I am done primary winding today of my toroidal core. Attached photo of transformer below. 755 turn of 0.914 mm magnet wire. Core size before winding OD-100mm, ID-60mm, Ht-50mm (M4,CRGO). Sir when I test for secondary voltage I get 2.7 volt on 10 turn of magnet wire. why I get less volt per turn. Because before making the primary winding I done partial test on core, wind 10 turn for secondary, I get 3.2 volt. And I am winding center tap for this transformer of 35-0-35 using dual winding method (2 wire at same time, bifilar.) If I want lower voltage from same winding can it be done? e.g for 25-0-25?
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.