Digitally modelled guitar amplifiers and effects have been around for a long time, but the sound of a real analogue circuit is still preferred by many guitarists. This talk will describe the development of a real-time analogue circuit simulator that can faithfully reproduce the sound of everything from tightly coupled transistor fuzz pedals to multi-stage valve amplifiers. The talk will cover the principle of operation, some challenges of optimisation, component models used, and will include a demonstration of the automated schematic to simulation code process.