However, because they "run hot", a common modification is to add a bias trim to enable adjustments to the idle current when switching tubes.Fender introduced the Blues Junior in 1995, and has revised and updated it periodically since then.
A set of three matched Groove Tubes 12AX7 preamp tubes and two matched Groove Tubes EL84 power tubes, graded according to Fender's specifications, are used in all factory Blues Juniors.
The Blues Junior uses a nonadjustable power tube bias (no way to vary the bias point), but it is fairly "hot" and can accommodate most matched pairs on the market.
The Fender Blues Junior is most similar to the Fender Blues Deluxe, which adds a "drive" channel, an effects loop, and uses 6L6GC output tubes for 40 watts of rated output.
Like many Fender amplifiers (particularly in the Hot Rod series), many limited edition versions of the Blues Junior have been manufactured since its introduction in 1995.
The original circuit board underwent a major redesign in 2001, when production moved from the US to Mexico.
The earlier circuit boards are green in color and are noted for a "darker," more bass-inflected tone.The label is often located on the bottom next to the reverb tank, sometimes it is placed on the side.The codes can be handwritten, which can occasionally make the letters 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.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.Many green board models have excessive noise in the reverb circuit, as the signal is inserted into the reverb after the master volume.