Tuning instability : possible structure problem?

A bass of that age should be relatively stable, so the wood probably isn’t moving as much as a newer bass might tend to.

The neck thickness and set look reasonable.

I assume the strings are steel.

If most of your tuning is in an upward direction and not equivalent across the strings, I’d look at the machines.

 

This alone is enough to cause the sort of change you describe, particularly the humidity (noting that you're in Puerto Rico). Is this a problem you weren't having under the same conditions up to some point in the recent past?

Regardless of the cause, this cycling probably isn't good for your instrument, it's hard on joints and soundpost. If you can't keep it climate-controlled at home, you might look into the possibility of keeping it at the hall between performances.

 

What is your tailgut material? I've had minor relative tuning issues with the nylon cord type, not so much with steel cable. The neck doesn't look bowed in the photos, has the scoop in the fingerboard increased at all (strings higher off the board near the heel but closer to the board in thumb position)? Also wondering if the instrument doesn't need a longer soundpost for its more humid environment, not sure if too short a post could cause this issue though.

Nice bass btw!

 

This sounds normal to me. Many halls are cold when you tune and get warm when the lights are on and the whole orchestra is there causing strings to go flat. Always tune with warm strings.
This issue is even worse in concerts at some halls. A drier hall will do the same.
How much varies from string to string, but synthetic strings like Evah are almost immune as they don’t expand with heat and their elasticity makes them react less to top movement.

 

If the neck was unstable in a way that affected the tuning, you should be feeling a clear difference in action/string height also.

 

Are you always re-tuning tuning up, or in both directions? If the former, the tailgut could be the culprit. Also, show us a photo of the pegs. I presume you know how to install strings, but we might as well rule that out first.

 

It would be useful to know if it sinks a few HZ which is normal or noticably more which is not normal.

 

May have no link to yours, but had a similar problem once. Took bass to Lisa Gass and she found lower half of fingerboard had come unglued from the neck. I couldn't see it or feel it until she pried them apart to show me.. Reglued lower half and problem went away. Strength of neck relies on "plywood" the two pieces form when glued together.

Most of what you describe seems normal behavior...tightening the low strings pulling the high strings flat, all flattening during initial play/warm-up, pitch change with location change...except on my basses all more or less stabilize after about 10 to 20 min max. An hour seems long.

 

With say 20% lower humidity I can have pitch drop for four hours easy.

 

At first glance, it does look like the maple in the neck has been thinned some. The fingerboard is the backbone of the neck and looks healthy enough in thickness. Do you have some measured thicknesses to offer us ?

 

My (limited) understanding is that the whole instrument swells with humidity, which slightly increases the distance between top and back plates. If the post was cut in a much different environment, there might be more top movement in the humid environment than ideal. Again I'm absolutely not a luthier but it's one hypothesis.

Might also check the fingerboard glue joint as Ben Green suggested, and you'd probably be having other big issues if the bass bar was loose, but might as well look in there with a flashlight and mirror and rule it out?

 

I'd get braided steel to replace the rope, which could be losing strength and stretching after years. If that doesn't help, then maybe time for new tuners if the rest of the possibilities are ruled out.

 

+1. I Had a fancy and expensive rope tailgut installed once. Everything was fine for like a month, but then the bass started losing pitch every day. I couldn't figure out why until the day it broke.

 

I have this issue on my '39 Kay as well...even just keeping it in my house. Always having to tune it up. I took it out to a gig last week and tuned it twice when we got there and again between the 2 sets we did.

 

Tailgut is a cheap experiment. If you’re handy you can replace it with a braided steel cable and a hammer-close fastener for under $3 at a big-box home improvement store.

I briefly tried a synthetic tailgut. Cute concept, but in practice it stretched excessively and sounded dull compared to steel cable.