[Amps] A question about grid bias in G3SEK's control board

[Hsu]

Good morning and evening!
  Here is   grid bias power supply in G3SEK's control board 
:http://www.zmdz.com/bbs/incomefiles/20144/1912574624747.JPG
  I do not understand how it works. I think it is not  a regulated power 
supply. If it work, should be modify as red line marked and replace NTW5142E 
by TL431(adjustable) or TL431 series a zener(higher volt)
          Hsu