The DC2/Pen from Mazda was introduced in 1931 on the B5 base and re-supplied on a B7 base from 1933-5. The characteristics generally mirror theose of the 1930 landmark AC/Pen but it has a slightly lower output than the AC2/Pen.
Many houses in the pre-war years in the UK had DC mains supplies and equipment for use on either AC or DC had to be transformerless. Thus the HT was limited to 230-250 volts and the valve heaters had to be run in series across the mains. This valve uses a 100 mA heater and drops 40 volts. To make the total drop of the set equal the mains voltage a dropper resistor would be included in the circuit. These were usually five or six inches high, wire-wound and about one inch in diameter. The ceramic former was a hollow tube and the windings would be painted, usually green.
Looking down into the anode the three concentric wire grids can be seen. The black structure above the top mica is a radiating fin to help keep the control grid from overheating.
The glass rod on the left provides an insulated fixing point for several of the electrode supports.
This image shows that the suppressor grid is supported on two supports but both the inner grids are supported on one side only. In this picture the support rod is on the left.
Here the bright wire of the grids can be clearly seen. The central cathode tube has a white oxide coating. The lettering on the envelope dome is also just visible.
The glass support rod is itself fixed by wires to the suppressor grid support struts and the anode supports.
The shape of the grids is shown and it can be seen that top wire of the screen grid is pinched close to the second turn. The anode is blackened to radiate heat more efficiently.
The balloon envelope is 58 mm in diameter, and excluding the B5 base pins is 119 mm tall.
References: Data-sheet & 1043. Type DC2/Pen was first introduced in 1931.