Loud pop right speaker, no right channel sound

Check the right channel amp fuse.

Check the back of the speaker for a fuse.

Swap speaker leads to check if the problem is with the amp.

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.

Few speakers have fuses or reset protection, but Jason is right, especially if moving tubes make no difference, check the speakers, and move the amp's right speaker wires to the left. 

You can use any spare small speaker to get some answers, to see if a problem moves?

First swap speaker cables to determine if it is the speaker or the electronics.  very easy unless your speaker cables are exact length.

Figure out what the problem is before you try to fix it.

Thank you to all of you for the excellent suggestions for troubleshooting!   It was the right channel tube fuse. I looked back at my listening notes and the same fuse popped 2 years ago. This time it sounded much worse so I feared the worst.