MIMF

I just completed my first instrument, a campfire mandolin kit from Stew-Mac. The E strings are distinctly twangy when played open, but seem fine when fretted, so I am guessing that the problem is in the nut slots. The other strings sound fine. Can anyone give me some advice on how to proceed?

If you cut the strings' slots too wide, the strings will rattle within them, lending a "twangy" tone. It's like a string buzz, on steroids. The other possibility is that you cut the slots a wee bit too low. Either way:

Replace the nut.

Did you make sure the slots angle down towards the headstock, so they strings are supported precisely at the edge of the nut just at the end of the fingerboard? Your file should angle less than the headstock and not flat like the fingerboard, but about half way in between. This helps support the string along the slot so it's force is spread over the width of the nut, and the result is a longer wearing nut and better sound.

Michael,
Your advice was right on the money. Adjusting the angle of the E string slots solved the problem.
Thanks!
Ralph

The same thing can happen on the bridge end of the strings.

Jon Whitney wrote:The same thing can happen on the bridge end of the strings.

Thanks Jon, I will keep that in mind. I appear to have lucked out this time.

On the subject of bridges, can anyone explain the theory behind having the crest of the bridge alternating about 1/8" between pairs of strings? It always seemed to me that this is contrary to the precise spacing of the frets. Just another newbie question.
Ralph

Intonation

To expand on Mario's reply, what you have is called a compensated bridge. The exact length of the string is adjusted at the bridge end to compensate for the slight stretching/tightening of the string when fretted. Fretting a string doesn't just shorten it by a prescribed length - it stretches it tighter by a predictable amount, which is based on the action height, the thickness of the string, and the string material. This stretching causes the string to sound a higher pitch than it ought to. To get the proper pitch (intonation), the string is lengthened slightly by both angling the bridge and notching the bridge saddle back towards the tailpiece.