PX25 no relation KT66
Prototypes and tests
Valve complement of the original Williamson amplifier was determined by scarce supply in wartime Britain. The two suitable and available output valves were either the PX25 triode, or a triode-connected KT66 beam tetrode. Williamson initially used the PX25, an already obsolete directly-heated triode introduced in 1932. In his second prototype, Williamson used the more efficient KT66, which became the valve of choice in post-war period. Powered from +500 V power supply, the KT66 prototype delivered 20 Watts at no more than 0.1% distortion. A less costly +425V power supply enabled 15 Watt output power at no more than 0.1% distortion; this arrangement became standard for the Williamson amplifier and defined its physical layout. The complete prototype system, including the amplifier, the experimental magnetic pickup and a Goodmans full-range speaker in an acoustical labyrinth enclosure, has proven to Williamson that a low distortion, deep feedback amplifier, indeed, sounded superior to amplifiers without feedback. The difference was particularly audible with the best available shellac records, despite the physical limitations of this low-fidelity format.The prototypes impressed the Marconi management, who granted Williamson unlimited access to the company's test facilities and introduced him to the people from Decca Records. The latter provided Williamson with precious, exclusive test material - sample records of the experimental Decca ffrr system, the first true high fidelity medium in the United Kingdom. These records, which exceeded any preexisting media in sound quality, helped Williamson with fine-tuning his prototypes. He was certain that he was now firmly on the right track, but neither Marconi, nor its parent the General Electric Company were willing to invest in mass production of amplifiers for the civilian market. The design was not interesting to company lawyers either, because it did not contain anything patentable. Williamson merely put together well-known circuits and solutions.Topology
The Williamson amplifier is a four-stage, push-pull, class A triode valve amplifier built around a high quality, wideband output transformer. Its second (concertina-type phase splitter, V1B), third (driver, V2A and V2B) and fourth (output, V3 and V4) stages follow Cocking's Quality Amplifier circuit. The added first stage (V1A) is a dedicated error amplifier, which compensates for the loss of gain caused by negative feedback. Williamson optimized operating points of each stage for best linearity with sufficient overload reserve. The output stage is biased into pure class A; traditionally it used triode-connected beam tetrodes or pentodes. With American 807 or British KT66 valves (Williamson recommended the latter type) and specified power supply the amplifier delivered 15 watts of output power. Further increase in output, according to Williamson, required use of four output valves; his 1947 article mentions construction of a 70-watt prototype.The plate of the first stage and the grid of the phase splitter are connected directly. This configuration, known since 1940, was still uncommon in 1947; American designers considered it a novelty even in the early 1950s. Phase splitter, driver and output stage are capacitively coupled. Cathode bypass capacitors are absent: Williamson, like Cocking before him, tried to linearize open-loop performance of each stage, and deliberately sacrificed gain for linearity; he was also concerned with potential low-frequency instability introduced by added capacitances. The circuit in either 1947 or 1949 variant contains no electrolytic capacitors; its power supply uses a CLC π-filter with two 8 µF paper capacitors, with a further LC filter feeding the first three stages.Derivative designs of the 1950s often deviated from Williamson's recommendations while retaining his four-stage topology. According to Peter Stinson, this alone is not sufficient to be called a Williamson amplifier. A true Williamson amplifier must meet five criteria simultaneously:
no relation