Posted by Steve Inman on December 25, 2000 at 21:31:02:
In Reply to: Hawkes EEb posted by Mo on December 25, 2000 at 20:19:03:
Steve's Compensation Mini-FAQ:
When the 4th valve is used (this concept is true with any combination of valves to a lesser degree) your Eb tuba "becomes" a BBb tuba, but with 1,2,3 valve tubing too short for a BBb, 'cause they're designed for an Eb. (These valves will lower an open tone 1,2 or 3 half steps when your tuba is an Eb tuba. They'll *almost* lower the open tones of a BBb tuba by the same amounts, but not quite.) As you add more valves, this error increases -- so much so that the note 1 step above the fundamental (for your horn LOW F) will play about 1/2 step sharp. So, you need to play low F using the fingering for low E, and then low E cannot be played at all -- there's no fingering left.
Enter compensation. The 4th valve tubing doesn't just make a loop, starting and ending back at the 4th valve. Instead it finishes up its loop by running back through an upper passageway through the other valves (the same total length is used, of course.) The trick is that the second upper (on my Besson 983) passage through valves 1,2,3 has a second set of tuning slides & tubing that will be activated if valves 1,2 or 3 are pressed while valve 4 is pressed. This extra bit of tubing happens to be the amount of extra tubing needed to allow each valve (1,2 or 3) to lower an open note of a BBb tuba by the proper amount, allowing your Eb tuba to play in tune as well as a 3 valve BBb tuba would. Hence, you'll still need a slide pull here and there -- especially with low F or E, but these notes are fairly easily played in tune. I usually lip them instead of pulling a slide.
If you don't have a compensating tuba, then you have to add the 2nd valve to the "normal" fingering of the very low notes whenever you can no longer lip them down into proper pitch. On an Eb tuba, this will occur somewhere around low G. If you have a 5V tuba with a flat whole step tuning for 5, then you'll substitute 5 for 1 to do your pitch correction in this region. Other techniques that you can use to make adjustments include trigger devices for the valve tuning slides or the main tuning slide. ANY of these options allows you to "compensate" for low register intonation problems. So one COULD claim that any tuba with a method to play all notes in tune is a compensated tuba. TRADITIONALLY, however, the technique used by Besson/Boosey/Hawkes is the one referred to as a compensated tuba.