Swap the tubes, if problem moves to other side, it’s the tube.
One stage at a time: Power Tube(s) one at a time; small tubes,1 at a time.
No need to check the rectifier tube, only one effecting both channels.
If not:
Most of my equipment has a single fuse. You could find/download the service manual, the fuse(s) will be shown.
No manual: unplug it, wait overnight; bottom/cover off; look for any fuses or blown parts which are often obvious.
I am like a Parrot who repeats: "Get Thee a simple tube tester!" No swapping involved, just test them 1 at a time. I check them all yearly, just before Thanksgiving.