Not exact mu between sections in same bottle is a very common thing. Due to construction process tolerances and/or age. You just swap tubes between positions or change and swap again for best gain and inter-channel match results.
You will not calculate or scope anything. Because I have already measured and posted that using a Zobel as the manufacturer recommends has a true and strong effect.
No need you discover America once again.
After you install with final cabling trim, you start with the Jensen recommendation for Zobel and Itch input load. You just listen to 200pF, 150pF, 100pF in the RC and you decide.
No need you discover America once again.
After you install with final cabling trim, you start with the Jensen recommendation for Zobel and Itch input load. You just listen to 200pF, 150pF, 100pF in the RC and you decide.
Obviously, but if you will install directly across the SUT just make the Itch's input load resistor 1Meg. Else you just install 18-20K as Itch's input load and you leave the SUT alone. When in same box without interconnects I see no real reason you damp right at the SUT's secondary.
One can try the 12SX7 that has a good reputation for very similar triodes within the same bottle. Different heater though and not so common/cheap.
Or a selected pair of 6J5's
It makes sense when the SUT is a self contained unit in own box, so the grid will not be flapping on the cool breeze when the input is disconnected.