The DE5 is a dull emitter triode designed to be used as an audio output valve to drive small loud speakers.
The DE5 was introduced in May 1924 and this exhibit carries the first BBC stamp etched into the glass. This has the words Type Approved by Postmaster General around the edge. This stamp was used from 1st November 1922 until September 1924. Thus we have good reason to believe that this form of the DE5 was the first to be introduced. The valve in the advert from May 28, 1924 has a metal skirt.
The words Marconi Valve are etched into the glass. Within the envelope the plain anode can be seen.
The central strut carries the tension spring to hold the top of the single inverted V filament. The grid winding is also visible through the silver gettering.
Between the words Marconi Valve is the Osram logo and the words Made By Osram Lamp Works can be read.
The balloon envelope is 56 mm in diameter, and excluding the B4 base pins, is 103 mm tall.
References: 4051 & 1003 Type DE5 was first introduced in 1924.