1 tube) 6B4G Sylvania rebranded Delco NOS 1952, separate plates

(81ma Gm=6700) strong Gm

Test Data: Vplate=225V, Vgrid=-30V

(NOS Ma range 80-100ma)

(NOS Gm=5200)

(6B4G=6.3V filament)
(6B4G is electrically equivalent to 6A3, but 6A3 has UX-4 pin base and 6B4G has 8 pin octal base)
(The best sounding 6B4G are 1940's-1950's: Ken Rad, Tungsol, Sylvania, Raytheon)