This if gunna sound so nooby but I was looking round my amp (5150) for the first time today. I can see the four 6l6gc tubes. Then theres two blocks either end, which one has the 12ax7s in? I didn't take anything apart coz I was scared I was gunna break it which I really can't afford to do.

No man has the right to be an amateur in the matter of physical training. It is a shame for a man to grow old without seeing the beauty and strength of which his body is capable

the preamp tubes are located in a small compartment at the back, bottom right. You have to take off a panel with a screwdriver to access it.
Peavey 5150, LTD EX400BD, tubescreamer, and a whole lotta fingers