A system for the control of an indirectly heated gas turbine comprising a primary system of controlling the temperature of heated compressed gas entering the expander, and an independent secondary system which includes a safety valve for instantaneous release of heated compressed gas to the atmosphere. The primary system controls system gas temperature and power output by modulating a flow of unheated compressed gas which bypasses the heat exchanger and mixes with the heated gas leaving the heat exchanger to produce a lower temperature gas entering the expander. The secondary system provides a backup means of overspeed prevention, and includes a safety valve to instantly discharge to the atmosphere hot compressed gas upstream of the expander by being responsive to the speed of the turbine. The safety valve includes a frangible membrane clamped between parallel flanges within the ducting, and further includes a dagger assembly for rupturing the membrane.