The PRE-73 and the PRE-73 MkII is not prepared for mounting the Carnhill mic and line input transformers. Modifying these units these takes some diy experience and a couple of hours of careful planning and work. The output transformer is very easy to replace though. A basic instruction is included with the transformer kit. It is not possible for us to give support on the modification and there is no warranty for modified units.
Here´s another way to upgrade mic input transformer in the PRE-73 MKI and MKII to the Carnhill 9045M: