Weird tuning problem.

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VomitHatSteve

VomitHatSteve

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The instrument I'm having trouble with is my abomination.

Right after I put that pickup in, I started having troubles keeping the bass string in tune. Specifically, the pitch of any note plucked on that string would shift by about about a quarter tone right after plucking.
At first I figured that it was just because the string was 5 years old, but I put a new string on this week and the problem got worse!
I know that pitch drops slightly on all strings as they resonate, but this is so pronounced that it's almost unplayable!

Other details:
The pitch change is most pronounced at the octave and above.
If I play the harmonics instead, there is almost no change.

Any ideas as to the cause or how to fix this?

Thanks!
 
1) I'm not sure exactly what you're describing. Is this a pitch change that stays flat, so that you tune to correct it, or is this like the floppy-string syndrome where the harder you hit the string, the farther out of tune it goes, but it comes back up to the original pitch? I wonder if it's a binding nut slot or you just need a heavier string.

2) This is the one with the homemade one-string bass pickup, right? How close is it to the string? 4 neodymium magnets stacked up could be pretty strong, and you may be getting some string pull. When you're fretting the higher notes, that would put the strings closer to the pickup -- that may explain why it's worse on the high notes but not the harmnonics. Other people I know of who make pickups with the neo magnets tend to use them like humbucker magnets, mounted underneath the pickup with a metal polepiece going up towards the strings. If that pickup's still held on by duct tape, it's easy enough to check to see if that's what's causing the trouble.
 
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I don't remember the gauge off the top of my head. I'll have to get back to you on that.

The pitch is rising momentarily and settling back down as it rings. So I'll pluck the string at the octave and hear a D# that drifts down to a D after a moment.

This is the homemade pickup. I was kind of worried that the magnets might be too close to the string, but if I move them much farther out, they don't get much of a signal. :(

And it is still held in by duct tape. I like the look! :D
 
I mentioned string gauge and string length because outside of certain tolerances strings will do funny things. A heavy string with too little tension will pitch a bit higher initially and then settle into it's correct pitch. Thats mainly due to the fact that it gets excited in the middle section of the string and the ends don't start to move right off the bat. It takes a while for the string energy to overcome the inertia. If the tension is higher in the string it is stiffer and transfers the energy more efficiently. The net result is that initially the effective string length is shorter in the initial attack of the note.

You can confirm or rule this out by simply raising the pitch of that string as high as you feel it will safely go and seeing if it still does it. If no0 you need a stiffer/heavier string or longer scale length. If yes then you need to start looking at other possible causes and the one that springs to mind is the pickup, specifically it's position.

After years of diagnosing weird problems on stringed instruments of all kinds the trick is to rule stuff out one bit at a time.
 
It is a short scale string (32.5") with a .095" (2.411 mm) gauge. It is tuned down a step.

Let me try the tuning up idea...

Yep, tuning it up 7 half steps had a huge effect. So I want to move the bridge farther from the head for that string?
 
VomitHatSteve said:
It is a short scale string (32.5") with a .095" (2.411 mm) gauge. It is tuned down a step.

Let me try the tuning up idea...

Yep, tuning it up 7 half steps had a huge effect. So I want to move the bridge farther from the head for that string?
Click to expand...

Yep pretty much or the other solution is to go for a heavier gauge string. Both solutions will have the same effect but from an instrument designers point of view you would start with a longer string length. I wouldn't have thought you have that option.

A lot of guitar players I deal with use a drop tunings and will come to me to have a guitar built with a longer scale length so that the lower strings can be dropped in pitch without suffering from the sorts of issues you have. Try the heaviest gauge string you can get your hands on.
 
The store I got the string at had a heavy and light gauge string, but I don't remember which I got... Some googling shows that I got a pretty light one. :(

I moved the saddle on that string farther out, and that helped.

Is there a limit to how far back you can move a saddle before it could damage something? The string is still nowhere near dangerously taut, but I want to make sure nothing else could go wrong if I tried moving the saddle all the way out.
 
If the neck is fretted then moving the saddle back too far will mess with the intonation. You need to get as heavier string as you can, that will increase the tension and hopefully get you were you need to be.
 
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