The CV4029 is a special quality EL71 sub-miniature audio output valve. The glass carries two designations: 5902 and CV4029.
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 two inner grids are aligned and pass flat across the cathode. G3 looks to be four rods that connect to the top screen, effectively a beam tetrode.
The heater wire is coiled and insulated.
The thin glass tube envelope is 10 mm in diameter and, excluding the base wires, is 41 mm tall.
Reference: Data-sheet. Type CV4029 was first introduced in 1954. See also1954 adverts.
