The PX25 and the PP4/500 were early audio power triodes that were significant as they were the first to provide a significant output. MAzda pipped GEC at the post by introducing the PP4/500 during 1931 whilst the PX25 appeared in 1932.
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 amplifier design utilising the PX25 and running at 400 Volts could produce about five to six Watts of pure Class A. Boost that to a push-pull pair in class AB1 will provide you with a heady 20W - enough to fill a dance hall and allow you to Stomp at the Savoy!!!!!
The Mullard DO24 has very similar ratings to the PX25 but many believe it to be an over-run PX4 clone with a lesser sound quality.
There were many iterations and improvements of the PX25 by M-OV over it's production life to take account of military usage. PX25 production ceased in 1962.