5b179a9d_Filament_Voltage_life__0
[1] Expensive USA or European tubes with excellent quality, good processing control methods. Reliable [2] Excellent tube quality, good processing control methods. Reliable [3] Import cheap tubes. Less-than-ideal material, processing, and testing methods and control [4] Less-than-ideal material, processing, and testing methods and control [0] 
Tube Quality. uncontrolled failures due to poor tube materials, processing, or construction
Filament Voltage life
0