This is an amazing exhibit. It was made by Mullard and the Mullard trade mark is on one side of the envelope. The design parallels the M-OV V24 designed by Captain Round but is much larger.
The label attached to the valve describes it as an F125 triode. However, this is a makers code and the Type designation is on the other side of the envelope from the trade mark and very hard to see. The inscription is A.T.25 followed by an X and 2. A figure 4 sits below. The AT25 was a transmitter triode made for the UK Army. The AT25 is described as being like the R Type in appearance with the cylindrical anode mounted horizontally. Clearly this is nothing like that externally but does have the anode and grid very much like the R Type. It is possible that the X2 is a variant for reaching higher frequencies.
This has been subjected to a destructive blow. The pip is broken and the electrodes have broken from their mounting at one end. The single strand filament connects to the end caps and the grid and anode connect to the side caps. The filament caps are of different size.
The trade mark in standard lighting is difficult to see.
The anode connects to the left and the grid to the right. The lower end of the filament is held by the fold in the support.
The bright anode with the grid helix just visible at the bottom. The grid is fixed to a single support all along its length.
An attempt to photograph the etched details on the reverse of the envelope. The X2 can be seen as can the two rings that form the full points between the A and T.
The grid is a close wound helix. It is perfectly circular but through the glass looks distorted. The pitch is much closer than the R Type.
The Mullard registered trade mark of the mid 1920's. For a clearer image seePA1.
The wide glass tube envelope is 44 mm in diameter, and including the base pins is 144 mm tall.