I can't understand this fret buzz?

[Mr H]

I recently restrung my Charvel 425SL acoustic/electric bass. I don't know what gauge the old strings were, but they looked thicker, so I'm expecting to need to tweak the truss rod anyway, but the fretbuzz I eventually discovered is a mystery to me.

There is minimal rattle all over the fretboard when fretting individual strings. When I fret the E string and the G string together near the 8th fret then there's plenty of rattle on the G string. It makes no difference if the E string note is played or muted (the A and D are being muted too).

Specifically the A rattles when the E is fretted at 7 and 8, whether I play the major or the minor on the G. Unless there's a capo lower down (just found this out when my daughter came in and grabbed the neck!), which suggests the rattle is on the lower frets.

It's not a problem, I'm just intrigued. Any ideas?

[NancyJohnson]

If the neck was set up for 'thicker' strings and you're now set with a lighter gauge, then yes, it's possible the truss rod might need adjusting. How straight is the neck generally (front bow, back bow, level), with the strings tuned up? How is the fret buzz near the neck and higher up the neck towards the bridge?

It's feasible that with the truss rod too tight, you might have some back bow on the neck which will cause the neck to arch away from the midway point; so somewhere around your 8th fret. The graphic below is pretty succinct. If you're doing this yourself, detune, turn the truss rod a quarter turn anti-clockwise and tune up :: lefty loosey, righty tighty :: repeat if necessary.

Edited June 13, 2015 by NancyJohnson

[Cyril]

If i have understood correctly, yo have rattle between nut and 8th fret, when you play at this 8th fret, right?
if yes, you have to adjust your trussrod to give your neck some relief too.
the dead part of the string is vibrating, like a sympathetic string, so if it's to close to the fretboard, it can hurt the frets.

[Mr H]

Thank you for the replies, gents. Clear descriptions and diagram.

I do understand the black arts of relief and trussrod tweakage to an extent, but it's the fact that the [s]A[/s] [b]G[/b] string rattle [i]only happens when I fret the E string too[/i], and in just those very limited positions, that has me flummoxed. I can only think that the neck must be flexing ever so slightly, but the rattle doesn't happen when I finger the fretboard adjacent to those spots on the E.

EDIT for critical typo

Edited June 14, 2015 by Mr H

[alyctes]

Cracked nut? It's a long shot, admittedly.

[Mr H]

I'll have to say that you're 75% a genius - I looked at the nut and discovered that the G string wasn't actually sitting in the groove.

25% lost though, because it made no difference to the buzzing!

[DaveFry]

I've had a similar thing happen on a fretless with roundwounds which I had set up for as little relief as possible for maximum mwah . At some times of the year the G string would rattle behind where I was fingering when playing 10ths around the middle of the neck. A tiny seasonal tweak of the truss rod cures it .

You can test to find out if this is what is happening by just bending the neck forwards fractionally by gently pressing the headstock against a wall and pushing back on the body as you play the offending notes .

[Mr H]

[quote name='DaveFry' timestamp='1434295529' post='2798377']You can test to find out if this is what is happening by just bending the neck forwards fractionally by gently pressing the headstock against a wall and pushing back on the body as you play the offending notes .
[/quote]

That did indeed stop the rattle.

I still cannot account for the specific circumstances which cause it, so I'll just give the trussrod a twist and forget about it.

[DaveFry]

You may find you can tweak it back come Oct/Nov .