A real Trainwreck Express is a fizzy, spitty amp. Some people like this. The same people who like fuzz pedals and that quasi-half-wave rectified sound. Personally I like more clarity.
The reason the Express is like this is primarily due to the last triode stage before the phase inverter (PI). This stage is biased very cold and drives the PI very hard due to the lack of any limiting resistor (grid stopper) between the triode and PI.
The lack of a grid stopper resistor and large coupling capacitor create significant bias excursion (blocking distortion). It appears the designer attempted to minimize the bias excursion by biasing the triode cold which limits its positive excursion and therefore limits the PI grid conduction. Despite the cold bias point the PI grid is still heavily overdriven resulting in massive blocking distortion.
The Axe-Fx allows you to reduce the amount of block distortion via the PI Bias Excursion parameter. Simply turn this down and the spittiness will go away. You can then also rebias the triode via the Preamp Bias parameter. Values closer to zero result in a more symmetric waveform.
The reason the Express is like this is primarily due to the last triode stage before the phase inverter (PI). This stage is biased very cold and drives the PI very hard due to the lack of any limiting resistor (grid stopper) between the triode and PI.
The lack of a grid stopper resistor and large coupling capacitor create significant bias excursion (blocking distortion). It appears the designer attempted to minimize the bias excursion by biasing the triode cold which limits its positive excursion and therefore limits the PI grid conduction. Despite the cold bias point the PI grid is still heavily overdriven resulting in massive blocking distortion.
The Axe-Fx allows you to reduce the amount of block distortion via the PI Bias Excursion parameter. Simply turn this down and the spittiness will go away. You can then also rebias the triode via the Preamp Bias parameter. Values closer to zero result in a more symmetric waveform.