Rick asked me to correct the post he sent where he said the 576 HV transformer is also used for the voltage applied to the DUT.
I haven’t looked at the 576 schematic in any detail because of my preference for 577s. What I can speak about is how the 577 works in specific terms, and in the paragraph following that one, how a common design concept, first applied by John Kobbe in the 570 for the plate voltage, was so simple and effective that it is used for the collector voltage in the 575, 576, and 577.
In the specific case of the 577 there is a separate winding for each collector supply voltage selection: 6.3V, 25V, 100V, 400V, and 1600V. Each of these individual secondary windings (not taps) are on the collector power transformer. The primary of this transformer is directly connected to the Variac. By controlling how much voltage is applied to the primary, the Variac controls how much voltage comes out of the secondary at each of these collector voltage windings. An elegantly simple design.
I am not familiar with how the 576 generates the collector supply voltage, but I am virtually certain it would be done the same way it was done on the 570, 575, and 577. A fundamental design principle of the 570, 575, 576, and 577 is they all supply full wave rectified power line frequency AC to the collector in the most commonly used configuration.
The HV generated in any of the curve tracers would be DC only. Reading the Theory of Operation section for any of Tek’s curve tracers would explain why they use full wave rectified AC taken directly from the power line. The HV supply would only be capable of a few microamps of current. Also, if the collector voltage came from the HV transformer then every time you toggled the A/B switch and connected a transistor to the high voltage, the CRT would dim and go out of focus due to the current drain.
Dennis Tillman W7PF