If I got Eric right he said:
"...First I believe Jim Williams used a similar arrangement to make the LT1088 rms to dc converter, but also when you start to look at the stability of any thermal feedback system the transit delay from the heater to the sensor plays a big role in determining the effective loop phase margin of that loop. So in theory a trade-off exists between the thermal gradient and the thermal stability, that is the closer the rings are to the core the less phase delay is introduced in the feedback, but that would cause a larger gradient between Q2, the zener and Q1. So the fact that this was a kit part makes me think that both, Jim and Carl were using this for the wrong purpose, Jim was using it to create an rms to dc converter, so he just wanted the thing to run fast, so he wasn't using the zener, he was using the temp sensor and the heater. However Carl had to be very concerned with this phase delay effect and also gradients. If you move the heater infinitely far from the sensor what you get is basically a bang bang controller. The heater heats up, it takes to long for the sensor to pick it up and consequently the control loop bangs back and forth inbetween the rails..."
What I hear out of that "kit part" is, both Jim and Carl basically used the same set of masks to create their individual solutions, Jim his thermal converter and Carl his heated zener reference. So that could explain why there are these additional heater rings, we again find in the LT1088.
Answer 2 seems wrong. In the zener mode the tempco would move into the negative area, wouldn´t it?
It's always a mixture of Zener and Avalanche effect (breakdown voltage <4V is kind of a pure zener effect, while with breakdown voltage > 6V the Avalanche is dominant). So decreasing the breakdown voltage by accident, as Eric said by "pure luck", its a little more of a Zener than Avalanche effect.
-branadic-