The DE5A is a dull emitter triode designed to be used as an audio output valve to drive small loud speakers. The DE5A has a lower impedance and can thus supply greater power than the DE5, but has only half the gain.
The DE5 developed into a family with DE5A and DE5B as well as the original DE5 valve. This valve is half silvered showing that the valve was magnesium gettered.
This trio of exhibits has Marconi valve and BVA on the envelope but not the BBC logo, suggesting these date from post 1928.
The base pins show the external attachment of the lead-out wires. The pin construction shows a central pin surrounded by an expanded sleeve.
The evacuation hole in the pinch is visible on the left and the fine wire grid as in the centre. The discolouration of the supports shows where the spot welds were made during assembly.
The envelope dome has the type designation etched into the glass.
The balloon envelope is 55 mm in diameter, and excluding the B4 base pins, is 92 mm tall.
References: 4051 & 1003 Type DE5A was first introduced in 1924.