An urge to fill up the mm-wave technology gap in the high-power regime, where a large number of civilian and military applications exist, has led to extensive research and development activities in fast-wave gyro-sources and amplifiers. Although one such gyro-source, namely, the gyrotron is now commercially available for applications such as in plasma heating and material processing, the gyro-amplifiers like the gyro- klystron and the gyro-TWT are still in the laboratory stage of development. The gyro-TWT using a non-resonant waveguide supporting propagating waves enjoys a wider bandwidth potential than the gyro-klystron supporting stationary waves using a system of resonant cavities. This has aroused considerable research interest in widening the bandwidth of a gyro-TWT for applications such as in high-resolution radar and high information density communication systems in the mm-wave frequency band. The author has explored the disc-loaded waveguide - a structure well known in the slow-wave regime for the application such as in the linear accelerator - as the interaction structure in the fast-wave regime for wideband gyro-TWTs.