The EL31 like the EL37 is a 25 Watt anode dissipation audio output pentode. The EL31 is designed for high anode voltages and lower current than the EL37 and can produce greater output power.
The EL31 is a true pentode. The anode is separated from the mica sheets by ceramic insulators.
The grids can be seen below the top mica. The control grid is fitted with a sizeable heat radiator.
The pinch and lower components.
The classic envelope is 43 mm in diameter and, excluding the IO base pins, is 119 mm tall.
References: Data-sheet. Type EL31 was first introduced in 1948. See also1948 adverts.
