The P27-500 is the Tungsram implementation of the M-OV PX25 design. This design is one of the all time classic audio output triodes.
The P27-500 is directly heated and feeding the filament to minimise hum is not straight forward. Williamson, who worked for M-OV and then Ferranti, indicated that the modern indirectly heated KT66 when strapped as a triode was almost identical in performance to the PX25.
The ratings given are for class AB1 push pull for a pair of such valves. Under these conditions the grids would require 108 Volts peak to peak to achieve full output. A single ended design running at 400 Volts at 63 mA (25 watts input) would produce about five to six Watts of pure Class A.
The Mullard DO24 has very similar ratings to the PX25 but the anode looks much like the PX4.
Note the hand construction with the band held against the pinch.
The anode dissipation is 25 Watts and the emission is given by four loops of oxide coated filament wire.
The filament tension springs have hooks welded to their ends.
A rare site - a pair of these iconic triodes together.
The balloon envelope is 64 mm in diameter and, excluding the B4 base pins, is 137 mm tall.
References: Datasheet, 1005 & 1043. Type P27-500 was first introduced in 1937.