Just to add something, whatever the values of Ra, ra and mu, the gain for Case 1 is always twice that for Case 2.
Attached is the preamp schematic for the amp (small enough file to upload). Will build it on my breadboard soon, and get some "empirical" data on how the triodes perform....
No voltage dividers at the inputs. Grid stoppers on both grids would be recommended. The cathodes are fully bypassed. A plate load of 120k would increase gain a bit compared to my calculation (for 100k), while the load of the tone control and volume control would reduce it a bit. It will be interesting to see if the calculations tie in with your measurements.
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