The EFF51 has a designation for a double pentode and was introduced in 1941. The two cathodes are strapped together and connected internally to G3. This leaves sufficient pins for the rest of the electrodes.
This valve was used as a voltage amplifier in VHF television receivers in the late 1940's.
The wide glass tube envelope is 30 mm in diameter, and excluding the B9G base pins, is 60 mm tall.
References: Data-sheet & 3002. Type EFF51 was first introduced in 1941.
Absolute Maximum Operating Conditions¶
Updated February 02, 2013. shape:wide.glass.tube construction:all.glass type:pentode age:1940.1950 base:b9g heater:6.3v pins:9 pin:1.h pin:2.a pin:3.g2 pin:4.g1 pin:5.k.k.g3 pin:6.g1 pin:7.g2 pin:8.a pin:9.h