Perform a thorough visual inspection of the tube socket to ensure nothing is damaged. You could also do a continuity check on the tube pins on both channels and compare the results from left to right channel. If everything checks ok, I'd agree with Gs556.
Help integrated amp is not working?
I turn off my amp and switch some tubes last night, now only one side of the speakers is playing. I re-installed the old tubes back in and its still the same issue. It's a Shanling STP 80, also, I tested the none sounding with the souding side speaker and the speakers are all good to go. Dont know if I messed something up HELP! What can be the issue?