The N108 is an output pentode designed for universal AC/DC receivers that operate at mains line voltage without a mains transformer. Small radios used a 100 mA heater chain.
The maximum output of the N108 was 4.1 Watts. A maximum dissipation of 12 Watts would make this small valve run very hot.
The Osram brand label.
The anode side view. All three wire grids can be seen through the top notch in the anode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 62 mm tall.
References: Data-sheet, & 1040. Type N108 was first introduced in 1952. See also1952 adverts.