VALVES For Diatonics. The rationale for half-valving diatonics is that this offers:
-- Normal bends on the bendable notes
-- Chromatic-style bends on the notes that usually don't bend
-- Better air conservation (i.e., louder sound with less breath) on the first 6 blow notes and last 4 draw notes, because the valves are preventing air from leaking out through the reed you're not playing (this is what valves were designed for in the first place).
However, you also can't play overblows on a half-valved harp. (overblows are different from blow bends).
PT Gazell and Brendan Power are the two most prominent users and advocates for half-valved diatonics. Both get great results, which you can hear on YouTube and at their websites. That said, neither is a dedicated blues player, and if you're looking to play blues, half-valving will take you away from the traditional sound. Whether that's a good thing or a bad thing is up to you to decide, of course.
Valves minimize air loss when playing blow or draw notes.
If both reeds of a corresponding reed pair are equipped with valves, the possibilities of >bending the notes is very limited. If a valve is only closing the lower tuned reed of the pair then bending notes remains possible.
In our valved serial models we do not valve the very high notes (e.g. holes 11 and 12 on a Chromatic in D). If you select valves which will note operate correctly given your selections, we shall not attach them!
Half-valving (of Richter-tuned diatonics)
The valves are put on the draw plate of 1-6 (valve is on the inside of harp hole) and valves on blow plate of 7-10(valve is on outside of upper plate).This leads to better air-tightness of the instrument - bending notes are still playable!
In addition a skilled player can get the blow-bending notes in holes 1-6 blow and the draw bendings in 7-10 in a certain range. So all notes of the chromatic scale become available without the need of the overblow technique !
PT Gazell is the master on the half-valved harmonica and showed what becomes possible on these instruments!We use the same material for the valves that PT uses in his instruments!
Helpful info from Seydel regarding reed life etc...Click here
More good Seydel info
More helpful info on Reed failure From the great Joe Spiers.