Double Bass Bridge replacement on Thomann / Hora with factory bridge flat side up

[Transistor]

I've bought an eight-year old Thomann 22, made by Hora, Romania. The action is about 15 mm and the bridge is non-adjustable. I've purchased an adjustable bridge and the plan is to copy to the bridge shape and notches onto the new one with the adjusters up by 8 mm. Then when I turn them down about 7 mm (so the threads are almost fully engaged) my action should be about 7 mm lower.

In checking the existing setup I've found two things:
- The (I assume) factory-fitted bridge is flat side up.
- The bridge is warped. I can get a 3.5 mm drill bit in the gap between the set square and the bridge mid-height.


Left: my bridge setup, flat side up. Right: image from Thomann seems to suggest that it should be the other way up.

What's the recommendation here? The bridge is centred on the f-hole notches. Should I install the new bridge the same way or turn it flat side towards the end pin?

If I do reverse the new one and lower the bridge the existing bends in the strings would move towards the neck. Will this be a problem or can the tail-piece be pulled further back from the bridge to compensate? (I can see what look like knurled nuts on the ends of the tail-piece U-bracket but I'd need to disassemble to have a better look.)

Many thanks.

 

[Greg Clinkingbeard]

Not an easy job but you can probably do it. The bridge should be flat on the tailpiece side and could possibly be straightened with steam and some clamps. Fitting the feet on a bridge is usually done before the adjusters are installed, and you will probably see why. You can either make a jig to keep them from turning or I have removed the adjusters and temporarily glued the feet to the bridge with CA. After fitting, they can be separated. Good luck.

 

[Transistor]

Thank you for the reply. I've taken the bridge off and can now see that it was actually installed correctly but has very badly warped.

The square on the "flat" side of the bridge (which was facing the end pin).

I'll carry on. I'm scanning in the original to make some 1:1 templates.

 

[Transistor]

Progress report and the photos might help someone else. Remember, I'm not a luthier. I'm a retired engineer with a good selection of tools in my garage.


1. Photocopy or scan in the old bridge. Print a few copies 1:1.


2. Cut one of the templates to mark out the excess material. I removed the bulk of this by holding the blocks on a belt sander mounted upside down in the bench vice. (I now know that when you do this it fills up with dust that will fall out when you turn it over.)


3. My first attempt at a bridge clamp to use during the sanding / shaping process. I've used a piece of aluminium machine frame extrusion but a block of timber would do the same job.


4. Another view.

5. The setup on the bass. They're not shown here, but mark out on the blue tape the centreline of the bridge and the edges of the feet so that you sand in the correct area.

6. I scribbled all over the underside of the feet. Here you can see the progress.


7. Here you can see a ratchet strap used as a precaution to keep a little compression on the post so that it didn't fall over. I don't think I needed it.
I also used the vacuum cleaner repeatedly on the sandpaper to stop it getting clogged up and ineffective.

8. The result of the previous steps was a bridge quite a bit of vertical. I laser cut the green parts (but they could be made of thin ply, etc.) to square up the jig before reprofiling. The large green piece is sloped to match the bridge taper of 4.8°.

9. Transfer the bridge profile from the paper template. Check the string spacing - 27.5 mm in my case. Mark them with the calipers.



10. I used a 1 mm drill to give me a guide where to file the slots after sanding.


11. Excess was cut off using a fret saw and the profiling was finished with a drum sander on the drill press. (A kit of these in five sizes with 20 sandpapers was €15.) I'm sanding flat-side down so that the profiling is square (for now).

12. The profiled bridge with filed slots for test fit. Now marked for sanding down from 11 mm to 6.7 mm to match the original bridge.


13. The end result.


14. I used an orbital sander for the taper and held the bridge in place with a bench hook and a gloved hand!

I'll post a photo of the result tomorrow.

 

[Don Kasper]

10. I used a 1 mm drill to give me a guide where to file the slots after sanding.

This is evidence of a great engineering mind at work...as is the complete photographic documentation of your process.
YMBAL - You Might Be A Luthier.
I doff my cap to you, Sir. Well done.
Thanks.

 

[Transistor]

15. The end result with pickups fitted. And the internal post is still upright!

I'm pleased with the result. It was probably > 14 hours work due to having to make the jigs, taking extreme care with sanding, and the first sanding jig failing to keep the bridge upright. Would I do it again? Sure!

See Identification of twin piezo-like pickup set with XLR style cable 1/4" jack socket for a little bit of detail on pickup installation.

 

[dperrott]

Bridge work is tricky and complicated. That was one complicated way you did it but looks like it came out ok. Good job

 

[notabene]

how does it sound?

 

[Transistor]

>How does it sound?

It sounds fine to me but I've only had it a few weeks and had an instrument on hire for the previous three weeks. (I've been an amateur electric bass player for fifty years and acoustic fretless for the past five or six.) I'll see what my tutor thinks!

 

[fdeck]

Marking the material to be removed like that is a great habit... it avoids getting confused and working on the wrong side.