I'm gonna take a guess and say its the one on the left since its the shortest signal path, but either way, you could test this with a multimeter couldnt you?
Something like taking all the tubes out (TURN THE AMP OFF FIRST AND LEAVE IT OFF), put one end of the multimeter onto the live terminal on the jack input and then pop the other end into the inputs on each valve fitting, whichever one provides the least resistance is the right one...
I'm not an electronics guy though, so I don't know whether this is a flawed approach?
What are you specifically trying to achieve with having varied tubes in the preamp stage?