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Gyrotron traveling wave amplifier (gyro-TWA) experiments based on a helically corrugated waveguide have been undertaken at Strathclyde University for the last few years. In this paper we report our latest experimental results using a longer pulse electron beam of energy 185keV and current 7.5A produced by a thermionic cathode. The coupling between the second harmonic cyclotron mode of the electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth. A saturated gain of 24dB which had a 3dB instantaneous bandwidth from 8.4 to 10.4GHz (i.e. 21% relative bandwidth) was measured. The gyro-TWA had a saturated maximum output power of 220kW, corresponding an electronic efficiency of 16%. Good agreement was found between the measured power and the value predicted from nonlinear beam-wave interaction simulations carried out using the 3D particle-in-cell computer code MAGIC.