It sounds like a tube, unless you’ve got backups for the rectifier and input tube you’ll need to wait for the new ones. This is why it is a good idea to have at least one complete set of spare tubes: you never know when one will go bad and the best way to check for a bad tube is replace it with a new one.

But until they arrive it wouldn’t hurt to reseat the tubes and speak cables. If you think there is gunk in a tube socket blow it out as best you can, with the amp unplugged of course.