Sir Martin . . . . .
Don't you just "LUVS" the Riders schematics draw ups on those old FADA's . . . . with their . . . " wild clustered wire paths " . . . but that is certainly better'n nuttin !
Your redraw was quite small, but sort of looks like you got the essentials all included.
Initially . . . . your supply voltages DO seem in order for your more premium type of set.
If my situation to solve, I would initially be thinking about the speaker, if it is the original and if it has / is being capable of producing a good quality of audio.
If untested, I would be getting out an audio signal tracer such that I could start at the plate of the detector and evaluate the audio quality and level, as I then sample while walking on down the stages towards the AF output.
Now if you DON'T have a signal tracer, its easy to utilize a separate AC powered receiver for its provided isolation, and run a shielded line from across its volume control (or phono input, in case it has such) and couple in at the shielded lines far end thru an .05 capacitor for providing DC isolation.
Then, that will provide you a test bed by using its stages of audio gain and its known good speaker . . . . however . . . VERY little gain will be required, after you get past the FADA's detector stage.
Now in evaluating this sets design criteria, looks like they used a first generation infinite impedance detector circuit.
Looking at the cluttered schematic I see neither a 1st gid coupling capacitor of ~ 125-250 pf nor do I see an ~ 2-3 meg grid leak resistor being utilized in that detector stage.
Therefore, they are dependent upon high level RF entering the detector stage . . . . and with 3 Rf stages . . . that is what you would have . . . on decent strength stations.
And with this early design of infinite impedance detector design . . ALONG with it utilizing a battery powered filament tube, it varies from the later I.I.D. circuit which is utilizing a tube with both filamant and cathode, in which case the detector developed audio signal is taken off via the cathode . . . . a la cathode follower style.
Look at the later versions of a FADA set , specifically the 475A or F 75 where they are back to a grid leak detector stage . . . . which you see circuitry sealed off in its shield pack between the final RF stages plate coil and the 1st grid of the detector stage.
Same holds true with the 480A and F80 where its schematic is clearer yet and easily showing the input couplng capacitor and the following grid leak resistor.
All in all, if the signal tracing of the FADA's audio path has revealed inferior audio deficiency through its path, with you first being sure that you have both the QUALITY and VOLUME POTS being optimally set.
If so, then I probably would 'speriment a bit and open that detector tube 1st grid to its incoming RF circuitry line and insert a 150-250 pf isolative / coupling capacitor in line, and a 2-3 meg on the grid side to ground to evaluate the set operating in that detector manner.
Just my thoughts so far . . . .
73's de Edd
