The NT40 is a pre CV designation of a UK Navy Transmitting valve. The NT40 is the GEC DET5 valve.
The valve is essentially a PX25 with a molybdenum anode to allow working at higher temperatures. It is also uprated to have a maximum anode voltage of 600 Volts. The anode dissipation remains at 25 Watts.
This design is pre mica electrode supports and the shouldered envelope that holds the top mica in place. So the electrodes are fixed to an insulating glass rod and to keep the large anode stable, four substantial supports rise from a collar around the pinch stem to hold the anode. The collar is held in place by a nut and bolt.
The ellipse or lozenge containing the Type designation clearly denotes an M-OV product.
The top of the anode with a glass rod behind holding the four filament tension springs.
A closer look at the filament retaining hooks attached to the spring arms.
The balloon envelope is 65 mm in diameter and, excluding the B4 base pins, is 141 mm tall.
