Initial design ideas took six months and initially I stupidly thought that I could create the amp on tag board or point-to-point like most of my other amps. I started with the Uno that I initially bought along with a 4x20 LCD display and tried looking at digital potentiometers to replace the traditional pots. This threw up more problems than I thought it would. The Uno didn't have enough PWM enabled pins, so I bought a 16 bit breakout board. This was successful but the digital pots were fiddly as they were SMD. It all looked promising until I realised that most of the signal voltages in a valve amp are a lot higher than that which can be handled by digital pots. Back to the drawing board. The picture shows my first breadboarded tone stack using a digital pot and three Silonex NSL-32.