From Mesa Boogie:
The MESA EL84 / 6BQ5 is a European classic and the tube type that led the British invasion. Tight and aggressive, yet smooth and warm, with an amazing chiming mid and top end. Brilliant focus with a frequency response that always brings your guitar to the front of the mix – This tube is true to its classic roots and is extremely consistent and reliable.
Stock in: Mark 5:25, Mark Five: 35 – Also: 20/20, Blue Angel, Express 5:25(+); F-30, Maverick, Nomad 45, Reverb Rocket, Rocket 44, Rocket 440, TransAtlantic TA-15 & TA-30