Does it use a transformer between the cartridge and tube grid?
There are 3 different types of magnetic pickups of that era:

Low impedance - 25 to 30 ohms (requires the transformer)
Standard impedance - 10,000 ohms
High impedance - 40,000 ohms

All have 0.5 to 1 volt output. What output voltage are you getting at the cartdige terminals?