The ARP38 is a 1937 RF beam tetrode for RF and IF applications. As the KTZ35 It was part of the M-OV 'Uniwatt' range of valves for use with mains or vehicle lead acid batteries. It was only supplied with external metal paint. This exhibit has lost the paint but the connecting wire and the adhesive ring that was the connection point remains.
A year later in 1938 the valve became the KTZ73/6J7G RF pentode. The main differences were the heater being standardised at 6.3 Volts and making it a pentode with g3 connected to pin 5.
The bright anode is a rectangle with rounded ends. The beam plates can be seen above and below the anode.
The grid supports appear white in this image but the reality is that it is only the cathode tube that is white. The residue of the paint adhesive on the glass makes seeing the internal structure difficult.
The classic envelope is 30 mm in diameter and, excluding the IO base pins, is 72 mm tall.