This is an amp I had in before and had serviced. The new tubes would not bias to 90mV as the Marshall service bulletin called for and ended up being biased too hot. In this video we'll show how to get the bias control set correctly even when the mini bias adjustment pots won't allow you to go high enough. It requires some board level soldering, so don't try this unless you have some experience as your iron can burn the traces off the board very easily on these.
NOTE: this solution will only work in amps where the bias pots don't allow you to get to the appropriate mV... It will not work to resolve issues with the phenomenon of 'thermal bias drift' as outline on this link: http://www.hullerum.de/Marshall/TSL122repair.html
Thermal bias drift occurs on some older Marshall DSL and TSL amps because the board material becomes slightly conductive and causes the bias to drift during high temperature operation.
Service bulletin: http://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Biasing%20Instructions%20-%20JCM2000%20Series.JPG