The mammoth rock 'n' roll celebration to pay tribute to Marshall Amps' 50th anniversary and the life of late mastermind Jim Marshall, has just gotten a little bigger. Former Dream Theater drummer Mike Portnoy has been added to the mix, along with Free founding member Andy Fraser and Slayer guitar god Kerry King.

The Marshall 50th anniversary event is set to be held on Sept. 22, at London's iconic Wembley Arena. Along with the addition of Portnoy, Fraser and King, previously announced guests which include members of legendary rock bands such as Iron Maiden, Judas Priest, Black Sabbath, Deep Purple, Slipknot and more will all come together to celebrate both the influence that Marshall Amps gave to rock music, and the man himself, Jim Marshall.

The 'Father of Loud' passed away in an English hospital on April 5, 2012, at the age of 88. Countless musicians paid tribute to Marshall after learning of his death, some of which you can read here.

"Family has always been at the heart of everything Marshall does and all Marshall owners are very much part of the 'Marshall family," says Paul Marshall, son of Jim and Marshall head of artist liaision. "It has been so heartening to see the amazing tributes paid to Dad over the past few weeks. He certainly made a huge impact on the world! This event aims to be a fitting tribute to Dad's life and we are honoured so many artists want to get involved!"

The Marshall 50th Anniversary Concert will take place on Sept. 22 at London's Wembley Arena.