(!!!!) (Best Pair #2) 6688=E180F RCA by Mullard UK gold pins 1974 (8.8ma and 9.0ma) (strong Gm)
6688 GE 5 Star used/good (7.2ma)
6688 GE NOS 1966 (9ma)
(!!!) (Best Mullard Pair) 6688=6688A=E180F Mullard Philips SQ and Mullard RCA gold pins NOS 1971-1974 (11.5ma and 11.5ma)
(!!) (Single) 6688=E180F Telefunken Germany <> bottom NOS gold pins 1968 (9ma)
6688=E180F National UK black plate NOS probably made by Amperex (9ma)
6688=E180F National UK gray plate NOS probably made by Amperex (16ma)
(!!!) (Single) 6688=E180F Amperex PQ USA NOS "O" getter 1968 (9ma)
