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Grounded-Cathode Transformer-Coupled (fixed bias) Amplifier

This is the classic push pull tube amplifier topology which 90% of tube power amplifiers use (and an additional 9% use the same topology, but with cathode biasing). The output tubes provide voltage gain and the idle current is set by a fixed negative power supply voltage. The output transformer works as an electrical lever that magnifies the load impedance presented to the output tubes to better match the output tubes high output impedance and magnifies the output tubes current swings, while it reduces the output tubes' voltage swings and output impedance to better match the loudspeaker's low impedance.

Transformers are usually specified by their plate-to-plate impedance. This is actually their reflected impedance, as in itself a transformer has no intrinsic impedance, only a winding ratio between primary and secondary windings (just as a magnifying glass has a magnification rating, but no intrinsic magnified image). The full plate-to-plate impedance is not the impedance used in drawing a load-line on the tubes' composite plate curves, as the composite "tube" only sees one quarter of this impedance.  This makes sense, as each output tube only sees half of the primary, which means that it sees an effective winding ratio only half that of the plate-to-plate ratio to the secondary. (Actually, the load that a single output tube sees is not constant in all but the strictest class-A push-pull amplifier, as the mutual coupling between primary halves works to present a curved load-line as tubes slowly switch on and off. The composite output tubes, however, do see constant-load impedance and, thus, a straight load line.)

Like all push pull amplifiers, this amplifier treats distortion harmonics produced by the output stage in opposite ways: it greatly reduces even order distortion harmonics, while passing odd order harmonics. On the other hand, even and odd distortion harmonics produced by the input or driver stage (or found in the input signal itself) are passed freely, without selective attenuation.

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