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Current Change

"Of course, all of the above applies to the case of a cathode resistor Rk whose bottom end is connected to a (finite) negative voltage source Vneg. In the limit where one approaches an ideal constant-current source (which could be viewed as Vneg goes to minus infinity and at the same time Rk goes to infinity so as to keep the quiescent cathode current at the desired value), the tube V2 still gets driven even though the changes in the two tube currents *do* now become equal. That is because it takes only an infinitesimal change in current to change the voltage dropped across an infinite resistance by a non-zero amount. "

I believe you're referring to the current through Rk, and that is clearly equal to (I1-I2). However, this is not responsible for creating the AC voltage that drives V2. That voltage derives from the V1 cathode. In fact, I'm bothered by Crowhurst's statement that, "This change is the input to V2 and, if Rk is very large, we can have (I1-I2) very small and still get some drive into V2." This implies that the drive for V2 is created only by (I1-I2) flowing in Rk. However, the drive into V2 is in fact provided by V1 as a voltage source. It only takes a few minutes with SPICE to demonstrate that the AC voltage at the cathodes remains relatively unchanged as Rk approaches infinity and the tube currents become equal.













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