Tube computers primarily used triodes and pentodes as switching and amplifying elements. A typical example, the 6AG11, contains two triodes and two diodes. It replaced the use of power triodes in public-address amplifiers almost overnight. In 1918, Edwin Armstrong used only triodes when he invented the superheterodyne receiver. Devices with three elements are triodes used for amplification and switching. Whether vacuum tubes or thyratrons, the triodes were the most expensive part of a counter. It has a total of about 450 tubes (mostly dual triodes). In technical terms, JJ produces triodes, beam tetrodes and power pentodes. Successful embodiments use the 6AS7G and the Russian 6C33-CB power triodes. British single triodes, four of which were used in each channel of his 1947 circuit. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915. Triodes and a few tetrodes were made. Triodes (and MOSFETs) produce a monotonically decaying harmonic distortion spectrum. The 6SN7 is basically two 6J5 triodes in one envelope. Triodes and MOSFETs have certain similarities in their transfer characteristics.