The DER (Dull Emitter Receiving) is a triode with a thoriated tungsten filament, and follows early valve design layout the of a horizontal anode cylinder containing a wire helix grid and a single thin wire filament. This example has a nickel sheet shell to the base. The first DER's produced had vertical electrodes but were microphonic and the horizontal arrangement was soon adopted.
The valve is marked to indicate that it is a Marconi Valve made at the Osram Works. The DER identification appears near the base below the type 1 BBC stamp indicating that the design has been approved by the Postmaster General. This valve was thus made between November 1922 and September 1924.
The filament voltage is 1.8 to 2.0 Volts at 0.4 Amps. Clearly designed for use with a lead acid accumulator for the filament supply. The anode voltage is to be 30 - 50 Volts. Early DER's suffered from poisoning of the emitting surface because of poor vacuum and consequent bombardment by ions. Gettering improved the vacuum and by running the filament at 1.8 volts via a rheostat and limiting the anode voltage the DER performed well for many years.
The construction follows the pattern of the R Type. The helical grid is welded to the support rod. On the left can be seen the feint image of the BBC stamp.
Through the etched image of the makers logo the other side of the anode is visible. The thoriated tungsten filament is intact and lights to a bright orange light.
The balloon envelope is 46 mm in diameter and, excluding the B4 base pins is 94 mm tall.
References: 1004 & private communication. Type DER was first introduced in 1921.