How to make a 50V regulator?
You can use a controller chip rather than a one-chip regulator. 

Edit: For example, the NCP1034

You can use a boutique high voltage switching regulator chip such as the LTC3639, or a similarly 'special' high voltage linear regulator such as a TL783 (if the power requirements are minimal). 

You could make a pre-regulator/voltage limiter with discrete parts. 

You may be able to use an off-line style flyback regulator with a DC input (be careful the UVLO circuitry does not interfere with your relatively low input voltage requirement). Or use an isolated off-line supply directly. 

If you're after a regulator for a high power audio amplifier (50V+ implies hundreds of watts output) you could use this impressively expensive and sophisticated boutique chip (LT8705). 80V max in and buck/boost. 71V is a bit too close for comfort to 80V but maybe you could get a lower voltage transformer since it's a buck-boost regulator.