The one hint that controversy loomed ahead is in Valley and Wallman's Vacuum Tube Amplifiers in section 11-11: "The circuit between Epp and Ep, comprising the plate load for the lower tube, resembles the constant-current source of Fig.11-18, but without the battery it is not actually a constant current device. It is merely the equivalent of a  simple resistance, of value rp + (mu + 1)R, returned to Epp." The key word was "resembles."

The first of two Views
    Today, the prevailing view (a new view) is that the SRPP is an SE circuit made up of a grounded-cathode amplifier that is loaded by a cathode follower that also functions as a constant current source; (a nice trick: providing a low and an infinite impedance at once); and that since the whole point behind a constant current source is high impedance, pentodes are seen as the best choices for the top active device; and that the resistor separating the tubes sets the bias voltage in the DC-coupled version and should be made as large as possible in the AC-coupled version to increase the gain and lower the output impedance; and, finally, that the bottom triode provides voltage gain to drive the top tube, but does not contribute to the heavy work of delivering current into the load.

    And since all agree that the grounded-cathode amplifier provides the most gain and lowest distortion when loaded by a constant current source and that the same holds true for the cathode follower, we kill two birds with one stone (one resistor, in this circuit).
    Of course, the resistor that spans the bottom tube's plate and the top tube's cathode is only a resistor and not a constant current source, but many do not realize this fact. However, a few circuit designers have understood this point and have replaced the resistor with true a constant current source, usually a transistor based one, which brings the problem of biasing the transistor with a capacitor and resistor that end up shunting the constant current source, substantially reducing its effectiveness.     

         Transistor-based constant current source, whose impedance cannot exceed resistor R's value

    A better approach would be to use a FET as a constant current source, as it would not require the shunting bias resistor. Unlike transistors, FETs conduct current when their gates are tied to their sources. In fact, many FETs conduct too much current for the average tube and would require using a source resistor to lower their current conduction, which would also--happily-- increase their output impedance.

         SRPP as a constant current source load

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