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.
This pair of valves both have ZF on the glass, however, only one has positive identification the other only has the ZF.
The PX25 with identification.
The distinctive M-OV ellipse with the feint PX25 inside.
The anode support system clamps to the pinch stem.
The balloon envelope has a maximum diameter of 64 mm and excluding the B4 base pins it is 143 mm tall.
References: Data-sheet, 1005 & 1043. Type PX25 was first introduced in 1932. See also1932 adverts.
