Solid state rectifiers don't "break up" as early, allow more clean headroom, and provide less compression than their tube counterparts.
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The codes are usually handwritten, and the letters can occasionally be hard to decipher. There is no reliable way to date 2003-2005 amps other than to ask Fender customer support to look up the date from the serial number, although you may find a date code on the speaker.
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.
The NOS Blues Junior is a current production, lacquered tweed amp with a Jensen reissue C12N speaker. There’s nothing limited about the edition; they built a bunch and when they ran out, they built another bunch.
Other Variants The Blues Junior chassis has also been used in the Two-Tone, a large amp with a 10-inch and a 12-inch speaker.