The NT40 is a pre CV designation of a UK Navy Transmitting valve. The NT40 is the GEC DET5 valve.
This low power transmitting valve was directly derived from the PX25 audio triode.
For transmitter use the same complex anode box was used but fabricated from Molybdenum. The NT40 has a directly heated oxide filament and is pumped to a hard vacuum to enable operation up to 600 Volts on the anode. The anode dissipation is 25 Watts and the operating frequency was of the order of 20 MHz. The design was popular and used in several designs.
The balloon envelope is 62 mm in diameter and, excluding the B4 base pins, is 143 mm tall.