This exhibit is a PL81/N152 from M-OV and branded Marconi. The earlier PL81/N152 had the straight tube envelope whereas here the envelope has been increased in diameter at the central section by 4mm. This will have the effect of reducing the glass temperature.
The glass carries the words 'foreign made'.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The high cathode power of the PL81 design made it suitable for use as a series pass element in voltage regulators, a circuit type with particular demands on the cathode.
The anode is shaped for greater surface area. The bright sheet seen through the side wall of the anode is the beam plate for although called a pentode, the construction is a beam tetrode.
The small plate above the top mica is the cooling fin for the control grid and is fixed to the copper grid support rods.
The thin glass tube envelope is 24 mm in diameter and, excluding the B9A base pins, is 72 mm tall.