Hexagonal spanner for Fidelity Research FR 66S tonearm

If you’re looking at the large round nut with flattened parallel facets, as shown in Elliot’s photo, I’d first try to turn it counterclockwise by hand, grabbing those facets. If that doesn’t work, then I’d use an adjustable wrench as I described above. Adjust it so it is in contact with those facets, hold it close to the head so you’re not exerting max leverage, and gently turn counterclockwise.

Thanks, the black knob seemed to have a grub screw but I could not find it or manage to loosen it. Have tried to use the provided small metal rods to loosen by hand but failed. Will try an adjustable wrench soon.

Have a pair of channel lock pliers that are just barely big enough to work but I failed to budge it no matter which direction I tried. Will seek help from local that may have better tools & experience.

This is an adjustable wrench. It has two flat parallel jaws that could engage that large nut with flat parallel sides.  For good measure, protect the nut with tape before engaging it with such a wrench.

"small metal rods". Are you referring to the small size Allen or hexhead wrenches that one could use to loosen the set screws in a B60? I guess that could work too, to remove the tonearm from the armboard without removing the B60 itself. To post a photo, you need first to post them on an external website set up for this purpose. Elliot can help you with that; I have never done it.

Channel lock pliers are very likely to "booger" the nut.  And there is only one direction to loosen a screw or a nut, to the left or counterclockwise when you are looking at the nut.  "Righty tighty; lefty loosy" is how I remember it.  There is the occasional screw that is reverse threaded for a reason related to its use, but I don’t think that applies here.

Took it to a friend with a clamp and proper wrench, all good now, thanks for all the positive responses, best wishes, LS