The circuit shown below dispenses with the zener-capacitor combination by connecting the output to the bottom of the split-load phase splitter' cathode resistor. "Surely the top output device is working as an unity gain follower in this circuit?" is what many readers are thinking. It may look that way, but the top device still provides gain and high output impedance. Here is why. A positive pulse to the output decrease the voltage across the cathode resistor, which implies a decrease in the voltage drop across the plate resistor, which results in the positive pulse expressing itself at the input of the top output device. The top output device cannot follow what it does not see and it cannot see the positive pulse if the output and the input track each other.

   Like the other long tail phase splitter based topology, this topology is in itself sufficient for building a complete amplifier, if solid-state output devices are used. In fact, if the load is high enough, for example headphones, this topology is sufficient with tube as output devices.

Conclusion
   We have covered quite a bit of ground on the topic of hybrid amplifiers. We have seen how symmetrical output stages work and how unsymmetrical one works. My regret is that there is far too much still left uncovered. A truly fat book is needed to cover hybrid amplifiers. A second regret I have is that I did not provide a name for each of the six common output topologies, as names would prove handy. But I cannot think of a naming system that would cleanly and definitively label each topology. And the usual audio practice of fluff names, such as "mega-ultra-supra-linear" sickens me.
    Next time, we will look into single-ended choke loaded source and emitter followers with triode front-ends. We must also look into the noise relationships within all the six output topologies. Until then, please send e-mail, if you have any questions or ideas to share.

         

                                     //JRB

    The bottom output device cannot see the positive pulse because the phase splitter's cathode is locked by the unwavering grid. This topology is the one used in the first Futterman amplifiers. The output tubes used were 12B4s and the amplifier used a current starved 6AU6 as its input tube.
    In the following circuit, the long tail phase splitter provides the phase splitting and the gain needed to drive the output devices. It also offers the full potential gain of the output devices working into the load and a high output impedance. It can be configured to accept feedback at the second triode's grid.

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