A new type of microwave source for the generation of high-power ultra-short coherent pulses at 263 GHz is presented. The source is based on the idea of passive mode-locking of two microwave tubes as it was first proposed in [1]. While passive mode-locking is well-established in laser physics, this idea is new for microwave electron tubes and was first experimentally demonstrated in [2]. As electron tubes, two helical gyro-TWTs are used that are coupled by a quasi-optical feedback system. The configuration proposed in this publication extends the original setup to enable the operation of the passive mode-locked oscillator in the hard excitation regime and to allow in addition the operation of the coupled helical gyro-TWTs as two-stage amplifier and as frequency-tunable, phase-locked backward wave oscillators. The performed simulation show an expected output power above 500 W and pulse widths below 0.1 ns for an operation as passive mode-locked oscillator
