The Cossor 220OT is a battery output Tetrode and dates from 1937. The 220OT could deliver 0.5 Watts of audio.
The electrodes are housed in a large bright box shaped anode and are held by a mica disc. This indicates an oxide covered filament as the older Azide process did not allow for mica supports. The filament can be seen under a glass as a pair of inverted 'V's and the grids are thin wire structures. The control grid is tight across the supporting rods and thus passes close to the filament. The other grids form spirals. All are made from thin bright wire.
The anode is in two parts either side of the grid assembly. The 'C' logo is embossed onto the sheet nickel. The anode is carbonised for more efficient heat radiation.
The dome and top mica. The grid shapes are visible.
The spacing is fixed by the mica supports. The serrated edge to the mica allows an interference fit into the bulb. The centre of the image shows the twin filament support springs. Four filament strands pass through the grids.
The filament tensioning springs hold the filament from the top centre point. Also visible is the construction of the grids. These follow the normal pattern of wire held in place in notches in the support rods.
The classic envelope is 50 mm in diameter and, excluding the B5 base pins, is 110 mm tall.
References: 1043. Type 220OT was first introduced in 1937.