BSA Standard sold STC valves during 1926. The P425 is a battery output triode and some operating details are printed on the box. What is not given is the output power.
Through a gap in the gettering the electrodes can be seen. The anode is a flattened cylinder with two side plates holding it. The side plates pass down to a collar around the pinch stem. The collar has small crimps around the circumference that probably allow some 'give' to enable the collar to be a tight push fit onto the cylindrical pinch stem.
The anode retaining collar is just visible on the left. The anode is a flattened cylinder and the control grid has rounded edges that match the anode. The filament is a single inverted V that is held in tension by the spring fixed into the glass bead at the top.
Here the anode support plates can be seen to have turned-up edges that add considerably to the stiffness of the structure. The helical spring has a hook at the lower end to hold the thoriated tungsten filament.
This image shows the filament and the tension spring more clearly.
The data-sheet packaged with the valve
The balloon envelope is 59 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: 1043. Type P425 was first introduced in 1926. See also1926 adverts.
