Calculations for Bb slides of C horn

[sirisobhakya]

I finally bought a bass trumpet, a Chinese-made 4-valve rotary C from Thomann. I wanted the Bb one, however it seems to be out of stock in every store I checked, including Schiller, Jim Laabs, Thomann, Reverb, and Ebay.

I can transpose Bb notes for C fingering, however not on the fly. I plan to train myself to be familiar to the C horn as much as I can. However I am not so sure I can do this all the way, and maybe, in the not so far future, I would decide to have slides made to convert it to Bb.

I tried to calculate the additional slide length, however 2-4 valve slides look like they can be tuned to Bb with their current available correction length, according to the photo from Thomann's website (https://www.thomann.de/intl/th/thomann_ ... rumpet.htm). I don't quite trust my calculation, though. Also, the base length may need more accurate value (for A = 442). So please help see whether all is in order, and advise where necessary. Your help would be highly appreciated.

A C bass trumpet overall length = 8'

The length of the first valve = 8' x (1-1.059463^2) = 0.9797' = 11.756"
The length of the second valve = 8' x (1-1.059463^1) = 0.4757' = 5.708"
The length of the third valve = 8' x (1-1.059463^3) = 1.5137' = 18.164"
The length of the fourth valve = 8' x (1-1.059463^5) = 2.6787' = 32.145"

A Bb bass trumpet overall length = 9'
The length of the first valve = 9' x (1-1.059463^2) = 1.1022' = 13.226"
The length of the second valve = 9' x (1-1.059463^1) = 0.5352' = 6.422"
The length of the third valve = 9' x (1-1.059463^3) = 1.7029' = 20.434"
The length of the fourth valve = 9' x (1-1.059463^5) = 3.0136' = 36.163"

Difference
Overall Length = 1'
First Valve = 1.47"
Second Valve = 0.71"
Third Valve = 2.27"
Fourth Valve = 4.02"

[brassmedic]

Instead of all that math, I would measure the first valve tubing with a piece of string. That should be the amount of tubing you need to add to the main tuning slide to lower the pitch by a step. Once you have your Bb main tuning slide in place, you can play test it and see if there's enough pull on the valve slides to be in tune.

[LeTromboniste]

The C=8' and Bb=9' are theoretical, "ideal" values. The ratio between the bore and the length of tubing will change the pitch somewhat (if I recall correctly, for an equal length of tubing, a smaller bore will slightly lower the pitch, a larger bore will raise it), as will conicity. There in the bell where it's so wide in relationship to the overall length where it doesn't really count as tubing anymore, as well. So your actual length of tubing will be somewhat different from the theoretical value. If you measure a bunch of trombones, every model will probably be a slightly different length, and none will be exactly 9'.