From Valve Amplifiers, Third Edition
Classic Power Amplifiers
Now that we can recognize and analyse individual stages, we can investigate the design of some classic amplifiers such as the Williamson, the Mullard 5 20 and the Quad II.
The Williamson
The design of this amplifier was published in Wireless World6 in 1947, and set a standard of performance that was years ahead of its time.
The input stage is the standard common cathode triode with 20 dB of global negative feedback applied from the loudspeaker output to the cathode. The phase splitter is a concertina circuit direct coupled from the input stage, and feeds a differential pair using both halves of a 6SN7. See Fig. 6.23.
Fig. 6.23: Williamson amplifier (by kind permission of Electronics World)
The output stage is a push pull pair of KT66 beam tetrodes operated as triodes that provide 15 W output in Class AB1, operating mostly in Class A. RV1 adjusts the DC balance of the output valves in order to minimize distortion due to the transformer core, whilst RV2 sets the quiescent current to 125 mA for the entire stage.
The linearity and headroom of each stage is excellent due to the careful positioning of operating points and choice of valves, but because this amplifier has four stages enclosed by the feedback loop, stability needs to be taken very seriously.
The input stage initially has an output resistance of ?7.5 k ?, but this is raised by the feedback to ?47 k ?. In...
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