Choosing the correct value for the Zener in the clamp circuit is important. Remember that both halves of the coil are wound on the same piece of iron so there is some transformer coupling between the two halves. When the left transistor is on, the right transistor will have a voltage equal to twice the supply voltage on its collector due to this transformer action. So the clamping voltage must be something greater than twice the power supply voltage or energy will be transferred from the power supply to the clamp.