Last year I shared a concept drawing on eLUTHERIE.org that explored the potential of having a fretless surface on the same plane as the top of the frets (fret crowns). This would lead to consistent action as the bassist transitioned from fretted to fretless.
It would be a fairly complex build, but theoretically could be more easily accomplished if the fingerboard was two pieces, each a different height. Click to enlarge.
Conversations with my friend Gary Culver — who is a skilled jazz upright bassist — yielded the insight that excellent fretless tone is derived in part by a relatively high action above the fingerboard. His opinion was fretless plane at lower height than the fret crowns would be more beneficial.
So, I chose to remain relatively conventional for this initial partial-fret test bass. The fretboard was fully slotted, and fretted to the 7th fret with remaining slots filled with maple veneer.
Knowing there would be a step down from fret crown to fretless plane, I leveled the frets on a descending incline, cutting a significant fret height difference from 1st fret to 7th fret. The 7th fret is about half as tall as the first fret. An unintended but pleasing consequence was to create an exceptionally low string action, without buzz. Down-stepping the frets increases the clearance for string vibrational arc toward the octave harmonic.
Concerns with the transition "hump" from fret to fretless vanished when Gary tested the completed bass this weekend. It was an amazing treat to watch and listen to him play Duke Ellington and Miles Davis, bopping between the two zones. He was able to coax clear deep and precisely intonated low notes coupled with expressive glissando techniques in upper registers.
I will build the alternate dual plane fingerboard version on a future instrument, to compare and contrast. Some musicians might prefer a low string height instead of ultimate fretless tone.
Another choice I faced was what position to make the transition. Fierce debate raged between Ken Kinter ("Monster") and Dale Storer regarding 5th vs. 7th fret. Both are accomplished 5-string players who approach the instrument from more of a hard rock/metal perspective. They ultimately decided 7th fret — on a 4-string bass — would be more versatile for those genres.
Their opinions contrasted with Gary's jazz perspective, where some musical keys would really benefit from fretless tone beyond the 5th fret. My prediction is the transition point fret will be matter of personal preference. There is no correct answer.
Read more about this claro walnut partial-fret Orchid bass.