Fender sold its first Blues Junior tube amp in 1995. The amp tries to capture the warm blues tone of the 1950s rock and blues era. The Fender Blues Junior has a popular following, and this amplifier is simple to modify. From changing out tubes or speakers to ordering custom modifications over the Internet, upgrading a Fender Blues Junior is a great way to achieve the sounds you want in an amp.
Change the tubes. As tubes age, they lose their brightness. Fender sells tubes, or you can purchase tubes such as the Electro Harmonix 12AX7 or JJ EL84. Preamp tubes (specifically the V1 tube) have the most to do with tone. Power tubes provide power to the amp. "Stain" marks -- cathode material burnt off onto glass -- on a tube is usually a sign the tube is nearing the end of its life. Replace tubes by pulling up on them. It is best to change all the tubes at once, as the age of tubes affects how much power they draw. It is unwise to mix and match tubes from different brands. Consult your amp's manual for tube use and replacement instructions.
Install a new speaker into your amp. By replacing your stock speaker, or aged speaker, with a new aftermarket speaker, you can upgrade the tonal characteristics of your amp. Speaker companies Celestion, Eminence, Weber and Jensen are known for producing high-quality sound in a Blues Junior, with the Celestion 30 a popular choice for that amp. What sound you are going for should determine your speaker choice. Remove the reverb screw, side screws and top screws from the old speaker. Lift the cabinet off its chassis and replace the speaker.
Purchase a foot switch. A foot switch allows you to click on your "FAT" switch without having to kneel over your amp to find it. The "FAT" switch gives the amp a louder, lead-guitar gain and tone, so a foot switch allows you to seamlessly give the amp that extra little kick.
Upgrade your reverb tank. The Ruby reverb tank, for instance, can replace your stock Blues Junior reverb tank to provide a richer sound.
Purchase custom modifications. BillM is famous for building custom mods for Blues Junior amps. Most of his modifications are fairly inexpensive, and his website describes how they work and what they will do. If you do not have amplifier modification experience, locate an amplifier technician to install them.
You should not attempt complex rewiring modifications unless you have amplifier modification experience. It is possible to be electrocuted while working on an amp. If you lack experience, contact a local guitar store to find an amplifier technician near you.