Z-Matic is Fisher's name for variable damping. Other makers called it different things but it generally worked in a similar way. Its not a straightforward variable NFB, but it has a similar effect in that it increases output impedance just like less NFB does.
Its not positive feedback exactly either, its more strictly speaking current feedback rather than voltage feedback. Typical NFB is a tap off the output transformer. It just takes the output voltage and feeds a portion of that back to the input. If the load drags the voltage down, you get less feedback signal so it bumps the output. Effectively it makes the amp less sensitive to output loading and flattens frequency response. Current feedback is typically done as a resistor between the output transformer 0 ohm lead and speaker negative. As the speaker draws current, voltage is developed across that resistor and thats where your feedback comes from. No speaker load, no voltage, no feedback. Typically amps used a combination of voltage and current feedback as part of the variable damping setup. The Z-matic control just adjusts how much of that current feedback you get.
This is an 80AZ schematic but it should be essentially the same circuit on all of these
The voltage feedback is the 2.2k off the 16 ohm tap. The 330 ohm to ground forms the voltage divider to set what % of the output voltage comes back to the input tube. Output of that goes to the junction of the 1k pot R5 and S2.
The current feedback system is the two 1 ohm resistors between the output transformer C terminal and ground.
Notice the speaker GND terminal pulls current through one or both depending what position S1-B is in.
As the speaker pulls current, voltage is developed across those resistors. That voltage feeds to the other end of R5.
Depending where you have R5 set controls how much of the feedback to V1A cathode comes from voltage and how much comes from current feedback.
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