The PP5/400 is an early audio power triode, and was one of the first to give a significant output. 26 Watts to the loudspeakers of the time 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 the PP5/400 and PX25 are much sought after by collectors, and New Old Stock (NOS) valves in their original packaging sell for over £100.00 each.
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.
The Type PP5/400 was the Mazda version of the famous M-OV Type PX25. However, whereas 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, Mazda seem to have been concerned only with public address service for which the original ratings (400V; 63mA; audio frequencies only) were sufficient. All Mazda-built PP5/400s are physically similar to that in the picture, with no evidence of redesign or uprating. However, some later M-OV-built PX25s were relabelled and sold as Mazda PP5/400s. Perhaps this is the origin of the extra voltage rating quoted.
The electrode structure of the PP5/400 showing the rigid mounting of the component parts. From Wireless World, April 22, 1931.
The balloon envelope is 60 mm in diameter and, excluding the B4 base pins, is 133 mm tall.
References: Datasheet & 1040. Type PP5/400 was first introduced in 1931.