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Right-Grounded Circlotron

Although quite different looking in topology, this circuit delivers the exact same output performance as the cathode-follower totem-pole output stage, as it exhibits the same voltage and current relationships, the same gain and output impedance. Two separate and floating power supplies are needed as in the conventional circlotron and the same ease-of-biasing and same single-heater-power-supply advantages obtain, as all the output tubes' cathodes are at ground potential.

(Actually, one output tube's cathode is literally grounded, but the other tube's cathode traces the output signal, which although unlikely to be more than a few tens of volts in the average OTL amplifier, can be too great in voltage swing in a transformer-coupled amplifier based on this topology.) Asides from a lower output impedance, this variation on the circlotron allows an easier negative feedback loop connection to be made, as the amplifier has only a single output relative to ground.

 

Both triodes function in a 100% input-signal-degeneration mode (cathode follower mode), as each output tube's grid sees all of an externally applied perturbation to the output, which results in a lower output impedance than the conventional circlotron. The drive signals for both tubes are no longer equal in magnitude (as referenced from ground and the driver stage), however, as the top tube input signal is referenced to ground; while the bottom tube input signal reference is the output. Viewed from the perspective of the cathode and grids, however, each tube sees the same signal magnitude; viewed from the perspective of ground, the top tube sees a much larger input voltage swing than does the bottom tube.

 

(A single power supply could be used for one or more channels, if two chokes and two power supply capacitors are added to each amplifier.)

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