Put amp into "Standby" mode, main power on, let it warm up 2-3 minutes, then turn standby switch to on position.
Tube amps that make use of a heater element circuit, it's a good idea to let it warm up 2-3 minutes before taking it out of "Standby" mode.
Swapping tubes,... I hope they are the specified tubes and that the power tubes are matched pairs,....
DT-50 Series:
12AX7EH Electro-Harmonix approved for the DT50 pre-amps.
EL34EH Electro-Harmonix (matched pairs) for the power amps.
... and that the power tubes were correctly biased.