A gas turbine combustor including: a primary combustion chamber; a secondary combustion chamber downstream of the primary combustion chamber; a venturi having a venturi throat; a transition piece; a cap assembly attached to the primary combustion chamber, and an external turbulator member in operable communication with the cap assembly, wherein the primary combustion chamber includes a mixing hole arrangement for improving homogeneity of an air and fuel mixture in the combustor; the venturi throat is disposed within a predetermined distance upstream from the downstream end of the primary combustion chamber; the transition piece is composed of a duct body, with a plurality of dilution holes formed in the duct body; and the external turbulator member includes a step positioned at the second end of the centerbody, the step defining a radial distance about the second end of the centerbody.