4.8V is the usuall voltage, but most will work with 6v for faster servo response and a bit more power at the cost of lower battery life before needing a charge again. Nothing that I'm aware of will use 12V. Are you confusing the receiver voltage with the voltage supplied to a BEC for electrics??? If so the the BEC drops the voltage supplied to the receiver and hence the servos to the 4.8V to 6V range.

Applying 12V directly to the receiver and servos will likely let you do some shopping for a new radio.

Don