Tom I think you are getting closer to my line of thinking. I am trying to look at modelling one basic SYSTEM issue of the amplifier. I'm not trying to analyse ALL potential modes of instability. I'm looking at the main reason the amplifier often needs the suppressor. This is INDEED because of the physics of the signal path outside the tube. You only need a simple model of a 'tube' with a basic internal feedback path to demonstate the instability mode that the suppressor targets. There are lots of other potential instability modes (same applies to transistors) but in this case there is a mode that is extremely easy to understand through basic modelling. The main contributor of this mode is the physics of the tank circuit. The tube is not the bad guy in this case.