Hi all
In case anyone's interested, I managed to get a copy of the schematics for the Cambridge P50 vintage amp that I posted about here a couple of weeks ago, and have gotten to the bottom of the problem. C12, the coupling cap between the third and fourth preamp stages, had dried out on both channels, and was almost open circuit. Apparently, the owner now tells me, it's been sitting in his roofspace for the last 10 years or more, so I guess that it's been alternately cooked and frozen each year, which sure as hell isn't gonna have done those electrolytics much good ...
Geoff