of the Williamson amplifier were published relatively late, when the original DIY enthusiasm had already faded - in 1952, 1957, 1961. Martin Kiebert, who built professional-grade Williamson amplifiers for his laboratory at Bendix Corporation, identified five sources of distortion caused by inferior components other than the transformer: Excessive noise and electromagnetic interference caused by noisy carbon or composition-type resistors and incorrect layout of the first stage. Replacement of resistors specified by Williamson with wirewound resistors could improve signal-to-noise ratio by . Replacement of 6SN7 with low-noise 12AY7 could gain another ; Frequency and harmonic distortion caused by asymmetry of passive components in two sides of a push-pull circuit. Typical components of the 1950s had 20% tolerances, which was unacceptably high for the Williamson; The 6SN7 driver stage was often unable to properly swing the KT66 grids, causing excessive distortion. According to Kiebert, the American 5687 dual triode was clearly superior. According to Talbot Wright, the 6SN7 was not at fault - distortion was caused by incorrectly set standing current, and could be improved by a simple increase in bias voltage; Distortion in the feedback voltage divider. This critical function required low-distortion wirewound resistors; Distortion was clearly influenced by the choice of output valves, however, Kiebert could not identify any specific rules. Kiebert rated the design positively but warned the readers that following Williamson's instructions is possible only in a laboratory environment. The amplifier reveals its potential only with expensive, properly matched components that were out of reach of an average amateur. Even a perfectly built and tested Williamson amplifier would sooner or later need valve replacement, which would very likely cause an unexpected rise in distortion. Variants and derivatives After 1950 the industry produced numerous derivatives of the Williamson amplifier, often deviating significantly from the principles outlined by its creator. In 1950 Herbert Keroes shunted common cathode resistor of his 807 amplifier with a large electrolytic capacitor which, according to Keroes, significantly reduced distortion at high output power. Contrary to recommendations by Cocking and Williamson, Keroes and his partner David Hafler used cathode shunt capacitors in most of their designs; by 1956 this approach became de facto industry standard. In the same 1956 Hafler used fixed bias in his EL34 Williamson. Later, fixed bias became a staple of Soviet and Russian Williamson-like designs that employed exotic output valves like the 6C4C directly-heated triode, the GU-50 generator pentode or the 6P45S