The 2C39A is a planar design of disc seal construction. This UHF triode was designed for use in a co-axial cavity with forced air or water cooling. Many designs exist for the use of this valve as a power amplifier at 1.3 and 2.4 GHz. In some designs it was also used beyond its ratings to provide some output at 3.4 GHz but varactor diodes were competitive on this band.
The labelling of the valve is on the anodised aluminium cooling fins.
The very small gap between the long copper anode and the grid mesh is clearly visible in the above picture.
The planar envelope is 32 mm in diameter and 66 mm tall.