It is often necessary to introduce a sever or attenuator into the slow wave structure of a travelling wave tube (TWT) to ensure stability against feedback oscillations. These are caused by signals reflected by mismatches at the input and output of the slow-wave structure and by the inherent backward wave. The position of the sever is an important design parameter because it affects the conversion efficiency of the tube. The paper describes investigations carried out using a one-dimensional large-signal computer model into the best choice of sever position in coupled-cavity TWTs. Results of the effects of sever position on saturated efficiency, tube length and bandwidth are presented. An improved procedure for determining the positions of severs in a tube is suggested.