Many thanks for the replies so far!
The main technical problem is that one of the OPT primaries has gone open circuit. I have found that Majestic have the winding details for the HF25 but I can’t find out anything about the PF91 OPT’s. The number of primary and secondary sections and impedance settings appear to be the same, though - so could it be reasonable to assume that the only difference between the two would be the UL taps? The plate-to-plate impedance seems to be the same for both triode and UL operation? (Mr Snooks suggested that I could install a switch system to a able the use of both modes in future if I use the HF25 spec opt’s.)
Another point of interest is the feedback systems, ie. 26dB GNF and positive feedback. This seems to be a most unusual arrangement which I don’t fully understand yet...! I’m also wondering if new opt’s will require changes to the feedback resistor bypass caps.
The more I find out about these things, the more there is to know and the more interesting it gets....
Thanks for your comments, brecklandboy, just the kind of thing I was hoping for. I’ve had these amps many years driving Lancasters with 15” Reds and have never heard anything easier to live with - different, yes, but nothing better overall. I think the real cause of the problems was that one of the mains voltage selection plugs corroded and caused intermittent connection, but because it was intermittent I failed to find it until the damage was done. I have since hard-wired the selection so it cannot happen again! When stripping down the amps, it never ceases to amaze me how they kept working so long with the drift achieved by some of the components. Having said that most of the really critical ones have been replaced by me over the years.