Sorry for the late reply. Everyone is one the right track here. The new network is literally a drop in fit for any of the old ones (PEC or discrete) in virtually ANY piece of Fisher stereo gear that has a magnetic phono preamp on board. Therefore, all of their applicable pieces will benefit from the new network for greatly increased RIAA accuracy.
Fisher did "fiddle" with the basic phono preamp design over the years, but all of this amounted to little if any ado. The vast lion's share of response shaping is created by the feedback network employed, so regardless of whatever other small changes you might see in a particular unit, the new network is still a drop in fit as is.
One other thing I'll pass along: George contacted me last evening to say that his continuing efforts with this (to produce high accuracy modeling for 12AX7 circuit design -- my insertion here) uncovered one additional slight glitch in the software used for this modeling exercise -- one that only affected response at the lowest frequencies. As a result, the response from his model is now a virtual carbon copy of my real world measurements produced from my working unit -- even right down to 20 Hz, which is quite incredible. The maximum deviation between his model results and my real world measurements from the new network are maintained within just .18 db now, across the entire 20 Hz to 20 kHz audio spectrum. This kind of adherence between modern CAD based vacuum tube circuit design and real world measured performance involving highly stylized EQ responses is exceedingly rare.
Understand that George is using one of the most widely recognized and accepted modeling programs out there to run these modeling exercises on. And yet, he has had to introduce a number of adjustments just to it to make it properly recognize accurate operation of the amplification stages used in the Fisher preamp design. It is this kind of required attention to detail that is absolutely necessary if modeling is going to be used to support vacuum tube circuit development work. If that level of attention is not employed, it would be much better to not use any modeling at all, as it can easily and unknowingly steer you in the wrong direction very quickly. It's the reason that I continue to be quite skeptical of the typical modeling results that are slung out there, while at the same time, utterly amazed at what it can accomplish in the hands of a truly skilled individual. We are all indebted to his contribution in this project.
Dave
Fisher did "fiddle" with the basic phono preamp design over the years, but all of this amounted to little if any ado. The vast lion's share of response shaping is created by the feedback network employed, so regardless of whatever other small changes you might see in a particular unit, the new network is still a drop in fit as is.
One other thing I'll pass along: George contacted me last evening to say that his continuing efforts with this (to produce high accuracy modeling for 12AX7 circuit design -- my insertion here) uncovered one additional slight glitch in the software used for this modeling exercise -- one that only affected response at the lowest frequencies. As a result, the response from his model is now a virtual carbon copy of my real world measurements produced from my working unit -- even right down to 20 Hz, which is quite incredible. The maximum deviation between his model results and my real world measurements from the new network are maintained within just .18 db now, across the entire 20 Hz to 20 kHz audio spectrum. This kind of adherence between modern CAD based vacuum tube circuit design and real world measured performance involving highly stylized EQ responses is exceedingly rare.
Understand that George is using one of the most widely recognized and accepted modeling programs out there to run these modeling exercises on. And yet, he has had to introduce a number of adjustments just to it to make it properly recognize accurate operation of the amplification stages used in the Fisher preamp design. It is this kind of required attention to detail that is absolutely necessary if modeling is going to be used to support vacuum tube circuit development work. If that level of attention is not employed, it would be much better to not use any modeling at all, as it can easily and unknowingly steer you in the wrong direction very quickly. It's the reason that I continue to be quite skeptical of the typical modeling results that are slung out there, while at the same time, utterly amazed at what it can accomplish in the hands of a truly skilled individual. We are all indebted to his contribution in this project.
Dave
and my head is clearer now. I've re-read Dave's post and I'm on board on what to do 
