The 3C45 is a (triode) hydrogen thyratron for pulse modulator duty. It would work with a peak anode voltage of 3 kV and a pulse power of 60 kW.
It was originally used in some WWII small airborne radars but remained in the lists until the late 1970s.
In-service life was rather short so the valves had to be replaced quite frequently. There are plenty of relatively modern specimens around, mostly in unused condition, but WWII originals are now rare. Modern specimens include a small hydrogen reservoir cylinder absent in the originals and this example.
The electrodes are firmly held, and the stout wire to the top cap is insulated by glass. The filament is vertical within the anode.
The anode is clamped to a glass tube that is fused with the envelope and has the anode connection within it. The anode is a circular flat plate partly seen through the mesh.
Looking up into the envelope reveals little detail beyond the bright cylinder that is the control element or grid.
The wide glass tube envelope is 38 mm in diameter and, excluding the UX4 base pins, is 107 mm tall.