The 25B5 is a dynamically coupled dual triode audio amplifier designed for the output stage of broadcast radio sets with series heater chains. The ratings are for AC/DC sets operating on 110 Volt American mains supplies.
There is a single input grid and the power triode is connected internally to the amplifier triode. All bias voltages are generated internally.
The 25N6G has identical electrical characteristics and is fitted with an IO base.
A closer view of the twin triodes.
The original Arcturus box.
Thanks to Han Bex for sending these images.
The thin glass tube envelope is mm in diameter, and excluding the base pins is mm tall.
Reference: Datasheet. Type 25B5 was first introduced in 1940.