It's about distortion here?The absolute max allowed supply voltages for the
A full-wave power supply uses a center-tapped transformer and a bridge rectifier. For plus and minus 18VDC when the mains is 10% high and the amplifier has no load then the loaded DC with a normal mains voltage is about 16.2VDC.
A transformer that has a 24V center-tapped winding produces a rectified and filtered plus and minus 16VDC. it will be about plus and minus 18VDC with no load and will be plus and minus 19.8V with the mains voltage 10% high and no load.