The PX25 is one of the all time classic audio output triodes. The PX25 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.
Today PX25 is much sought after by collectors, and New Old Stock (NOS) valves in their original packaging sell well.
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.
M-OV redesigned and uprated the PX25 over its 30-year production life, so that it was fit for service in more demanding applications including RF. This exhibit has the shouldered envelope and mica separators. The original versions had glass rods to hold the electrodes in place and the heavy anode was attached to a collar around the pinch stem. Here the top mica holds the upper part to the glass envelope and thus no heavy supports or collar are required.
The PX25 inside the etched M-OV lozenge on the glass.
The anode box has to dissipate 25 Watts and the flanges at the centre of the working faces add to the radiating surface area.
Looking along the grid axis.
The upper mica is thick to support the four filament tensioning springs. The filament is oxide coated.
The circular mica disc has small points that grip the inside of the envelope.
The classic envelope has a maximum diameter of 50 mm and excluding the B4 base pins it is 137 mm tall.
References: Data-sheet, 1005 & 1043. Type PX25 was first introduced in 1932. See also1932 adverts.
