The type VMS4B typifies the swansong of the screen-grid receiving valve with anode top cap. It is an improved, vari-mu RF/IF tetrode intended for domestic superhet receivers but it represents the final milestone on the road which started (barely 10 years earlier) with the Type S625. Like the earlier valve the envelope has a straight side at the base to facilitate mounting through a chassis screen.
The balloon envelope was replaced by the shouldered or classic envelope and this can be seen on our other VMS4B exhibits.
The zinc paint is showing some corrosion. The Marconi brand is printed on to the metallising. At the left of the image above the base can be seen the wire connection to the paint.
The construction is hidden but the anode connecting to the top cap reveals supports going either side of the rectangular screen cage. the anode is thus a pair of thin sheets placed either side of the screen. With the later move to the classic envelope shape and the use of construction between mica discs the designer had the freedom of choice as to the electrode brought out to the top-cap connection and the control grid moved to the top connection. These changes were made possible by the move to improved methods of oxide coating of the cathode and the consequent move away from the azide process
The balloon envelope is 45 mm in diameter and, excluding the B5 base pins, is 119 mm tall.
References: Data-sheet & 1005 Type VMS4B was first introduced in 1933.