[Radio Wrangler]

The ultra-linear circuit puts the valve part way to acting as a triode. The data sheet gives figures for the anode impedance of tetrodes as tetrodes. So the impedance the anode presents to the transformer will be different and lower.

The impedance the transformer presents to the valves has nothing to do with the valve's anode impedance. The designer made plots of the anode voltage versus current and constructed a 'load line' on the plot, planned to swing the anode over a reasonable range considering the valve and the HT voltage, while taking a swing of current designed to take the chosen amount of power with that voltage swing.

Have a read around some classic valve-era design books like Terman and the radiotron design handbook. Look up load lines.

Get comfy with how plain triodes and plain tetrodes anodes are biassed-up and loaded. Look at ultra-linear once you're clear on the plain versions. You need to understand them before you can appreciate what the ultra-linear circuit does.

David