[freakaftr8]

Right. I was able to achieve low current with a CTC 17 but the linearity was the same as you described. if I were to get it better to my liking, the H cathode current was not happy about this. I would look to the capacitors in the horizontal section, including filters and see if any are off spec?

Also have you checked cathode current?

Are any of the filter can electrolytic capacitors getting warm? This would have to be checked before the main transformer radiates heat into the first can capacitor.

Also do you have any crawl in the picture? I had a crawl on the CTC 17. Once I isolated the filters and replaced some high ESR caps, my crawl was gone and I was able to establish a better linearity with low cathode current from the efficiency coil.