For the bass player aficionados amongst you we have here two quite special basses, the first a 6 string Peavey Cirrus fretless bass, the other a custom luthier made Jerzy Drozd 5 string fretted bass. Hence a total of 11 strings...had to get a Spinal Tap reference in there somewhere :)
Both basses are owned by local jazz/ funk musician Mark Foxhall who needed super low action on these instruments to facilitate his fast playing style. This required getting the neck as straight as possible with little neck relief and lowering the action at the bridge as far as possible ... whilst minimising any string buzzing.
The issue with the Peavey fretless was that the whilst the strings were already very low (virtually laying on the fretboard), they were buzzing all over the place as the neck was actually in "backbow". Immediately I started work there was a problem - the truss rod had poor access and the truss rod nut could not be adjusted as it was very stiff and the hex hole had been rounded off meaning that the usual allen key could not get enough grip to turn the nut and adjust the truss rod.
I managed to loosen the nut by tapering an old allen key I had that was slightly larger than the hole so that it could grip the edges of the hole enough to loosen the truss rod nut sufficiently to straighten the neck and get rid of the backbow. Immediately the buzzing disappeared. The rest of the set up was relatively straightforward.
Next up, the Jerzy Drozd bass presented a different problem. The truss rod worked fine on this bass, however even when the neck was adjusted as straight as it would go, the action at the bridge was still too high, even with the bridge saddles lowered as far as possible in the bridge pieces. The bridge saddles were removed, and the bottoms filed down so that they could sit lower on the bridge. This improved the action somewhat, but now at this lower action buzzing was occurring at various places across the neck.
On closer inspection it was apparent that the neck itself was not actually straight. It presented a sort of "S" shape along the length of the neck, with a dip at the nut end and a hump over the body. Also a number of frets were too high in relation to neighbouring frets.
One way to deal with this would be to take the frets out, level and refinish the fretboard, and re-fret the neck to provide a straight and level playing surface. However the frets still had plenty of height and life left in them so I recommended a fret dress to level the frets and see whether this would also take up the variance in the neck itself sufficiently to get rid of the buzzing. Using the precision Fret Jig the neck was adjusted as straight as possible prior to levelling, crowning and polishing the frets.
This worked, and with a few additional tweaks at the saddle/bridge, an acceptable low action with no (or minimal) string buzz was achieved........ Where very low action is required there is always going to be a trade off between how low you can go and some string buzz, which is also dependent on the players playing style, attack, and the type and gauge of strings used.
The next time round the bass will need a re- fret, which will be the opportunity to level the neck itself prior to re-fretting. What a lovely looking bass though! A highly figured body with a birds-eye maple fretboard and a fabulous sounding active pickup system.... nice.