Dating vintage fender amps by serial number
2006 amps have a small metal “Fender 60th Anniversary” button on the back plate. Early Tweeds The early green board tweed-covered amps were not lacquered. It has the same cream board inside with the same components as every other current-production Blues Junior.The serial numbers for these amps begin with “LO,” indicating that they were made in the Fender/Sunn Lake Oswego factory. It usually applies to old vacuum tubes/valves that have gone unsold for decades. All cream board tweed Blues Juniors have a “Limited Edition” plaque on the back.
In 1953 the cabinet designs were changed to the so-called "Wide Panel" design, with a 5 inch wide tweed covered panel above and below a wider swath of grill cloth.Fender ceased production of 5B6 Bassman amplifiers during the spring of 1954.During November 1954, Fender introduced the newly designed 5D6 Bassman amplifier offering four ten inch speakers and was designed utilizing two rectifier tubes.The cream-colored board is laid out entirely differently (and better) than the green board. But the old ones sound darker, while the new ones are brighter, with more emphasis on treble tones.
One is not necessarily better than the other; the dark tones are nice for blues and jazz, while the new amps do brighter tones better.Fender introduced the Blues Junior in 1995, and has revised and updated it periodically since then.