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Double Bass Tuning problem

M

MarkH

Apr 12, 2003
67
1
U.K.
#1
Hi,

I’ve recently bought a beautiful 30-year-old Roger Dawson bass with a great pedigree (London Symphony Orchestra, BBC Concert Orchestra etc....) but something strange is going on.

I’m finding that the harmonic on the G string is much sharper than the open string itself.
I’ve changed string a number of times (…I’m currently using a Flexicor G)
The bass has a new bridge and has been set up in the last month or so.

All the other string’s harmonics are totally in tune with their respective open string.

Using a tuner, which I often do, the open string seems to be about 2-3 cents flatter than the harmonic.

The next thing…

When I tune up with the tuner using harmonics, the bass feels nicely in tune and very comfortable.
When I tune up using open strings, notes played on the G string are consistently sharp.

Clearly I’m going to waste no time in getting the bass looked at, but I was wondering whether anyone had any experience of this, or indeed might be able to suggest a reason/solution?

Thanks.
 
#4
With the typical I am not a luthier, do not try this at home disclaimer:

It almost sounds as if there is some anomaly that is causing a different mensure for the G string than the others.

Are you certain that the bridge is straight, well-aligned and the G is witnessed at the same point as the other strings on the bridge and nut?
 
#5
Thanks...

It looks pretty straight and I certainly can't see any problems with the bridge itself.

Surely, though, a harmonic is a 'subdivision', in a sense, of the open string itself.
Therefore the string would in effect be stretching over a longer distance (...if the bridge wasn’t straight) but the harmonic itself would still be in tune with the open string??

As you can tell, I'm no luthier either!!
 
#6
MarkH said:
Thanks...

It looks pretty straight and I certainly can't see any problems with the bridge itself.

Surely, though, a harmonic is a 'subdivision', in a sense, of the open string itself.
Therefore the string would in effect be stretching over a longer distance (...if the bridge wasn’t straight) but the harmonic itself would still be in tune with the open string??

As you can tell, I'm no luthier either!!
Click to expand...


I understand that. I was speculating that perhaps there was something creating a false witness point on the string.

For example, if the groove in the nut or bridge on the G string has become rounded over time (sloping to the fb), it would make sense to me that the change in break angle caused by fingering a note might alter the witness point at the nut or bridge. I've actually had this happen to me on an acoustic guitar.

This would certainly explain the idea that stopped notes are sharp when the open string is in tune. The harmonic issue is a little to harder to explain.
 
#10
Rob asked the right question. Tuning open strings with the bow and a tuner is very interesting to say the least. The absolute pitch is not only dependent on string tension, but also (and significantly so) on bow speed and weight as well as where you bow the string. This can easily make a 5-8 cents difference either way of the pitch center. It's not as obvious with harmonics, since these are typically bowed lighter so that bow weight does not account so much for pitch variation. I'd say there's nothing wrong with your bass or strings, you're just looking at one of the may ideosyncrasies of this instrument.

Another possibility that can wreak havoc to intonation is the various wolftones that especially older and better instruments tend to produce. Although it is possible to tinker with soundpost and 'wolf-eliminators' to improve this, it will more often just change the wolfs rather than eliminate them. The only way to deal with this is to really become familiar with your instrument and adapt your playing where needed.
 

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