The NR47 is the UK Navy version of the classic directly heated 25 Watt anode dissipation audio power triode PX25 designed by M-OV.
The box anode is supported in each corner and the struts converge to a collar clamped around the pinch stem.
The lozenge shape containing again the NR47 Type designation is classic M-OV style.
In this image we see the broken thoriated tungsten filament first strand and the way the tension spring stands under no load conditions. The filament tension is thus considerable in order to maintain the geometery of the valve between hot and cold conditions. Thoriated tungsten runs at about 2000 °K and glows bright orange.
The balloon envelope is 65 mm in diameter, and excluding the B4 base pins is 140 mm tall.