Tubes Tested on Amplitrex AT1000 tube tester (fixed bias mode). TWO GE VT-4-C vacuum tubes. Tubes test at 4320 and 4340 where 2340 micromhos is pass/fail. 120% / 120.5% Gm mutual conductance. Type VT4C tubes: Spec (new) = 3600 micromhos and 100 mA plate current.