half valved diatonic harmonica

Do you mean how half-valving works or what PT specifically does?

As you probably know, regular bends on diatonic depend on both the blow reed and the draw reed sounding the bent note. The higher pitched reed bends down to slightly less than a semitone above the lower pitched reed. The lower pitched reed does not bend down except for a few microtones. Having two reeds sounding the same note in diferent ways enroches the tone of the note and increases the stability of the bend,

Let's say you have Hole 4 of a C harp. BLow C, Draw D. The D bends down to a slightly flat D-flat.

The C doesn't bend down, unless a valve is placed on the slot of the draw reed, placed on the inside, on the side of the slot opposite the reed. That way, when you blow, the valve is flattened against the draw slot, preventing any air from pasing through the draw slot and directing all the air to the blow reed, isolating it as a true single reed.

The result of this type of half valving is to

- make the blow reed louder,

-to make it more sensitve to hard attacks - it will blank out of you blow too hard,

- to allow it to bend down as an isolated reed, which differs from a dual reed bend in two ways:

1. It will be harder to control
2. It will not b limited in range by another reed; bending range will be determined partly by gapping and partly by player skill
3. The tone of the bend will not be as rich as with a dual reed bend.

When I listen to PT play, the tone of his harps remind me of the tone of a chromatic, largely because of the valving.