linear power supply preregulator
Here is a concept of adjustable linear power supply based on descrite components and 723 voltage regulator:
**broken link removed**
For 30V output you will need to ensure that the unregulated voltage is >35Vdc and, of course, change th values of resistors around the potentiometer ..
Pre-regulation can be added to the above project and there are several options here but thy can be discussed on another occasion ..
Regards,
IanP