Fractal Audio Systems
Any triode stage is a "simple clipping circuit". IMO it's biased cold because if it were biased in the center it would cause so much blocking distortion that the amp would be unusable. You can't just look at a triode stage in isolation. You have to consider how it interacts with the other stages.It's a simple clipping circuit, copied from the Marshall 2203/2204. It's biased super cold, so the positive swing amplifies (although the AF is pretty low), but the negative swing drives into cutoff - the effect is not unlike a single-stage diode clipper.
Most amps use a grid stopper between the last triode stage and the PI, often with another resistor to ground as a divider. Typical values are 220K. If the amp has a tone stack before the PI then that resistor is unnecessary as the tone stack provides the grid stopper action.
And I don't agree that it's copied from a JCM800. In a JCM800 that stage is before the final gain stage so it doesn't get driven nearly as hard. In a JCM800 that stage only clips for strong signals. For a typical mV signal the stage is only mildly distorting.