A cartridge assembly for connection to the frame of a high pressure, high temperature press, comprising a front end. The front end may comprise a back up with a conical portion intermediate and coaxial with an anvil and a piston. The anvil may comprise a proximal end in contact with the back-up and a distal end being adapted to form part of a pressurized chamber within the frame. The back-up may comprise a truncated cylinder comprising a first and second interface that are joined by a peripheral cylindrical wall. The cylindrical wall may also comprise a portion extending normally from the periphery of the first interface to a net concave portion of the cylindrical wall. The net concave portion may extend from the normal portion of the cylindrical wall to the periphery of the second interface which abuts the anvil.