The F2 from Louden is a bright emitter triode that was introduced in 1924. The F1, F2 and F3 were introduced as detector, RF and AF amplifiers. The blue top F2 had a higher AC anode resistance than the others. All were made with wire electrodes including the anode. Later the FER series were made with similar construction but fitted with thoriated tungsten filaments. SeeFER1.
The electrodes are obscured by the gettering. The blue top is distinctive.
The Base shell is black composition and the pins are simple split-pins with the lead-out wires soldered at the top.
The blue designation of the F2.
The basic electrode configuration. A is the anode spiral, B the grid and C is the single strand filament. D, E & F are the supports back to the pinch.
Thanks to Ron Deeprose for sending the valve pictures.
The balloon envelope is - mm in diameter, and excluding the B4 base pins is - mm tall.
Reference: 1003 Type F2 was first introduced in 1924.