The CV4006 is the UK government procurement specification for the commercial 6059 which is itself a special quality version of the type 6BR7 low noise sharp cut-off HF pentode.
The specification gives the prototype as the VX7081. The valve was carefully designed for low internal noise and, especially, for low internal hum pick-up from the 6.3V AC heater, thus making it unnecessary to provide a DC source of heater current.
The valve is protected against vibration damage by the use of four mica sheets. Commercial valves with the minimum of mica separators will, under vibration, elongate the holes in the mica and eventually fail.
The considerable number of screens around the working electrodes effectively hides them from view.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: Data-sheet & 1040. Type CV4006 was first introduced in 1957.