Sorry guys for late reply, strong cold hit me down...
Finally found the fault, it was one socket of power tube.
Sockets are old but looks like new.
After I checked all mentioned and unmentioned possible sources I look at the sockets with magnifier and penlight and there were anode pin and bias damaged. Bias pin was stretched and anode slightly burned, black.
After I changed sockets, output signal is symetric and sound is amazing.
Thanks for suggestion!
Finally found the fault, it was one socket of power tube.
Sockets are old but looks like new.
After I checked all mentioned and unmentioned possible sources I look at the sockets with magnifier and penlight and there were anode pin and bias damaged. Bias pin was stretched and anode slightly burned, black.
After I changed sockets, output signal is symetric and sound is amazing.
Thanks for suggestion!