The Accordion Amplifier
A new single-ended topology

     It's time to stir things up a bit: how about a single-ended amplifier that doesn't look single-ended. Single-ended amplifiers have come in basically three flavors: conventional, parallel, and para-feed. The conventional arrangement is shown below. Consisting of just a few parts, this amplifier is conceptually the easiest to understand. (Of course, "easiest" is relative; many do not understand how, when in use, the plate can swing to a voltage more positive than the B+ voltage.) The para-feed arrangement places the output transformer between the plate and ground and uses a choke to mimic a constant current source. (In fact, a current source can replace the choke at the cost of needing a twofold increase in B+ voltage.) This variation provides a better PSRR figure and allows the use of a non-airgapped output transformer, which means that a nickel core material can be used, furthering the transformer's performance. The parallel arrangement is the conventional arrangement but with multiple output tubes.

2A3-based parallel single-ended amplifier

2A3-based para-feed single-ended amplifier

    The new accordion amplifier also uses multiple output tubes and should be called "the series single-ended amplifier," but why start being sober after almost a hundred years of nomenclatorial whimsy. Besides, the accordion's expanding and contracting nicely symbolizes this amplifier operating principle.

2A3-based conventional single-ended amplifier

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