• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Output transformer problem - SE TH100 - James Audio Transformer

So,to finish the story,problem is fixed and amp works fine.
After many many nights and "operation",problem was in voltage changer N-FET 2SK2700. He is changed now and the buzzzz is gone.
Big thanks to everyone helping me in detecting problem!!
Cheers!