Hi
I don't have a tube chart here, but here are the functions as I remember them for the respective channels. Each preamp tube controls two functions:
V1A First gain stage of both channels (first side of preamp tube)
V1B Second gain stage of both channels (second side of preamp tube)
V2A Third gain stage, Ch1 & 2 (first side of preamp tube)
V2B Third gain stage, Ch3 & 4 (second side of preamp tube)
V3A Fourth gain stage, CH3 & 4 (first side of preamp tube)
V3B Tone stack driver CH3 & 4 (second side of preamp tube)
What I do, is buy a new preamp tube from Mesa, warm the amp up, put the amp in standby, replace one of the preamp tubes with the new tube, take the amp out of standby, test it with the new tube in place, and repeat this process until I have found the failed preamp tube.
If a new preamp tube does not solve the issue, I call Mesa.
Hope this helps and let us know what you find.