If the preamp comes set up with a stepup transformer, about the only means available is to load the secondary of the SUT. If set up right, the load will load the cartridge and the transformer for optimal results.
IME things are rarely 'set up right'. So its a good idea to load the cartridge at critical damping, and then load the transformer for critical damping too. This can be a bit tricky, but any technician with a square wave generator and a 'scope can do it.
You can also do it by ear, but the values you arrive at will be more approximate (this happens to be one of those nice cases where the test equipment and ears agree, BTW). FWIW, the correct load for the cartridge will be the same for a transistor preamp as it will for tubes, assuming that both otherwise have the standard 47K input.
IME things are rarely 'set up right'. So its a good idea to load the cartridge at critical damping, and then load the transformer for critical damping too. This can be a bit tricky, but any technician with a square wave generator and a 'scope can do it.
You can also do it by ear, but the values you arrive at will be more approximate (this happens to be one of those nice cases where the test equipment and ears agree, BTW). FWIW, the correct load for the cartridge will be the same for a transistor preamp as it will for tubes, assuming that both otherwise have the standard 47K input.