(!!!) (Recommended Pair) 6C8G National Union round black plates 1950 era used/very good (3.4/2.8ma and 3.3/3.4ma) (very close Gm)

1 pair)  6C8G National Union round black plates 1950 era used/very good in white boxes

1 tube) 3.3/3.4ma and Gm=1600/1700 small section of getter flaked off glass - see photo - does not effect performance of the tube

1 tube) 3.4/2.8ma and Gm=1700/1600 

Tested in Amplitrex

Vplate = 250V

Vgrid = -4.6V

Good NOS Ma = 3ma

Good NOS Gm 1500-1600

dual-triode

You an buy an adapter socket on EBAY to sub this tube for a 6SL7GT