After the establishment of the valve as a reliable amplifying device, improvement in circuitry was a continuous research task for engineers. Over the years, many amplifier circuits appeared in technical literature. The most significant circuits used for high-quality sound system are listed chronologically as follws:

1913 - Push-Pull circuit:

Conceived by E. H. Colpitts, this basically a balanced circuit. The two valves can operate in several modes, class A, AB or B supplying great output power while canceling much of distorsion.

Simplified output circuit of a conventional push-pull amplifier

1927 - Negative-feedback circuit. This circuit was developed by H. S. Black. Please see the section "Classic Circuits".

1947 - Circuito ultra linear:

D.T.N. Williamson, in England, projected this circuit for ultra-linear operation.

The Williamson circuit used in high-quality amplifiers.

1949 - Symmetric Push-Pull circuit:
In 1949, F. C. McIntosh patented an efficient amplification circuit. This simmetric push-pull circuit consists essenssially of beam-power valves operating with a bifilar-wound output transformer, with the beam-forming plates in the tube cross-connected.
The McIntosh circuit

1950 - Hafler-Keroes circuit:
David Hafler and Herbet I. Keroes developed an ultralinear amplifier configuration generally known as the Hafler-Keroes circuit. It is conceived in such away tht the screen grid of the valve is returned to the output transformer at a point representing about 18,5% of the impedance of the primary winding. This circuit represents an intermediate mode of operation, giving the power output of a pentode and the low output impedance of a triode.

Transformer connections in the Hafler-Keroes  circuit.