Drone Valves from Highland Reeds – set of three.  These drone valves are screw adjustable which means that they can be accurately set to suit your blowing strength and don’t fluctuate with bad blowing.
 
Benefits are:
Assisting with strike in and cut off
Accurately set to suit your blowing strength
Regulate air flow to the drones giving a much steadier drone sound
A real help for pipe majors who need a uniform sound from their unsteady blowers