About CT secondary winding on toroidal transformer
But before primary winding I test core with sample winding with old
  transformer which have 12 volt secondary.


With 12 V and a turns ratio of 37:20 you should expect about 6.5 V. You measured 6.66 V and this sounds OK.


  after actual wind full 755 turn on primary i test secondary voltage
  again with 20 turn this time I got 5.5 to 5.7 volt


755:20 with 5.6 volts on secondary implies a primary voltage of 211 volts. 

Maybe you thought you were feeding 240 V but you only have 210 V.