The electron beam of a gyrotron is typically generated by a magnetron injection gun (MIG). A single anode MIG capable of mechanical tuning was developed. Previous gyro-TWT assumes that the operating point of the mechanically tunable MIG is in the temperature limited region. The cathode current is thus dominated by the cathode temperature. However, a slight adjustment in position of the center electrode of the mechanically tunable MIG can significant modify the electric-field profile. The beam current of the mechanically tunable MIG is thus not only dependent on the cathode temperature, but also the relative position of center electrode.