Thanks for the comprehensive answer. Hehe, and on your final point I had actually started to look at other amps, how did you know?!I've actually found the culprit I think, one of the plates/cathodes(?) in the output valve/tube had come loose, and was floating about freely inside the glass envelope!!! Is this a common fault?
Anyway I think I'll get a new pair of output valves and get an engineer to bias it properly and to give it a pat on the head. Thanks for your help ![](http://www.audioroundtable.com/emoticons/smiley.gif)