The DER was a landmark valve and marks the move away from the bright emitters of the early years. The DER stands for Dull Emitter Receiving and was introduced by M-OV in 1921 as a general purpose valve. This exhibit came from an ebay auction in 2009.
The lead-out wires pass through the composition base and are wrapped and soldered to the two part pin. The base has a moulded ridge to denote the correct alignment, this ridge marks the anode and the letter A is moulded into the base and filled with white paint.
The DER and BBC stamp carries the words 'Entirely British Manufacture' all wording is etched into the envelope.
The base has the M-OV logo and the pins consist of a central pin surrounded by the bottom section with the petal like split top.
The electrodes are mounted horizontally. The DER has the same construction as the R-type but the single strand filament is thoriated tungsten and not pure tungsten.
The filament is held at either end but there is no tension spring to counteract the sag as the filament lengthens when operating at 1,800°C.
The pinch and supporting wires can be seen. The red platinum at the point of the pinch seal is clearly visible.
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 77 mm tall.
Reference: 1003. Type DER was first introduced in 1921. See also1921 adverts.