it could be as simple as replacing the preamp tube, likely the PI tube right before the power tubes, or the fuse which may be blown and keep to tube from turning on. I'd check all the fuses for continuity and then swap known good tubes in. The power tubes ARE lighting up? That's good as they are more expensive! Find a schematic for the amp (maybe here The Unofficial Ampeg Page) and be sure the fuses, where present are not blown. Past tube replacing you probably want to bring it to someone who knows these dinosaurs
The Marshall sounds like either a dirty,/bad pot or bad solder joint on the pot, you could try some fader lube/ pot cleaner spray available at radio Shack, and if that doesn't help you could check the solder and replace pot if bad.