The PL38 was designed for use as the line output stage of television receivers. Mullard also made the PL38M, and this differed from the standard valve in that it had an external metalising.
The peak anode voltage surge could be up to 8,000 Volts. The screen grid dissipation was 8 Watts.
The anode face-on. The ceramic insulators between the anode supports and the mica sheets add extra insulation - required because of the high peak voltages involved. The control grid has heat sink fins at the tops of the support rods.
End on the grid wires can be seen. Full size all three wire grids can be seen and they do not align.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 121 mm tall.
References: Data-sheet & 1040. Type PL38 was first introduced in 1948.