(!!!!) (2nd Best Single 1950's) ECC33=6SN7GT Mullard black plates platter getter halo NOS 1950-1955 (8.3/7.5ma)
(!!!!) (Best Single 1950's) ECC33=6SN7GT Mullard black plates platter getter halo NOS 1950-1955 (10.2/12.2ma)
(!!!!) (Best Single 1961) ECC33 Mullard gray plates "D" getter halo NOS 1961 (9/9ma) 1-2% section balance