The PX25 is one of the all time classic audio output triodes. This example is branded as a Marconi product as can be seen below, note also the hand construction with the band held against the pinch.
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.
All the PX25 exhibits in the museum have oxide coated filaments but a 1936 (ish) Leak amplifier that came into the author's possession in the mid 1960's had a pair of PX25's with thoriated tungsten filaments. These thoriated tungsten filament version were usually found in transmitter modulators where they were more durable than the oxide filaments of the time.
The Marconi name and the Marconiphone Company identification,
The mechanical support for the electrode assembly is afforded by the wire frame attached to a wide collar around the glass pinch stem.
At the top can be seen the oxide coated filament.
The glass rod support for the tension springs and control grid.
The balloon envelope has a maximum diameter of 65 mm and excluding the B4 base pins it is 140 mm tall.
References: Data-sheet, 1005 & 1043. Type PX25 was first introduced in 1932. See also1932 adverts.
