The 6GM8/ECC86 is very special high frequency double triode. At a time when AF power transistors were replacing valves but no VHF transistors were available the 6GM8 was designed to operate directly from vehicle battery supplies for car radio applications alongside transistors.
This valve was used as the VHF input stage of car radios. The corresponding IF stages were EF97 and EF98 which were also designed to operate from vehicle supplies. This technique was used until the early 1960s, but the ECC86/6GM8 was especially problematic. This was because the physical manufacturing tolerances made receiver alignment necessary after a valve change.
This German made valve has the Amperex branding.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
Reference: Datasheet. Type 6GM8 was first introduced in 1959.