I have two V-77's, one from the first production run (more champagne than silver) and one of the later ones. Both have the stock tube in them.
The tube is soldered in, so it's not a simple swap. You'll either need to unsolder the old tube and solder the new one in, or install a ceramic socket for easier swapping.
If you decide to replace the tube, a nice "new old stock" 12AY7 will do the trick very nicely. I prefer RCA and GE 12AY7s personally. I don't use new/current production tubes.
Good luck!
Cheers,
Wade