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Truly mixed modes/topologies
The amplifier shown above combines Class-A and Class-B with single-ended and push-pull. Two output transformers are required, but only a single power supply and input circuit is needed. This amplifier relies on the 300B to deliver the majority of that all important first watt of power and on the 6550s to deliver the wallop missing from so many single-ended amplifiers.
I would love to see some research on the Class-A-SE/Class-AB-PP amplifier that Nelson Pass has made famous in solid-state circles. In past issues we have covered possible tube implementations of this style of amplifier. The attempts relied on using a pair of output tubes and controlling the grid signals to these tubes. An alternative approach would to add a third tube to an existing push-pull output stage. In the schematic to the right, we see a constant current source loading half the primary with a current equal to that being drawn by the bottom EL34. This balanced current ensures that the transformer will not saturate at idle. In the absence of the top EL34, the amplifier would be purely single-ended, but the top EL34 allows the to break out single-ended operation.
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