(1 Pair) 31 RCA/Cunningham NOS 1930's - 1940's (12.0ma and 13.2ma) (Same Gm)

1 pair) 31 RCA/Cunningham NOS 1930's - 1940's

1 tube) 12.0ma and Gm=1100 RCA/Cunningham engraved base

1 tube) 13.2ma and Gm=1100 RCA red print taller bottle

Tested on Amplitrex

Vplate=180V

Vgrid=-30V

Good NOS Ma = 11 -12ma

Good Gm = 1000