The PP5/400 is an audio power triode, and was one of the first to give a significant output. 26 Watts of push pull audio to the loudspeakers of the time was suitable for use in 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.
This exhibit has Mazda etched in the dome of the envelope.
It also has been labelled by a re-seller.
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 (400 V; 63 mA; 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.
The structure of the filament supports can be seen in the picture.
The balloon envelope has a maximum diameter of 60 mm and excluding the B4 base pins it is 133 mm tall.
References: Datasheet & 1043. Type PP5/400 was first introduced in 1931.