When changing power tubes on 70's Marshall take great care as to the proper orientation of the power tubes in the socket. All Octal power tube bases have a key-way or index pin in the middle of the tube that is supposed to prevent incorrect insertion.
The problem is that Marshall amp often used porcelain tube sockets that are easily chipped. It is very easy to insert the tube into the wrong pin sockets if you do not take care to properly align the tube. Newer tubes often do not respect the original specifications of such things as tube bases and tube pins. Often these components are made smaller, probably in order to economize on materials.
Before turning on the power on an amp that has just been re-tubed, leave the amp and standby and check to see that all of the power tube filaments are lit. If one of the tubes is dark, it is probably incorrectly inserted.