A nonode is a type of thermionic valve that has nine active electrodes. The 7AK7 is a pentode vacuum tube (thermionic valve). Secondary emission can be undesirable such as in the tetrode thermionic valve (tube). This type of circuit was used for thermionic valve (vacuum tube) technology. The EL84 is a thermionic valve of the power pentode type. In 1916, Ediswan set up the UK's first radio thermionic valve factory at Ponders End. The thermionic valve was invented in 1904 by the English physicist John Ambrose Fleming. The EL34 is a thermionic valve or vacuum tube of the power pentode type.