Fx37 is it the same as Adox fx39ll ?

Vetus said:

Anyone compared fx37 side by side with Adox fx39 ll ?

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Going by anecdotal evidence (here) and MSDS info (here), it seems they're totally different animals, FX37 being a PQ developer and FX39 an MQ type. I don't know the accuracy of the first source provided.

Note that despite technical differences it's very well possible that real-life results from both developers are indistinguishable from each other, at least under certain conditions. So even if they're technically different, they may still be functional substitutes - depending how much attention to detail you're willing to spend...

It is reported in "Amateur Photographer " [UK], 27 Aug 2005 p25 that a reader asked which of the public domain "FX" formulas is closest to FX-39 [I believe it was made by Paterson at that time].
One of the editors asked Geoffrey Crawley who was quoted as replying "the closest published formula to FX-39 is FX-37".

From the formula and msds posted by @koraks it looks like FX-39 contains potassium carbonate and sulfite rather than the sodium in FX-37 and metol instead of phenidone. Also from the recommended dilutions FX-39 appears to be about 3x as concentrated as FX-37 though the working solutions may have similar activity. Adox may have tweaked FX-39 II further, but IMO it seems likely that in terms of results Crawley's comment is likely still true.

Alan Johnson said:

FX-39 contains potassium carbonate and sulfite rather than the sodium in FX-37

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Alan Johnson said:

FX-39 appears to be about 3x as concentrated as FX-37

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Yes, and that is nicely consistent as well, since the solubility of the potassium species is generally higher, so a much stronger concentrate can be made with them.

could FX39 be DK50 with substituted K carbonate? Crawley preferred both over phenidone and Na carbonate.

DK-50 gets only a formula and one line in the film developing cookbook 2020 p164-5. Maybe it was considered too grainy when the 35mm era arrived. Crawley seems to have been the first to notice that this type of formula could be well suited to the finer grain high tech grain films, BJP 03.27.96.