<p>External applied magnetic field plays the main role in gyroklystron operation. Even if a gyroklystron is designed for a specific magnetic profile, it is still possible for an alternative profile to have better performance. This unexpected result may be due to a number of reasons, such as manufacturing imperfections (e.g., variations in cavity dimensions), misalignment issues (e.g., off-axis electron beam injection), and many unknown factors in gyroklystron theory and design. Systematic theoretical investigation of magnetic field variation in the gyroklystron RF interaction structure is of great importance for optimization of gyroklystron operation and better understanding of its beam wave interaction behavior. In this paper, an analytical approach has been developed for studying the effects of drift tube magnetic field detuning on the gyroklystron performance. An experimental, two-cavity 35&#xa0;GHz fundamental harmonic gyroklystron amplifier, as reported in the literature, has been used to conduct the present study. First, the results of developed analytical approach are validated with the experimental device results for the case of optimal magnetic field detuning. After this, the analysis has been extended for studying the impact of drift tube magnetic field detuning on gyroklystron performance. Both the device gain and bandwidth decreased due to changes in the drift tube magnetic field detuning parameter. Specifically, the device bandwidth falls from 0.43 to 0.08% as the detuning variation parameter varies from 0 to ± 0.16. Similar observations were also made for the device output power and efficiency. The effects of drift tube magnetic field detuning on device bandwidth are more significant as compared to device gain. Hence, this study clearly demonstrated the need to consider the role of the drift tube magnetic field detuning parameter when optimizing the performance of a gyroklystron amplifier.</p>