The KT61 audio output valve, part of a series of very successful valves from M-OV, features the standard 6.3 Volt heater that was introduced from America in the mid 1930's.
Characteristic plot taken with the μTracer.
The Marconi label. The other side has the M-OV etched lozenge with the KT61 in the centre.
The classic envelope is 42 mm in diameter and, excluding the IO base pins, is 102 mm tall.
References: Datasheet, 1043 & 1040. Type KT61 was first introduced in 1939.