Revision 6. Got it running last night. Seems to be a distinct improvement over the previous one. This does not imply that the previous one was in any way poor...just this one is bloody excellent.

Going to try some different valves in the CCS circuit. gm of the 12BY7 is ~15 mA/v and the 12GN7 I've got waiting in the wings is 35 mA/V. It is dificult to imagine it getting much better.
regards,
Douglas

....damned Thrintun!