geobee:
My Yamaha keyboard only has a ''B'' type USB to host output, no midi output.
Details please. Which Yamaha keyboard? My old one only had a USB port, no 5 pin DIN MIDI ports. But what is carried over the USB is MIDI. And that's exactly what those adaptors are intended to work with.
But if you do want to use an Arduino, since you want MIDI over USB in then it might be easier to use a different Arduino like the Leonardo or Pro Micro which has the USB connection already built in.
Steve