Depends on the Servos and the reciever. Type the model numbers into Google and look at the Manufacturers specification sheet....that will tell you the maximum voltage that you can operate the device at. Most recievers are rated for voltages up to a 2 cell LiPo (7.4vdc nominal....8.4vdc peak) however most servos not specifically rated "High Voltage" are limited to 6vdc.
CB