GT-12AY7

This tube is closest in specification to a 12AX7 and can be exchanged with a slight drop in gain.  Most companies changed over to the 12AX7 as it became more common. 

This tube can be tried in later Marshall amplifiers in the first gain position, where the owner of these amps would like to drop the background noise and lower the gain level to plexi or early 70's vintage gain levels with its returned wider frequency response.