2C51 BLACK ANODE NOS Double triode
The 5670 is a rugged special quality double triode developed from the 2C51 the 5670W should indicate a special quality variant but there is little visual difference between them. It is possible that the suffix has less meaning when the original Type was a special quality type.
Applications include: mixers, oscillators and amplifiers operating to VHF. Also for use in multivibrators, synchronising amplifiers and industrial control. For airborne use it was certified up to 80,000 feet. It additionally had low microphany. The latter attribute brought it to the attention of audio amplifier builders.