The influence of the X-rays preionization on the performance of a 312 cm3 active volume phototriggered HF laser has been studied for Ne/SF6/C2H6 and SF6/C2H6 mixtures at a fixed SF6 partial pressure (78 mbars). Parameters are the Xrays dose (0.1 -10 mR), the pumping current pulse duration (60-155 ns), the electric charge deposited in the medium (< 3 mC), and the ethane partial pressure (< 10 mbars). It is shown that, without ethane in the mixture, arcs develop close to the discharge peak current as soon as the pumping pulse duration exceeds 100 ns at a fixed deposited charge value, in the whole range of X-rays dose values studied. An efficient discharge stabilisation effect can be obtained by addition of ethane, even without X-rays preionization and with a pumping pulse duration value as high as 155 ns. Nevertheless a low ethane concentration in the mixture is required in order to optimise the laser performance. In such conditions an arc free discharge, and correlatively high output laser energy and efficiency values, can be obtained only with use of the preionization. Optimum laser performance can be achieved with a low X-rays dose.