The KT42 is a beam tetrode audio power output valve from 1937 and it replaced the pentode N42 released the previous year in 1936. The rated anode load is 7,000 Ohms.
The classic envelope is 43 mm in diameter and, excluding the B7 base pins, is 100 mm tall.
References: Datasheet & 1040. Type KT42 was first introduced in 1937.