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?

Tone Comparison - Part 2

This is the first opportunity to consider the layout of the parts before holes are drilled.

The parts for my capacitor project had finally arrived and it was time to sit down at the workbench and think through just how all these components would fit into the Hammond enclosure.  I knew it would be a tight fit considering there would be at least twenty two capacitors inside the box once it was complete and each time I installed a cap there would be less room to work with.

Eleven different brands of capacitors will fit inside the box.  I'll be able to compare really common ceramic and mylar capacitors to other more expensive capacitors like Orange Drop, Mallory 150, Mojo Dijon, Oil-in-paper, Vitamin Q and a few others.

For smaller holes I simply chose the correct size bit and used a stepper bit for the larger 3/8" diameter holes.

I managed to find twelve position rotary switches made by Alpha and a CTS concentric potentiometer with a 250K and 500K resistances which are the two most common pot values in electric guitars.  Single coil pickups will generally have a 250K pot whereas humbuckers will usually have 500K.  I wanted the left side of the box to be .022uf capacitors and the right side to be .047uf capacitors.  It turned out I had eleven "sets" of capacitors in both of those values meaning each pair were made by the same company.  There were more capacitors in stock but this covered many of the common capacitors being sold today and left me one position which would be "off".  I used a multimeter to label each lug of the rotary switches and decided the order of capacitors ahead of time.  The two Carling On/On mini switches would be used to select which potentiometer resistance was in the circuit and the other switch would send signal to either rotary switch (choosing capacitor values).  At this point I was still undecided about using terminal strips or buss wire for the grounds which didn't really matter since the first steps of construction would be locating and drilling the holes for all the switches.  I wanted to get the these mounted before finalizing the location of the input and output jacks.

It was at this point I knew the terminal strips (bottom) would take up too much room so I took them out and replaced them with simple buss wire.

Once all of the switches were mounted I could verify enough room existed to put the jacks where I wanted them and get to wiring up everything except the capacitors.  I wasn't 100% sure where I could pull resistance measurements from but assumed it would be somewhere on the mini switches.  I also wasn't sure I could have the multimeter plugged in while using the box with an amp since there would likely be direct current from the battery injected into the circuit.

This was the initial concept and I knew there were mistakes in the drawing but it was a good start.  Whatever problems I couldn't foresee would show themselves as I got wiring.  The idea was to get all the switches wired as well as the the meter probe jacks.  

The time had finally come to start soldering so after removing the terminal strips I used the existing holes to mount star ground tabs which could be soldered to.  I took 18 gauge buss wire and ran it through the tabs and around near the perimeter of the box with the ends soldered to the potentiometer.  The aluminum box is conductive but soldering directly to it would be difficult since the thickness of the metal would absorb the heat before melting a ground bead.  There wasn't room to use the 200 watt soldering iron like on an amp chassis.  I started at the input jack and wired up all of the guitar signal path including jacks for the multimeter probes so I could see if I had it right before the capacitors went in.  I didn't want to have to fix mistakes once the enclosure was completely full with parts and capacitors but assumed there were corrections in my future.

The first few capacitors being wired into the enclosure.

Not only had I determined which capacitors would be soldered to each position of the rotary switches, but I also measured each cap for their actual value ahead of time.  As I started wiring in capacitors I began to wonder about just how much noise would picked up by all of the wires in the box.  Each capacitor would have heat shrink on the leads to prevent any shorts in the signal chain but that does nothing for noise.  It didn't concern me enough to stop working before the capacitors were in so I wouldn't know until it was completely done.

The final three capacitors just happened to be some of the largest ones to be installed.  It looks like a rats nest but the wiring is nice and neat.

It took a few hours to wire all of the capacitors and I was fairly lucky how the order of things worked with the space inside the Hammond enclosure.  The buss wire really offered versatility for grounding since the leads of the capacitors differed in length.  All in all the process went fairly smooth and I was anxious to plug in and give it a listen.  

One snafu appeared right as I was finishing.  Early in the process I had wired all of the switches and meter jacks and confirmed the meter would work to reference either 250K or 500K resistance.  However later in the process and after a few "on the fly" design changes I noticed something odd....153K of resistance.  That meant the two potentiometer values were now wired together in parallel instead of being separate.  I took a break and went back to the design checking what had changed.  The meter jacks were disconnected and I used some jumpers to correct the problem before soldering new wires back in.  

Here is the final product with Mogami shielded cable.  The ends with heat shrink are using just the center wire for the guitar signal.  The other end uses both the center wire and shield soldered to ground like a grid wire in a tube amp.

On the initial testing with a guitar and amplifier I was mortified for a minute when I heard the noise.  As I had suspected the meter was injecting 9 volts into the circuit and was making a lot of noise.  When the meter was turned off the vast majority of extra buzz went away.  However it seemed like there was still a little more buzz than I would have hoped for.  I took a short break and grabbed some Mogami shielded wire and decided to replace the original cloth covered push back wire with shielded cable.  I would use the center of the wire for the positive signal and the shield soldered to ground on just one end.  This way any electromagnetic interference picked up by the shield would be sent to ground.  So while it felt like a setback I was fairly happy with the design and probably being overly picky about the buzz.  This was never expected to be exactly like a single tone control in a guitar.  There are a lot of parts in a small space and plugging a guitar that already had a tone control would be just adding more load to the circuit before getting to the amp.  What I wanted all along was a fast, easy way to listen to one type of capacitor to another and see which ones I liked best.  After all the shielded wire was installed the design was very quiet and subtle differences from one cap to another could be easily heard.  Currently there is one capacitor not working and I haven't taken time to try and reflow the solder or just swap the capacitor out with another one.  Eventually the plan includes a video so people can listen for themselves to see which capacitors they like and before that I'll fix the one problem remaining.


Tone Capacitor Comparison - Part 1

A variety of capacitors some of which will be installed in the capacitor comparison box.

A variety of capacitors some of which will be installed in the capacitor comparison box.

There are a variety of capacitors made from different materials from different manufacturers and there are even more opinions about which ones are the best.  Over the years I've used different types of capacitors in modifications and instrument builds but it was always difficult to determine which ones sounded better than others.  I had thought about different ways to sample capacitors but never set aside time to plan a way to do it.  Once I started to work on tube amps it became fairly obvious one capacitor might sound better than another even if they had the same value.  In an effort to hear differences I first sought out videos on YouTube.  Kudos to the people taking the time to solder in different capacitors and make some pretty interesting videos.  To learn a little bit about tone controls in an electric guitar check out a short article on my website here.

This style of rotary switch is used in the Gibson Varitone.

This style of rotary switch is used in the Gibson Varitone.

In one particular "Trade Secrets" video from Stew Mac, they had a simple rotary switch setup in the spirit of a Gibson Varitone that made it quick and easy to hear different capacitors so the customer could choose what they like the best.  It was a great idea that lead me to go through my parts to see how many brands of capacitors I already had in stock.  It turned out there were way more than a Varitone rotary switch could offer so it was clear a new project was just taking root.

At this point I knew I'd have to spend some time looking for a different rotary switch but before I did, it seemed to make sense to consider what other parts I might need and determine what exactly was I hoping to achieve with the final product.  

  • A way to quickly switch between different brands of capacitors
  • Compare .022uf capacitors to .047uf capacitors (matching brands
  • Plug in a guitar with single coils or humbucking pickups
  • Reference the resistance of the tone control

Whether it was over-thinking a simple thing or good idea in the works I wasn't sure, but I did know it was time to work on a drawing.  It wasn't going to be an elaborate electrical design but it would require time to build not to mention some expense.  I wanted to have a solid plan before ever ordering the parts and it wasn't something I wanted to do more than once.

These 2mm jacks will allow a multi-meter to be plugged into the capacitor box to measure the resistance of the tone controls.

These 2mm jacks will allow a multi-meter to be plugged into the capacitor box to measure the resistance of the tone controls.

I already had a Hammond enclosure which turned out to be too small for a different project.  Rather than return it I'd try to put it to use and that would establish the space I had to work with so I began working on the drawing using a small dry erase board.  There would be two rotary switches (one for each value of capacitors), a concentric potentiometer (250K/500K resistances), two mini switches (resistance selector, capacitor value selector) and two panel jacks (input and output).  Each capacitor would need to be grounded so I could either use terminal strips or simple buss wire.  In early planning the terminal strips would be easier to modify after the fact but buss wire would be almost certainly save space and offer greater versatility during the build.  After briefly looking for knobs for a concentric potentiometer it occurred to me it would be possible to reference the resistance of the concentric pot with a multi-meter instead of relying on numbers on a knob and fortunately I also had these in stock.   Either way, once it was built I could simply plug a meter in and get the actual resistance present in the tone circuit.

Once it seemed the idea was pretty solid and the initial drawing was complete it was just a matter of waiting on the parts t0 figure out the actual layout before drilling any holes in the enclosure.



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