Abstract : This paper deals with the real time simulation of a class A single ended guitar power amplifier. Power tubes and triode models are compared, based on Norman Koren's work. Beam tetrodes and pentodes characteristics are discussed, and displayed as Norman Koren's model parameters. A simple output transformer model is considered, with its parameters calculated from datasheets specifications. Then, the circuit is modeled by a nonlinear differential algebraic system, with extended state-space representations. Standard numerical schemes yield efficient and stable simulations of the stage, and are implemented as VST plug-ins.