Circlotron Amplifier

GlassWare Audio Design Software

Bare-boned and currently in vogue, this circlotron circuit is a greatly simplified version of the classic Wiggins circlotron circuit, forgoing tetrodes/pentodes, screen-to-plate cross-coupling and output transformers. It is also known as the balanced amplifier. Advertising claims to the contrary, this circuit is 100% push pull in operation and it can be run in class-A, class-AB, and class-B mode. Two separate and floating power supplies are used per channel and both output tubes' cathodes sit at the same ground voltage potential at idle, greatly simplifying both the heater power supply and grid bias adjustments design issues, as all the output tubes can share the same heater and negative bias power supply.

Both triodes function in a modified input-signal-degeneration mode, as each output tube's grid sees only half of an externally applied perturbation to the output. This configuration results in a higher output impedance than the pure cathode-follower connection would realize. For example, a one volt pulse applied to the output, forcing the positive output half a volt positive and the negative output half a volt negative, will produce a countervailing decrease in conduction from the top tube and an equal, but countervailing, increase in conduction from the bottom tube, resulting in an countering of the pulse voltage. (The left tube's cathode will be half a volt more positive in relation to its grid-effectively making the grid half a volt more negative-and the right tube's grid will be half a volt more positive in relation to its cathode when undisturbed.) On the plus side, both the drive signals for both tubes are now equal in magnitude (although differing in phase) and both are referenced to ground, greatly simplifying the driver stage design.

This circuit delivers the exact same performance as the midpoint-referenced totem-pole output stage, although differing in topology, but not in voltage and current relationships.

(A single power supply could be used for one or more channels, if center-tapped chokes and extra power supply capacitors are added to each amplifier. Additionally, an output transformer could be used to couple low-impedance loads to high-rp tubes.)

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