In one aspect of the invention, a cartridge assembly is adapted for connection to a frame of a high pressure, high temperature press having a front end with a back up intermediate and coaxial with an anvil and a piston. The back up has an anterior end proximate the anvil and posterior end proximate the piston. The cartridge assembly has a hydraulic system adapted to apply axial pressure to the back-up through the piston to axially move the front end with respect to a cartridge body. The assembly also has a centralizing assembly with a rod rigidly attached to the cartridge body at a first end and a second end adapted to slide within a peripheral bore formed in the front end.