ceases being push-pull, and becomes two single-ended circuits cascading from one to the other while sharing a common constant current source. To better see how this works we can deconstruct the circuit into two circuits:

   The result is an economy of parts that works at least as well as the previous version. The bottom triode still draws a constant current and the top triode still operates in perfect single-ended fashion. In fact, this version enjoys one advantage over the previous version: a potentially greater output voltage swing.

   In the previous version, all the potential upward voltage swing was strictly defined by the amount of voltage developed across the bottom constant current source, as the current source was tied to a fixed voltage. In the single constant current source version, the current source is tied to the top triode's cathode, which if it is not attached to a too low impedance external load, will follow the upward voltage swing of the bottom triode's plate. The maximum upward voltage swing is roughly equal to the voltage across the constant current source times the transconductance of the top triode times the external load impedance:
    +Vmax = Vcs x Gm x Rl.
The negative voltage swing is equal to the constant current source current value times the load impedance:
    -Vmax = Ics x Rl.
   What if we want to use a constant current source, but do not want to use a solid-state device? Here is where a new circuit is needed. By using three triodes we can use the center triode as a constant current source. All that is required is a large valued resistor Rak to span the center triode's cathode and the bottom triode's plate and a capacitor to relay the bottom triode's plate movements to the grid of the center triode. The effective impedance of this constant current source is given by:
    Z = rp + (mu +1)Rak.

    In the circuit above, the bottom triode's current is held constant by the constant current source and its gain approximates its mu. The signal then cascades into the grid of a Cathode Follower loaded by a constant current source. If we restack the circuits, we get the circuit below:

    In this circuit, like the previous version, the bottom triode operates independently of the top triode's operation with its current held constant by its constant current source, while the top triode works in in perfect single-ended operation with its idle current set by its constant current source. In the circuit below, the constant current source count has been reduced to one.