It was created in the early 1970s using an amplifier circuit board found in a skip by Deacon and fitted into a Hi-Fi speaker cabinet, powered by a 9-volt battery. The amplifier had no volume or tone controls for most of its history and was never broken or repaired. It was used along with May's Red Special guitar and his treble-booster to produce sounds reminiscent of various orchestral instruments, such as violin, cello, trombone, clarinet, or even vocals, starting from the songs "Procession" and "The Fairy Feller's Master-Stroke" from the 1974 album ‘’Queen II’’.
KAT Deacy Amp replica
In 1998, Greg Fryer with the help of UK amp specialist Dave Petersen undertook the job of trying to make three copies of the Deacy Amp with May's full backing. These amps produced a similar compressed and saturated sound but lacked the tonal characteristics that were so crucial to the unique Deacy sound. In 2003, Nigel Knight became involved in the amp's development. Several prototype amps were built that were continually edging ever closer to the sound of the original but all fell short of the mark when compared side by side with the real amp. It was only in 2008 when Knight was given permission to take the original Deacy Amp apart and test and analyse each individual component that he finally began to understand the intricate workings. 
With this new information in hand, Knight called on the vast experience[according to whom?] of speaker manufacturers Celestion, who developed and produced nearly 30 prototypes for testing and analysis over a two-year period. Custom transformers were produced to exact winding and laminate specs, obsolete components were sourced and made RoHS compliant and bespoke cabinets were constructed at great expense from Sapele veneered chipboard, exactly as the original.
In 2010, some 12 years after the project commenced, the Brian May Deacy Amp Replica was given the official approval and blessing of both Brian May and John Deacon.
The first production run of KAT Deacy Amp replicas were shipped in March 2011 and sold out within one month.
- Fryer, Greg (30 June 2005) History and background of the Legendary Deacy Amp. brianmay.com
- A Strings: KAT Deacy Amp product page. astrings.co.uk