In fact, this resistor only furthers the circuit's functioning, as it adds to the infinite impedance postulated in the constant current source view of the circuit and it furthers the balanced operation between the tubes in the push-pull view. Examining the three resulting traces reveals the current relationships within the circuit while it is being used and answers the question. 

    However, the constant current source aspect is still false, as even in the case where no load impedance is attached, the bottom tube is still not working into a constant current source, as the impedance represented by top tube falls quite short of infinity. Even without an external load, the bottom triode sees a load impedance no greater than the top triode's rp and the top tube's cathode resistor against the mu plus one:
         Z = rp + (mu + 1)Rk.

  The external load impedance effect on the load impedance seen by the bottom tube.

    Certainly, a pentode just might offer a higher impedance, although the screen bias circuit (the resistor and capacitor), because they are in parallel with the pentode, shunt the pentode, reducing its effectiveness as a constant current source.

  Testing the current relationships within the SRPP

     So what does the scope show? It depends. With or without load? The load is all important. It is the load itself that creates a second possible current path, as with a load, the current can flow from the bottom tube into the load or from the load into the top tube.  In other words, if the load is a 32-ohm or even a 2,000-ohm headphone, then the traces show that the push-pull view is correct. On the other hand, if the load is infinity, or what is close enough, 1-meg for example, then the single-ended/constant current source interpretation is half correct, as when the SRPP attaches to no load, the only possible current flow is straight up through both tubes, making differing current conductions impossible; thus, it is true, in this case with no load, that the circuit can only be single-ended in operation.

    Resistor R is the spoiler in this version of the SRPP, as its resistance shunts the pentode's.

< PREVIOUS   Copyright © 2002 GlassWare   All Rights Reserved