I am in the process of building some 5' x 16" Tube Traps. I have one almost complete. In the past I have built IC's, speaker cables, DIY Hydra power conditioner, various isolation devices and a TT wall shelf.
For those of you looking for a place to start: try AudioXpress magazine (formerly Audio Amateur.) You'll find articles on scratch-built amps, preamps, phono stages, subwoofers, electrostatic arrays, etc. Electronics cover tube and solid-state projects. There is also extensive reporting on modifications to older and classic equipment. I've built some of their projects. One was an add-on moving-coil phono stage for a low gain pre-amp.