The Mullard S3 triode is fitted with a small version of the I/S1 interservice base. The Mullard KB and later ORA 'B' used the I/S1 base to improve performance at HF.
Mullard's design aim with the S3 and S5 was to replicate the performance of the M-OV V24 and Q respectively. So that the S3 could fit into a V24 holder the smaller I/S1 base cap was designed. This is 25 mm in diameter and 23 mm long.
The S3 could be operated from a six Volt battery and needed little HT on the anode and found use as a local oscillator in superhet designs.
The etched Mullard logo with the 3 of S3 clearly visible. The S is very faint. The red spot marks the anode connection.
E314 is etched on this side of the glass. Above are the words Protected by Patents and above that is the type 3 BBC stamp. The presence of this stamp dates this exhibit to between September 1924 and the end of 1927.
The filament connections pass through the pinch and into the base but for the I/S1 type of base cap a copper strip on the outside of the glass moves the contact to the other end of the valve.
With different lighting the single strand filament is easy to see.
The lower connection on the ebonite base cap.
The anode here is at a slight angle to the axis of the grid and filament - not what was intended.
The top of the filament and grid within the cylindrical anode.
Optimised for the filament strand.
The copper strip is highlighted.
The S3 and V24 together. With these exhibits the V24 is 73 mm long and the S3 is 72 mm long. The fact that both will fit the same holder id obvious.
The thin glass tube envelope is 18 mm in diameter, and including the base is 72 mm tall.
Reference: 1003. Type S3 was first introduced in 1919.