Which has a bigger cup — Bach 18 or Kelly X-SMALL?

The Bach 18 has a cup diameter of 32.10 mm / 1.2638 in compared to 30.73 mm / 1.2098 in on the Kelly X-SMALL — a difference of 1.37 mm / 0.0539 in. A wider cup generally produces a fuller, darker tone but requires more air support.

How does cup depth compare between Bach 18 and Kelly X-SMALL?

The Bach 18 has a medium cup while the Kelly X-SMALL has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.

Bach 18 vs Kelly X-SMALL

Name: Kelly X-SMALL
Brand: Kelly

The Bach 18 has a wider cup diameter, the Bach 18 features a deeper cup.

Overall Comparison Cup Diameter Cup Depth Rim Width Throat Diameter Backbore Finish Material

The taper behind the throat leading to the shank. Affects projection and tone character.

Mouthpiece A

Bach 18

• Tuba

Backbore—

Best suited for

📣High-volume projection — open throat for power

Mouthpiece B

Kelly X-SMALL

• Tuba

Student

BackboreS

Best suited for

📋Student / Eb tuba / euphonium doubler

Which is better?

Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.