The S625 RF tetrode was a design classic, and became a bench mark for later developments. It was introduced in 1927, and had a dull emitter filament (Thoriated Tungsten). The amplification factor was 110. The valve was designed to be used horizontally with the grid and anode circuits in different metal compartments. The valve would be aligned so that the screen grid lay in the circular cut-out of the chassis for maximum effect. However, here we see that Marconi also made a horizontal plastic base for the S625.
The electrodes are arranged as a set of discs along the length of the valve from one base to the other. The 'V' shaped filament is enclosed by the wire cage of the control grid, this is loosely wound on notched rods. These electrodes are taken to the base three pins. The screen grid is a metal mesh disc that is held on the top of a cylinder that encloses the anode. The anode is not visible. The top pins connect to the screen and anode.
Each of the five pins is held by a pair of springs that fix into brass supports that are held in the base. External connection is via the screw on the side of the base.
The filament and control grid end.
The base without the valve. The four holes are for the fixing screws but the two screws in the centre screw into the base itself. The ridge in the base has a flat vertical face and probably the design idea was that an L shaped metal plate with a hole for the valve would be fixed to the base and aligned with the screen grid thus shielding the grid end from the anode.
The function of each part of the holder is present moulded into the plastic. The base is 122 mm long and 45 mm wide the overall height is 44 mm.
The wide glass tube envelope is 32 mm in diameter and, excluding the B4 base pins, is 97 mm tall.
Reference: 1005 Type S625 was first introduced in 1927. See also1927 adverts.
