This rig uses a pair of 4CX250B tubes in parallel. The plate tank is made from a piece of copper sheet 5 inches wide by 7 inches long. The plate tuning cap is the small piece of copper sheet located on the right side of the box. If you want to build this amp and enjoy an exercise in frustration you can click here to view a PDF copy of the construction article from QST magazine. This is an 18 meg file so be patient.