I put a star in the title of this post because my analysis is going to be really rudimentary and amateur. It’s just where I’m at in my understanding.
That said lets look at the Vulcan, which is a great overdrive/distortion with a very unique circuit design. The circuit comes from Joe Davisson. Sadly Joe has passed away, but his site is archived over at diystompboxes.com.
Here is my drawing of the Vulcan.
This looks like three Common Emitter Amps in a row. The innovation is the diodes D1-3, which sit between the lower base resistor. Explanations I’ve read say that this prevents the transistor from becoming saturated.
Here is a link to a simulation of the first stage of the Vulcan. The whole circuit is three of these in series.
The output has a soft clipping, similar to a Tube Screamer. Try removing the diode and replacing with a wire. Notice the clipping sharp and asymmetrical. With the diode the clipping less sharp and more rounded.
Driving this with higher input, try changing the AC source to 500mV or 1V, with the diode between the base the lower bias resistor, the output is more rounded, and symmetrical. Try removing the diode and replacing it with a wire. As you drive the input with a hotter signal the output gets a hard square clipping.
Since the output of the CE amp is inverted, the effects of the “Vulcan Diode” will occur at the top of the wave form. Clipping occurs when the base of Q1 becomes saturated. The purpose of the “Vulcan diode” is to prevent the base from becoming saturated. With the diode in place the base can never get below 0.7V.
Removing the diode we need to adjust the bias for optimum performance, I don’t think this changes the clipping behavior, so the experiments here are still valid information.
These images show the simulation with and without the diode. The input is 100mV AC. Notice without the diode the clipping harder along the top edge. With the diode distortion shows as a mild rounding on the bottom edge.
Lets try 200mV. The clipping is stronger with the diode but is still softer. Without the diode the wave gets clipped hard on the top edge.
With a 500mV input the effect is even more extreme.
Conclusions
This is a sort of amazing innovation. I’m surprised no one else thought of this. Seems to remove the hard clipping of silicon transistors and turns it into a more tube-like clipping, that is becomes more pronounced as the input is driven harder.