The Large Circuit Board was an item from the Forestry Mod. It was used for the crafting of various machines and was also used to upgrade Electrical Engines (table below) when Electron Tubes were added with the Soldering Iron.
To add tubes to a Circuit Board, hold a Soldering Iron and right-click towards the sky or any block which is not a machine or inventory. A menu will appear with slots for tubes on the left and a circuit board slot on the right. Place the tubes in the slots on the left first, then the circuit board in the slot on the right. Once the circuit board has been placed in the slot on the right, it will have the effects for the respective tube applied to it. The effects for each tube on a Circuit Board (which can then affect an Electrical Engine) are listed below.
Type of tube
Effect (Max stack)
Change in input EU
Change in output MJ
Providing that there is enough space on the Circuit Board, each effect can be applied more than one time (for example, if Bronze Electron Tubes are in two slots and a Circuit Board is added, Boost II will be applied two times over) for additional stacking effects. However, this is limited by the Circuit Board's capacity; a Small Circuit Board can only hold one tube, a Medium Circuit Board can hold two tubes, and a Large Circuit Board can hold three tubes. Each effect also has its own stacking limit shown in the table above in parenthesis after the effect name.