A linear model for use in determining stability, relative stability, and dynamic response of phase-angle modulated SCR systems is presented. The model is based on sampled-data control theory and assumes 1) a system bandwidth that is small relative to sampling frequency, and 2) limited excursion of the SCR triggering angle. These assumptions are justified for practical systems. The theory leads to a simple equivalent transport-lag representation. If the triggering angle varies over a relatively wide range, a "worst-case" analysis is necessary using the linear model. An example is included to illustrate use of the model for system design and is supported by experimental data.