 Some, but not all, were versions of existing American beam tetrode tubes or European power pentodes, such as the KT66 (6L6 similar), KT77 (EL34 and 6CA7 similar), KT88 (6550), and KT63 (6F6, pentode but almost identical characteristics).Although the RCA 6L6 of 1936 (the result of a license agreement between RCA and M-OV) was the first true "beam power tube" on the market, the later KT66 became almost equally famous, at least in Europe.The two tubes were nearly interchangeable, except that the KT66 was somewhat more rugged than the early metal 6L6.