Take a look at this page for Marshall-JTM-45 and '62 Bluesbreaker Reissue Modifications.
Page details restoring the current BB/JTM45s reissues to period correct component values.
The mods get it closer to the original issue of the amp, which was modeled after the 5F6-A Bassman.
The schematics for the Marshall reissues and originals are at the bottom of the page.
Bassman 5F6-A schematic
pdf from Schematic Heaven.