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.

The PX25 is an early audio power triode, and was one of the first to give a significant output. With the loudspeakers of the time a pair in push pull producing 20 Watts was suitable to fill a small hall. With transformers of the period being less efficient than later designs an audio power amplifier using a pair of these valves was a substantial piece of equipment, and heavy.

Today PX25 is much sought after by collectors, and New Old Stock (NOS) valves in their original packaging sell for over £100.00 each. This exhibit was £40.00 in 2003.

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.

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.

The balloon envelope has a maximum diameter of 60 mm and excluding the B4 base pins it is 137 mm tall.

References: Datasheet, ¹⁰⁰⁵ & ¹⁰⁴³. Type PX25 was first introduced^¶ in 1932.

 
Pin Connections

B4	1	2	3	4
	a	g1	f	f

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	Pdiss	D
	4.0	2.0	500	-54	82	26	4%

Thanks to Frank Philipse for supplying the above PDF datasheet