The AT40X is a UK Army transmitting valve dating from the early 1920's. The AT40 dates from 1921 and also had a 7.0 Volt filament that consumed 1.5 Amps. The AT40 had a maximum anode voltage of 1.2 kV. The design is based on the R Type receiving valve.
The envelope carries the letters WD for War Department.
The filament support spring is held in a glass pip in the centre top of the envelope and offset is the sealed evacuation tube. The pinch tube at the base is used for multiple supports.
The serial number of 2967 can be seen on the glass. The figure 7 can be made out after the words Fil Volts.
The other side of the envelope carries the M-OV logo and the letters GEC (The parent company). The grid is a helix supported on two rods. A fine wire wraps round the grid wire and the support to fix the grid in place. The pinch has two glass 'horns' to provide additional fixing points. The anode is attached to a band clamped found the base of the pinch and the grid is supported on a band higher up the central extension to the pinch tube.
This image shows the way the anode is formed into a cylinder and one end of the sheet is wrapped round the support. The fine wire holding the grid to its support is clearly seen. The filament is a single hairpin of quite substantial diameter compared to receiving types.
The wide glass tube envelope is 52 mm in diameter, and excluding the B4 base pins is 124 mm tall.