OK, first, off how are you running your el84’s. As pentodes with a screen supply, ultralinear, or triode connected.
The output transformer (+speaker) is the load and for modeling, is the plate resistor and they are fixed.
So, the Z-out is determined by the tubes dynamic plate resistance. If you add a unbypassed cathode resistor the DPR increases. For a triode it is Rk(u+1) "cathode resister x mue(amp factor) + 1. This is the effect of degenerative feedback.For a pentode, I don’t know ask someone else.;)
Now if you bypass the cathode resistor with a big cap. From a AC (impedance) point of view it’s not there. So you are more or less back were you started, from a Z-out point of view.
So is any of this to worry about. No. there is not much you can change, so go with what you like and don’t worry…John
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