Seeking advice: String Height even between 1st and 20th frets

[ob641]

Hey everyone, would love some opinions on this.

Recently I changed my strings on my short scale Serek bass. The new strings completely changed the action, and so after various saddle turns/truss rod adjustments, I am finding that the height from the 1st fret to the 12th fret is perfect, but the 12th fret to the 21st fret is higher.

For context, I had on a pair of DR Hi Beams for a few months before putting on some GHS bass boomers. The bass is 30.5'' scale, but I don't believe the Hi-Beams that were put on were any sort of specialised short scale strings.

Any tips or tricks would be greatly appreciated. I'm aware that the height between the bottom and top of the lengths will never truly be "even", but I'd like it to be as similar as possible. I'm trying to solve this problem myself before going to a luthier; self knowledge is empowering!

 

[Turnaround]

I am finding that the height from the 1st fret to the 12th fret is perfect, but the 12th fret to the 21st fret is higher.

That's normal. Might be able to help if we had some measurements. What relief is there now? What is the string height at the 12th fret, and at the 21st fret?

 

[KidAmnesia]

You need the string height to be very low on the first frets for the extra tension of pressing the string down to alter the pitch as little as possible. But if you keep that extremely low action even all along the fretboard you will have a lot of buzz because the strings move a lot more near the center of their length.

But anyway, if the action is fairly even between the first and 12th but then rises noticeably until the end of the fretboard, I'd think you have the trus rod too tight and it's forcing the neck into a back bow.