The CV807 (commercially the 3A4) miniature output pentode is specified for both audio and transmitter use. The CV807 is the same as the commercial 3A4 and the envelope has both Type designations. For a small dry battery valve 1.2 Watts output in the HF range would be quite an achievement. Clandestine radio springs to mind.
The KB/D identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the D identifies the maker as Mullard Radio Valve Co., Mitcham.
The twin filament strands are held in tensioning springs and exit at an angle from the top mica thus keeping the strand vertical as it passes through the valve.
Little detail is visible.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
