[PaulR]

Thank you Al. I looked up the typical operation stats for ultralinear operation and, although the suggested bias is -20v at 400v Ht the other measurements seem ok. I might increase R34 a bit just to calm it down further and I will add the suggested stability resistors to the screens.

Well, are we nearly there then? Many thanks to all who have contributed. I haven't forgotten PJL's suggestion regarding the runaway valve but I will have a play with that outside the set using a new valve socket and my HT power supply. Here's to the next 60 years of life for this amp!