173.48.30.29
A common-cathode circuit with an unbypassed cathode resistor is an example of series-series negative feedback. This means the feedback signal is sampled from the output current and summed with the input voltage. The effect of current sampling is to raise output impedance, and the effect of voltage summing is to raise input impedance.
It's easy to show that the series-series feedback model is a good fit to the tube circuit in question. That doesn't mean you are obliged to use the model to predict or explain circuit operation. But to claim that the model doesn't fit (that is, that the unbypassed cathode resistor does not introduce negative feedback) raises a logical inconsistency that's pretty much impossible to reconcile with accepted rules of circuit analysis.
-Henry
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