A variable cross-section distributor device for inputting a mixture (M) of fuel and oxidant into a premix burner , wherein the distributor device comprises a distribution element having an axis (A) of propagation of the mixture (M) and is provided with a first surface (Si) for inputting the mixture (M) into the distribution element and with a second surface (Su) for outputting the mixture (M) from the distribution element , wherein the first and the second surface (Si, Su) are located opposite each other along the axis (A) of propagation such that said axis (A) is normal to both surfaces (Si, Su) at the respective points where said axis (A) of propagation crosses said surfaces (Si, Su), wherein the distribution element comprises a plurality of openings putting the first surface (Si) into communication with the second surface (Su) so as to allow the passage of the mixture (M) through the distribution element , the openings being mutually separated and distributed angularly about the axis (A) of propagation in such a way that the position of each opening on the second surface (Su) is defined uniquely by a respective azimuth angle (φ1, φ2, φ3), measured relative to a second axis (B) perpendicular to the axis (A) of propagation of the mixture (M), the distribution element further comprising deflector elements distributed on the second surface (Su), each of said deflector elements being located proximate to a respective opening and provided with a guide surface positioned above the respective opening and configured so as to be able to direct the mixture (M) output from the second surface (Su) in at least one deflection direction (D1, D2, D3), wherein the at least one deflection direction (D1, D2, D3) forms the same azimuth angle (φ1, φ2, φ3) with the above-mentioned second axis (B) as the respective opening , and wherein the deflection direction (D1, D2, D3) furthermore forms a deflection angle (β1, β2, β3) with the axis (A) of propagation, wherein the deflector elements comprise: a first fixed deflector element the guide surface of which is oriented in such a way as to direct the mixture in a first fixed direction (D1) forming a fixed, non-null first angle (β1) with the axis (A) of propagation; a movable deflector element the guide surface of which is oriented in such a way as to direct the mixture in at least one second direction (D2) which is variable and different from the first direction (D1) and wherein said guide surface is provided with means for varying the second direction (D2), said means being configured to allow a relative displacement of the guide surface away from or towards the second surface (Su), the relative displacement being a function of the pressure (P) exerted on the guide surface of the movable deflector element by the mixture (M) output from the respective opening . a first fixed deflector element the guide surface of which is oriented in such a way as to direct the mixture in a first fixed direction (D1) forming a fixed, non-null first angle (β1) with the axis (A) of propagation; a movable deflector element the guide surface of which is oriented in such a way as to direct the mixture in at least one second direction (D2) which is variable and different from the first direction (D1) and wherein said guide surface is provided with means for varying the second direction (D2), said means being configured to allow a relative displacement of the guide surface away from or towards the second surface (Su), the relative displacement being a function of the pressure (P) exerted on the guide surface of the movable deflector element by the mixture (M) output from the respective opening . <EOS>