Blues Junior Mods

A few years ago someone I knew was moving and asked if I was interested in a Fender Blues Junior amp.  My initial thought?  I needed another amp like I needed another hole in my head but in reality, the price was too good to pass up and  it would be a good little combo for rehearsals.  After careful consideration and deliberation lasting about fifteen seconds I purchased the amp.

Fender Blues Junior with Tweed covering and new leather handle.  It was made in 1995 and is a Rev-B board, one of the earliest Blues Juniors.

Right away it was a joy to load up in the car and bring to a rehearsal however, the sound lacked high frequencies and seemed (to me) void of any classic Fender sound.  Of course, this was a used tube amp after all so I hoped it might benefit from some new tubes and magically spring back to life.  Since preamp tubes require no bias adjustment it was easy to swap them out and try  several different 12AX7's in the first position to see if there was any appreciable change in "tone".  The first preamp tube is going to amplify the voltage from the guitar to a usable level and sends the  signal (usually) to the tone stack.  Changes made to the first stage will be amplified by each successive stage so improving the signal in the earliest circuits is often better.

After leaving the tone controls, the signal needs to be increased again and sent out to (possibly) the reverb.  Each time the signal is sent through some type of control circuit there's a voltage drop so it gets recovered by another preamp tube before going to the output tubes where current is amplified to drive a speaker.  The Fender Blues Junior uses two EL84 output tubes which are fixed biased as opposed to cathode biased.  After about a half hour of "tube tasting" some 12AX7's I did end up trying a few different matched EL84 to see if any noticeable change occurred but I wasn't ready to give up on the pair of RCA EL84's that were in the amp when I bought it.  After all the strategic tube sampling was over the amp seemed to be a little better but nowhere near an amp I would want to play even at a rehearsal.

Next it was time to jump online for some research because I wasn't very familiar with a Fender Blues Junior or  how it "should" sound.  For many years my main amplifier was a vintage Fender Super Reverb which was an early Silverface design prior to the "master volume" era circuits.  It didn't take long to stumble across the BillM Audio website to find a wealth of information on Fender Blues Junior designs from its debut in 1995 up to current models.  Bill has a perfect way of describing the sound I was hearing..."The biggest issue with the Blues Junior is that it sounds small and boxy. Cup your hands around your mouth and speak or sing. That’s boxy."  It seemed the issue wasn't the tubes but could quite possibly be the design itself, so I began to consider which of the BillM Audio Blues Junior mods would be worth trying.  After all, there wasn't much to lose giving mods a try considering the good purchase price of the amp itself and the very reasonable cost of the mods themselves.

Perhaps it was worth mentioning at the outset but...if you do NOT have any soldering experience, have little to no electronics experience and most importantly lack the understanding of potentially lethal voltages within a tube guitar amp - DON'T ATTEMPT THESE MODS YOURSELF.  It is not to say a savvy DIY'er can't possibly do these mods but there are a lot of things that could go wrong even if you do a lot of research ahead of time.  Also, the mods will void any manufacturer warranty so you assume all risks doing it yourself.  In my opinion, it's best to have a qualified technician do the work for you.  The BillM Audio website  has a lot of instructions (including videos) on how to work on these amps but you risk hurting the amp or more importantly, hurting yourself in the process. amps have potentially LETHAL voltages.

Fender Blues Juniors prior to 2001 had a green circuit board.  Test markings on this amps circuit board show May of 1995 which is when the "Rev-B" design first appeared.

Before ordering any mods I had to  find out which circuit board the amplifier had (green or cream).  Like any amplifier, the Blues Junior has had revisions over the years to address design issues and make the amp "better".  Once I determined the circuit board was an older green board the first round of modifications were ordered from the site:


The BillM Presence and Sparkle controls require drilling holes in the chassis which is a permanent mod to the original design.

  • Basic Kit - As described on his website this modification addresses the "boxy" sound these amps are prone to have.  There are several capacitors which get changed to improve the first gain stage and tone controls.  Also included with this mod is an adjustable bias pot which simplifies and speeds up the process of biasing the amp for different pairs of EL84 tubes.  Some amp techs will argue against the idea of adding a pot this way because it requires drilling a couple small holes in the circuit board.  I think their point might be stated this way:  "Do some math to address bias circuit corrections with a different (correct) value resistor instead of drilling hole in the circuit board".  This is a valid point of view but...there's a case to be made for convenience.  The new precision pot included in the kit allows you to make subtle changes to output tube bias and listen to the changes without soldering in different resistors to get a desired result and the kit includes a new, sharp drill bit to do the job.  There is also a resistor change in the power section that smooths the ripple current going to the output tubes.
  • Presence Control - This mod adds a potentiometer which controls the amount of negative feedback coming from the speaker and he give a wonderful explanation on his site here.  It requires drilling a hole in the chassis which some people might not want to do (permanent mod) but I wasn't concerned about it on this particular amp.  
  • Sparkle Control - Another mod which adds a potentiometer to alter the high end to give you different flavors of high frequencies and upper mid range frequencies.  You can read a far better explanation on his site here and although I wasn't really sure it was necessary for the amp I had, it couldn't hurt to have more flexibility in the frequencies that needed the most attention.  I do like the way he describes the location of this control within the amp's design and why it differs from simply using the treble control.
  • High Voltage Preamp Mod - This increases plate voltages which creates more headroom and "headroom" translates to a cleaner tone.  For a small amp that might need to get cranked up once in a while I like the idea of more headroom.  Distortion can be added others ways but if the amp distorts too quickly at lower volumes it won't lend itself to being versatile for different situations.
  • Standby Switch - There isn't a neeeeeeed for a standby switch on a Blues Junior but it is nice to power on the amp in standby so it's ready to go as soon as you flip the switch to play.  When you take a break, need to swap out pedals or unplug for some reason, it's great to quickly put it in standby and  get right back to playing without waiting for the tubes to get warmed up again.

The blue rectangular part shown here is a precision potentiometer for adjusting bias and to the left are a couple of the resistors that have been changed.

After installing the first round of modifications the amp really came alive and I couldn't have been happier with the results.  Well, it sounded so good I decided to make a couple final "tone touches" and ordered the following:

Low profile is the key to this output transformer.  Notice the clearance on the lower right of the transformer as it relates to the upgraded speaker.

  • Low Profile T020 Output Transformer - Transformers are usually the single most expensive part in a tube amp so "mass produced" amplifiers often have more cost effective transformers.  These less expensive transformers aren't necessarily bad and it's understandable from a cost analysis (production) perspective but a better transformer can make a significant improvement to the sound.  This mod is going to perform at its best if you also do the basic mods.
  • Upgrade Speaker - There's nothing particularly special about the stock Blues Junior speaker so after reading up on speaker options I chose the Eminence Cannabis Rex which I ordered from one of my normal vendors.  

The process of performing the mods was a slow and steady process carefully following the instructions that come with the kit.  After all of the mods were done the amp went from flat, honky and lifeless to borderline amazing.  It was and is an entirely new amplifier tone and the reverb modifications really opened up the stock reverb pan.  Since purchasing the tweed version originally I made a point of purchasing another used U.S.A. made Blues Junior to make some comparisons to and eventually perform the mods again to ultimately offer for sale.

I no longer simply use my Blues Junior for rehearsals.  It performs wonderfully in any bar type setting performing in a blues/rock and roll band and I've used it at outdoor shows with a small P.A. primarily running vocals.  At a larger outdoor venue with full p.a. system the amp sounded amazing being able to set the controls exactly how I'd like them to be and let the mic and p.a. do the rest of the work.

To resolve the loud and intermittent buzz each solder joint was re-soldered for the tube sockets.

After using the amp for a couple of years a loud buzz and intermittent signal developed.  The output tubes were the suspect and after trying a known good pair of EL 84's it seemed the sockets were the culprit.  Each of the tube sockets in the Blues Juniors are soldered to a printed circuit board (PCB) and considering there's a fair amount of heat present along with stress and vibration it seemed logical to work on them.  I first cleaned the sockets followed by re-tensioning so they held the tubes more securely.  Finally, the original solder was removed, connections cleaned and each pin was re-soldred.  The amp was back up an running in no time so there's no excuse not to sit down and compare a stock Blues Junior with a BillM Audio modded Blues Junior.  Perhaps I'll dip my toes into the YouTube pool and let people decide for themselves.  Now, where to find the time to do that?

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