Posted by C(G) on November 04, 2003 at 21:48:17:
In Reply to: 3 valve comp? posted by Matt G. on November 04, 2003 at 21:29:00:
On every auto-compensating horn (i.e. Blakely system), it falls upon one valve to serve as the "master" which triggers the addition of short lengths of tubing to the other valves.
On an Eb 4-valve compensator, the 4th valve serves as the "master", re-routing the air path back through the first 3 valves, adding sufficient tubing to each of the first 3 valve branchs to make the compensated length of the slides equivalent to a 3-valve uncompensated BBb tuba. In other words, the "pitch" of the instrument has been lowered to BBb and an Eb compensating instrument with it's 4th valve tied down will play as if it were a 3-valve BBb tuba. This means that 1+3 and 2+3 combinations will be slightly too short, but not nearly as short as they would be if this were an uncompensated Eb tuba.
On a 3-valve compensated Eb tuba, the master valve is the third valve, which has the effect of lowering the "pitch" of the instrument to CC. The only out-of-tune valve combination (in theory) will be 123, which will be a teeny bit sharp.
What this means is that a 3-valve compensator has the ability to play nearly all of its valve combinations in tune, whereas the 4-valve compensator will exhibit some degree of mistuning on 124 134 234 and 1234 combinations.
The downside is that the 3-valve Eb is playable without false tones down to a somewhat sharp A natural, where the 4-banger can take you more-or-less chromatically into the pedal range.
For brass band use, both 3-valve Eb and BBb instruments make a lot of sense--and they tend not to be as stuffy as the 4-valve variety.