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This page presents details of triode connected and ultra-linear "Williamson" amplifiers produced by the Acro Products Inc USA

Acrosound Williamson Amplifiers.

In 1936, Alan Blumlein was granted UK Patent 496883 covering a design concept describing a particular form of loading configuration for power tetrodes, pentodes and beam power tubes in electron tube amplifiers whereby the Screen Grids (Grid 2) are fed from a tapping in the primary Plate winding, or from a separate discrete Screen-Grid winding, of the output transformer.

Later, writing in the November 1951 edition of the US "Audio Engineering" magazine, David Hafler and Herbert Keroes of the Acro Products Company, USA, used the term "ULTRA-LINEAR" to describe a specific form of this mode of operation.

It could also be reasonably said that the 1950/1951 Hafler and Keroes of the USA upgrading of valve hi-fi amplification to their ultra-linear output stage configuration was yet another stage in the evolution of the remarkable Williamson Amplifier project.

US Patent 2680218, granted 1 June 1954, covered their ultra-linear transformer design. Then on 7 June 1955 they were granted US Patent 2710312, covering the 43% turns ration for optimum U/L performance.

Then on 7 May 1957, Acrosound was granted US Patent 2791646 covering an improved ultra-linear confiuration whereby the screen grid windings were separate to the plate windings, thereby enabling the application of different plate and screen DC voltages - a useful feature for high power applications.

Acrosound said: "The Ultra-Linear Williamson arrangement with the Acrosound TO-300 transformer doubles the power of the triode version for a given distortion and also provides better transient response as evaluated by square waves. This circuit has been described in Audio Engineering, June 1952 and Radio and Television News, February 1953."

An even higher powered design using US made 6550 valves instead of the British KT66 was published in the November 1955 edition of Radio and Television News. (Copy below).  Note the KT88 valve was yet to come.

The 1955 Acrosound USA Transformer Catalogue offered no less than three variants of the original Williamson - 12 watt triode or 25 watt ultra-linear with (interchangeable) KT66 or 807 valves, and a 50 watt ultra-linear version with 6550 valves delivering 50 watts output. It also offered a 100 watt ultra-linear amplifier (not Williamson based) having the latest transformer design using 6146 valves with separate plate and screen supplies - one serious piece of valve audio amplifier !!

For full sized images open in new window.


Acro Triode or Ultra-linear connected Williamson with KT66 or 807 Valves:

Acro Ultra-linear connected Williamson with KT66 or 807 Valves:

Acro Triode or Ultra-linear connected Williamson with 6550 Valves

Performance of the above 6550 version of the Williamson amplifier is stated by Acrosound as:

Acro Ultra-linear connected Williamson with KT88 or 6CA7/EL34 Valves


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This page last modified 06 July 2010