ok, I think I figured it out after I did some testing.
there are 4 types of circuit boards, there's the Basic Circuit Board, Enhanced Circuit Board, Refined Circuit Board and Intricate Circuit Board.
the basic has 1 slot, the enhanced 2, refined 3 and intricate 4 slots. you need to fill all the slot for each board, if you do not do that, it won't work.
then there's 4 electron tubes that you can use on a board for electric engines, the copper (electric choke), the tin (electric boost 1), the bronze (electric boost 2) and the iron (electric efficiency)
you can have only 1 copper tube on a board, the same goes for the iron. for the tin and bronze you can have 2 on a board.
so for you, you can use 2 tin tubes and only 1 iron tube, and since that is 3 tubes you'll need a Refined Circuit Board
