Continuing saga with my 6L6 build - Global feedback to a Floating Paraphase front end

[wstein25]

I have made great (ok, good) strides, in the refinement and performance of my 6L6 build. Schematic attached. It shows both feedback connections. One labeled NFG!, the other working. I replaced the output transformer with a serious hunk of iron from a Hammond Organ AO-63 model amp. From the size of it, it was probably designed for 35 watts or so. I also replaced the Paraphase input tube with a 6SL7 with the hopes of using more feedback to clean up the amp. But I just don't seem to be able to get the feedback right using the typical design of feeding the transformer output back to the cathode of the input tube. The results are oscillation or increasing total output with more feedback. Being unable to resolve this I decided to implement the summing junction at the grid. Rather than using 1K in series with the feedback signal to the cathode, which would oscillate, I now have 220K in series with the feedback signal sent directly to the grid At least with a signal generator/socpe or using Arta on a windows laptop, everything makes sense. On attaching the feedback, the output drops measurably, and I cut the 2nd harmonic from about 2% to 1% over most of the 20 to 20kHz sweep. And very close to flat response. It does show roll off starting at 15KHz, but I think that is an artifact of Arta/laptops, and a cheap USB sound card I/O. Mostly I would like to figure out what I can possibly be doing wrong with feedback to the cathode.