungsram tubes  ECC83 12AX7 

= matched pair



Excellent test result 
S/gm = 1,75 + 1,6 and 1,7 + 1,7mA/V (100% = 1,6mA/V - usable starts with 1,05mA/V

Final assessment: Only the slope/gm S/gm provides information about the emission
Further explanation: According to military standards, tubes with 60% (minimum value) were classified as usable.