Scooping out rotor core - What can possibly go wrong?

[sirisobhakya]

I recently tried the Yamaha YBL-835 and like it, especially the larger sound and more open feeling. I don’t have or earn that much money to burn for a second new horn, though.

Seeing from the part list that the 835 only differs from my 830 in just a few aspects: gold brass tuning slide (which I have already), new leadpipe, bored-out no-step gooseneck (which should be very small difference), brass rotor caps, and scooped out rotor cores (the port is now U-shape instead of ʊ-shape), I have been toying with the idea about changing the rotor cores. However, Yamaha would not sell only the core, rather the full rotor assembly at almost $600 each. The potentially cheaper, but much more destructive way, is therefore cutting the rotor cores of the 830 into the desired shape.

I understand that there is no turning back, and if the result turns awry, I have to buy the rotor assemblies to fix it, sacrificing $1,200 plus labor cost.

Therefore I would like to ask: what can possibly go wrong with this modification? I understand that the response would change, probably drastically, but how about the intonation? And the change in the sound, would it be larger as I hope? Assuming that it is done by a competent tech, would there potentially be more friction/sticking problem? Overall, is it worth the risk? Or should I just do more OT and save up for the real deal?

[ithinknot]

Bad idea on a modern horn with fundamentally good valves. The difference is unlikely to be as "drastic" as you imagine, and you can't know which of the changes between the 830 and 835 had the effect you like (...and then there's sample variation...).

All you know is that you liked the 835 as a whole, so sell your 830 and buy one.

[Burgerbob]

Just get a new horn.

[hyperbolica]

The upside seems non-existant or small, and potential downside could leave you without a horn and wind up costing a lot of money . I'd let it be, and switch horns eventually if you still think its worth the money.

[tbonesullivan]

The valve caps on the 835 are also brass, which can change things compared to the standard nickel silver. The casing and knuckles are identical, so part of me really questions how much more "open" the valves are, and removing the "lip" on the edges of the valve cores also means that the air passage on the inside of the valve is now D-shaped, instead of oval.

Another thing to consider: when is the last time you had your entire horn chemically cleaned? Mineral deposits and oxides develop over time on the interior of most of the tubing on the bell section, which can have some effect on how it resonates. You were comparing your daily driver, which would definitely have some oxidation, along with pretty much brand new horns.

Then you also have "new horn" effect, which definitely is real.

Honestly, I would agree with the others that you should either leave your horn as is, or trade it in for an 835.

[harrisonreed]

Or get an Edwards

The new Yamaha basses and large Xenos were such a strange step for Yamaha. They had really good, lightweight designs, built well, and they moved to really heavy overbuilt designs, still built well.

I bet you could trade in or sell that Yammy for enough to offset the cost of a used Edwards bass.

[CheeseTray]

Years ago, I switched out the rotor core of a Bach 42B with one from a 50B, given that the 42 is known to have smaller than optimal tubing and rotor assembly for a .547 horn. While I found that it did create a noticeable difference in the "blow" of the valve, it was not a "game changer" or even all that significant in the scheme of things. After a while, I ended up putting the original core back in. Your results may be different. In the end, I think there are so many other variables with a valve's construction that increasing the diameter of a few centimeters of distance (even at a critical point like the valve) doesn't always provide the big results hoped for.

[Mamaposaune]

Years ago, I switched out the rotor core of a Bach 42B with one from a 50B, given that the 42 is known to have smaller than optimal tubing and rotor assembly for a .547 horn. While I found that it did create a noticeable difference in the "blow" of the valve, it was not a "game changer" or even all that significant in the scheme of things. After a while, I ended up putting the original core back in. Your results may be different. In the end, I think there are so many other variables with a valve's construction that increasing the diameter of a few centimeters of distance (even at a critical point like the valve) doesn't always provide the big results hoped for.

If the outer diameter of a Bach 42B and Bach 50B are the same, I assume that the 50 rotor is bored out more, so the inner diameter is larger? My husband's 42B had the rotor bored out by Osman by the previous owner; would that essentially make it a 50 rotor?
An aside, we were surprised to find that our 42B's, 36B, 88H, 78H and 72H all share the same size rotor cap - they are actually interchangeable!

[OneTon]

An aside, we were surprised to find that our 42B's, 36B, 88H, 78H and 72H all share the same size rotor cap - they are actually interchangeable!

Anyone try a 42B rotor cap on a Conn 8D?

[CheeseTray]

If the outer diameter of a Bach 42B and Bach 50B are the same, I assume that the 50 rotor is bored out more, so the inner diameter is larger? My husband's 42B had the rotor bored out by Osman by the previous owner; would that essentially make it a 50 rotor?

Mamaposuane,

The whole thing was done on a lark. I confess that I made no attempt at measuring the ports themselves but the 50 ports certainly looked bigger than the 42 ports. The outer diameter of the both cores is the same; I just dropped the 50 core in... no issues.

I would say that whether your husband's rotor became a defacto 50 core would depend upon how much metal you had taken out. As I mentioned, for me, while the feel of the horn's blow changed with the 50 core, it wasn't dramatic. I didn't feel compelled to stick with it and put the original back in... It was a good 42 with its original rotor core.

I'll add that I put an Instrument Innovations valve on another 42 and that was a night and day difference for the better.

[Posaunus]

I don't know about "scooping out" (increasing the cross-sectional area of the flow passages) a too-small rotor, but I do know that larger, more free-flowing rotors help the F-attachment range on some of the "classic" pre-1990s trombones (not just Bach 42s). The (King?) valve on my Benge 165F (early 2000s?) makes a noticeable improvement vs. my 1972 Conn 88H - so much so that it (or something very similar) is apparently now the valve on modern 88Hs. And the much more open Lindberg CL valve and wrap on the Conn 88HCL create a similar, positive difference.

I'm pleased that valve design (rotary and axial) have continued to progress - so much so that many of you have your favorite "upgrade."

[hornbuilder]

Here is a picture of a .594 ball in the port of a Bach 50B rotor. There is very little material in the center of the port to remove, when you realize both sides need to be opened up. There is a little bit more material on the sides of the ports, but that doesn't really help. Any benefit would be minimal.

[CheeseTray]

Matthew,
Just out of curiosity, what is the actual size difference between the stock 42 and 50 ports?

[braymond21]

I actually did this on a Chinese copy of a Yamaha 613H that I had lying around. It did change the way the valves played but not enough to really be worth it. It was a cool experiment and definitely helped the horn play a little better, but not enough to make it worth it.

[jjenkins]

Here's a video of Dana Hofer (Chicago) performing this service:

[peteedwards]

That method doesn't seem like it would be very effective. There is not enough metal just between the ports to come anywhere close to opening them up to "full bore"
When I scooped my rotors 20 some years ago I used a 3 axis CNC mill with a "lollipop" cutter (a ball endmill with undercut above the ball. I did not take any material out between the ports, but "squared out" the circular bore so it would be uniform cross sectional area throughout the passage (effectively, full bore size) I also smoothed out the lead-in/out areas where it normally encounters a sharp corner (I think Yamaha rotors do this already)
Here's a cross sectional drawing of what I did with the material removed in red:

scoop.png

The results were quite good, but now I have an even better way:
https://www.trombonechat.com/viewtopic.php?t=36910

[jjenkins]

I read your 3D printed rotor core post yesterday and found it to be fascinating to say the least.