In order to suppress high order mode self-oscillation in a high gain amplifier, some special procedures, such as decreasing the coupling between beam and rf field in the forepart of the amplifier, are taken such that the tube works in a fully amplified state in an operation range. In simulation, the rf output power is 1.7 GW with a beam of 7.5 kA at 750 kV when input power is 6.8 kW and the corresponding gain is 53.9dB. Based on the results from 2D PIC simulation, the experiment is performed on the Sinus accelerator. In experiment, the amplifier is driven by a beam of 8 kA at 800 kV, and the maximum output power is 2.04 GW when input power is 62 kW; the maximum gain reaches 46.7dB when the input reduces to 39 kW, the corresponding output power is about 1.84 GW. Both the simulation and the experiment prove that a kW-level rf power can drive the tube to generate a GW-level output power.