Am obliged, gentlemen, for the above. I didn't know about valve testers, so that's a plus point; shall investigate my local Maplin's on Saturday. However, they sold me a multimeter a couple of months ago specifically for testing continuity. After scratching my head at home for a few days I realised that that was the one thing it couldn't do! Useless oiks! Had to make my own.

Anthony, I doubt there's a problem with the biassing circuit; partly because of a service 10 months ago and partly, as I mentioned, the biassing imbalance shifted when I swapped valves.

Dave W. Sensible suggestion and that's precisely what I'm gonna do. I've no idea where to source further replacement valves from, though. E.A.R., possibly? Or just search the net? Are there valve stockists around nowadays?