A new "indirect" laser-driven ultrashort hard-x-ray source based on the combination of a high-repetition-rate femtosecond laser system with a conventional x-ray tube is demonstrated. By irradiation of a metal cathode with femtosecond laser pulses electron bunches are produced and accelerated toward a high-Z target material in a high voltage electric field. Hard x rays are generated via electron bremsstrahlung and characteristic line emission inside the target. The influence of laser parameters on thermionic electron emission and on the hard-x-ray generation efficiency is studied. A theoretical analysis and a comparison with available ultrashort hard-x-ray sources are given.