I need a voltage regulator that input voltage is 85Vdc up to 100Vdc and output is 72Vdc (+-5%) with 6A (no current limit). May be linear or switching. This power supply is for testing metro electronics modules like lighting, audio and video system, PLCs, etc...
Switching will be the way to go - else you have to dissipate 168 watts worst case, whereas a switcher even at 85% efficient will only dissipate about 65W.
A simple buck converter will serve, perhaps with a crow bar ckt to blow a fuse if the series pass xtor in the buck fails short ( it can happen )