Electron tubes are used in most of the recipes for farms, as well as being used to upgrade certain machines. They can only be crafted in a Thermionic Fabricator, requiring Molten Glass. When placed into Circuit Boards, they reduce the EU intake of the attached Electrical Engine by 1 EU/t. Iron Electron Tubes are among many items treated as Chips by GregTech.
Note that the output slot will not show the output until the Thermionic Fabricator is hot enough to produce Molten Glass.