All About Your Blues Junior

These mods have become known as the “Billm” mods from my name on the Fender Discussion Pages. and the Telecaster Discussion Pages. Fender’s Blues Junior is a great little amp–it’s small, light, priced right, and all tube. It has spring reverb, flexible tone controls, and it’s loud for its size.  It has a couple of shortcomings, however, that can be overcome without too much difficulty.

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.” Some think that this undesirable tone is due to such a large speaker in a small cabinet. But most of it is due to the components selected by the original designer for the tone stack and the lame coupling cap values. In my opinion, the original designer (long gone) sandbagged the Blues Junior so that I wouldn’t compete too much with the Blues Deluxe.

The next issue is bass performance. The low notes are fuzzy and indistinct. Often called “flub” or “farting out,” it has multiple causes. The speaker is partly to blame, so is the small, not-great-quality output transformer. Surprisingly, the limitations of both can be mostly overcome by just increasing the amount of capacitance in the power supply, commonly known as “stiffening.”

The third issue is output tube bias. Bias is a small negative voltage on the grid that sets the operating point or “idle speed” of the tube. On amplifiers with push-pull output like the Blues Junior, the tube manufacturers’ recommendation is for the operating point to be set at approximately 70 percent of the full power rating. The original designer of the Blues Junior chose a much higher operating point, and most BJrs run at 100 percent of their rated output, sometimes higher. Hotter bias is not automatically better. Hiss increases and the amp becomes mushy and inarticulate. Output tube distortion occurs earlier, but this is not always a good thing. My basic mods kit includes a simple way to make the bias adjustable.

Old BJrs Sound Different
Fender has used two different circuit boards in the Blues Junior. The original green circuit board was in production from 1995 to early 2001. The later cream-colored board has been in production since mid-2001. Although the circuits are virtually identical (except for reverb), the cream board is much brighter, with livelier treble, than the green board. Where applicable, I’ve separated the mods appropriate to each. The green board reverb is prone to hiss and hum. See my Improving Green Board Reverb page. The Green boards were made in USA; production shifted to Mexico in 2001. There is no difference in quality.

Some Experience Required
If you can wield a screwdriver, soldering iron, and multimeter, you can do these mods, but it’s a good idea to have some experience soldering and desoldering printed circuit boards.  Gaining access to the back of the circuit board is not all that easy–amp techs hate these amps for a good reason. Here’s how to minimize the pain:


  1. John Monk says:

    Hi Bill great improvements on BLJR I was wondering about a stand by switch, for unplugging input with out turning power off, or is it necessary. Thanks John

  2. Tim Jones says:

    I’ve got two BJrs. The first is a 1996 or 1997-ish black one with the standard “Specially Designed” speaker. The reverb is so weak I leave it all the way up to get a marginal echo. Otherwise, I like the amp. I’ve also got a 2007 Tweed BJr. Just plain too beautiful to pass up.

    Anyway, I’m thinking of hot-rodding the black one. I play blues and classic rock using a Stratocaster with 57/62 PUs or a custom with Eric Johnson electronics, sometimes with fairly loud bands.

    I’m reasonably good with a screwdriver and soldering gun, but I’m a little confused by your kits and other hot rod tweaks–which ones go together, etc.

    Which mods do you recommend?

    Many thanks for the time and effort you devote to this great little amp!


    • bill says:

      None of the mods conflict with one another. You could get “the works” and it would only improve the amp. Note that some are for tone and some are for player convenience, so, for example. if you never record or play through the PA, you wouldn’t need the line out kit. The audio-taper master volume is great for home players who need to play quietly, useless for playing out. The information you need is in the descriptions on the kits page.

  3. Mike Johnson says:

    Hi Bill,

    I have a Fender 2 tone amp, Is this the same as a BJ but with the 10″ & 12″ ? I have had the amp for a year or so, I really like this amp but its not as loud as i would like it to be, i have read that changing the speaker would help. What tubes, speakers & MODS can you recomend to make my 2 tone kick some A$$, i think this amp can be great.


    Mike Johnson

    • bill says:

      The most effective way to make your amp louder is more efficient speakers, but you’d drive them better with a general upgrade from the basic mods, presence control, and Clean Boost module. An upgraded output transformer would help, too. Tubes are pretty much all the same as long as they’re reasonably fresh–no magic tubes will make the amp louder. A 10/12 pair like the Eminence Ragin Cajun and the Wizard would be loud and aggressive. A Copperhead and a Swamp Thang or Cannabis Rex would be more mellow/clean, but still louder than stock.

  4. Dan says:


    I have a BJ NOS Tweed with the Jensen speaker. It sounds great, but the bass is lacking. What steps should I take to give the amp a better low end? I don’t have any experience with modding amps, so your suggestions for fixing this issue were somewhat confusing to me.

    [Surprisingly, the limitations of both can be mostly overcome by just increasing the amount of capacitance in the power supply, commonly known as “stiffening.”]



    • bill says:

      The mods really do improve low-end tone quality, but if you don’t have the mechanical and soldering skills, you shouldn’t experiment on your Blues Junior. You can find a local tech to install the mods. I can do them for you too, but I have a lengthy backlog. You can email for more information.

  5. Louie Warren says:

    I still need to take my BJ apart and identify the mods on board… one day! In the meantime, I’ve had to go back to my old Line 6 AX2 for one project. The other guitar player’s amps are all in the shop at the current time. He asked if he could borrow mine. He’s been using it for over a month in that project and loves it! I see a purchase and some mods in his future… however, mine breaks up a little too easily for him and he wants a super clean tone for Tele twanging. I suggested the TwinStack mod. Would that be correct? I’m sure he’ll consult with you once he makes the purchase.

    Another thing… is there anywhere on the net that has A/B comparisons of stock and modded? I’d love to be able to hear that somewhere.

    • bill says:

      I need to get some recordings up. I just finished an amp that’s going to Brazil, all of the mods focused on being a chicken picker’s amp, with major twang. Of course it has the tone stack, stiffening and TwinStack mods, but it also has lower gain in the preamp, an upgraded output transformer and JJ 6V6s instead of EL84s, with an Eminence Swamp Thang. It’s bright, clean, incisive, and sounds absolutely killer with a Tele. I was playing a ’69 Thinline through it today, and it was awesome.

  6. walt says:

    I did the basic mods for my green board b jr. and am impressed. I am thinking of now doing the presence control mod and the output transformer mod. I am wondering, is the stock output transformer really that bad? besides the sound quality, does the one you offer make the amp any louder? I’m sure it is an improvement, but is it really a $60 improvement.

    • bill says:

      The stock OT is undersized and limits bass performance. The Heyboer OTs I offer have more clarity as well as better low-end performance. An upgraded OT seldom makes the amp louder; it’s mostly a quality thing. Whether it’s worth $60 is up to you!

  7. Troy says:

    Bill, I recently purchased a Blues Jr. and I was told the mods were done by you personally. It does have a small Bill M. sticker near the on/off switch. Is there any way or really knowing the serial number is b-138724 if you keep a log of it. Thanks, and I have tried this amp side by side with a Blues Jr. with no mods. There is a big difference. The tone is a lot warmer and the reverb is so much better. Do you recommend a replacement speaker? Thanks.

  8. Russ says:

    Hi Bill, Before I start doing mods, I’m wondering if Fender did any of your mods or just fixed some of the problems on the Hot Rod Tweed version? I only ask because I only see black BJrs. I’m definately doing the bias mod. Thanks Russ

  9. Andy says:

    Hi Bill. I just purchased a new Blues Junior Tweed. After about 5 minutes of me playing and the tubes getting nice and hot, I noticed a slight burning smell with a little haze. Is this normal for new tubes to give off that smell the first few times of using the amp? Could it be the protective film that’s on the tubes or is it something I should be consirned about and have the amp fixed or replaced?



    • bill says:

      It’s not unusual to smell some burning/cooking smells with new tubes. The EL84s run very hot. I’m a little surprised by the haze, though. I would pull the back off and look at the resistors to make sure there’s no burning or charring. A defective tube can cause one of the power supply resistors to burn. That would be covered under warranty.

  10. Luke says:

    Hi Bill,

    I’ve been reading your many articles and comments on the BJ – Thank you so much! I have a BJ, cream board that I would like to set up for as a lighweight harp amp. Can you recommend mod’s and speaker to get me full lows and any mods that may improve the “fartiness” of my current stock setup. I definitely would want a line out set up as well.


  11. Chuck says:

    Hey Bill, I recentlly purchased a used Blues Junior and I removed the back panel to replace the tubes. I put the back panel back on turned it on and started playing and now I get a very loud rattle type buzzing noise. It never rattled before I took the back panel off. Any ideas on how to stop the noise?

    • bill says:

      No idea what you did. Maybe an output tube was on its last legs and the handling caused something to come loose internally. If you poked around inside, it could be a bad ground, ungrounded speaker jack, broken connection at the main filter cap ground. If the buzz is unaffected by the master volume, it’s definitely a power supply/filtering or output tube problem.

  12. Hello, Bill.

    I just wanted to say that i loved my amp after I made the basic kit mod and got the TO20.
    The instructions were very simple and clear.
    I took some pictures of the modding process and posted them in my blog (it’s in portuguese, I’m brazilian).

    This is the link to post:

  13. Todd says:

    Hi Bill,
    I’ve ordered some upgrades for my green board Blues Jr. and had one question about hum.
    I didn’t notice that this page was moved from your old site to your new site and wondered if the fix still was applicable?

    Also, if I pickup a new transformer will that negate the need for this fix or will hum still be an issue?

  14. Torbiz says:

    Hi Bill,

    I´m thinking of doing some of your mods. Is the output transformer you sell usable for a 220V version? If not, what should I get?

  15. Paul Beecroft says:

    Hi Bill,

    Thanks for all your work on the BJnr. I first saw your site around 2006 when I had just bought my 2006 Anniversary Tolex Blues Junior (now with Jensen c12n) which I was very pleased with at the time. After much pondering I ordered a kit of parts from you a few months back receiving them here in UK a few weeks later, and have gradually done the mods one at a time since. I installed the adjustable bias, TO20 output transformer, power supply stiffening, twin stack mod, re-voiced tone stack, presence control, and cathode follower mod. Just yesterday installed the replacement filter caps you supplied even though there was no visible leakage from the standard IC caps; I must say that each mod has been a significant improvement in the sound quality – even the last one which I didn’t expect would make much difference: there is now much more note definition, clarity and sustain, less noise and hum even at higher volumes. Thanks once again your work is much appreciated.

  16. TP says:

    Hi Bill
    You were talkin about lame coupling cap values. Your new basic kit includes orange drops, do you support bigger values or do you leave the values the same (0,0022µ) to keep the bass tight and not too early go into distortion. A Deluxe Reverb or a Vibrolux has much bigger values like 0.047 or 0.02. But these amps are not intended to play overdriven (at least Leo didn’t want them to get “abused” thet way 😉
    and merry christmas from Austria

  17. Wil says:

    Hi Bill ! I can’t post in the speaker section so I post here.
    I have a Blues Junior serie III, and I don’t find many reviews about the Stock Lightning Bolt speaker. It doesn’t sound bad to my ears, but I’d like to get closer to an real american sound. I was interested in buying a Jensen speaker, but you don’t seem to like them much. And since I fully trust you because the mods I’ve done are definitely a real improvment, I’d like to have you opinion on my question : What speaker for a real american sound ? (SRV, Hendrix (yes Hendrix, is not american at all for amps…)) But that’s the kind of tone I’d like to get.
    Thank you very much, I’ll order a TO20 in a couple of days to get the best out of that little box !
    Happy new year from Belgium !

  18. ron brown says:

    Bill…just puchased a BJ. No matter how I adjust the master and Vol controls, I get no breakup. Sounds clean regardless if vol up master low, or Master up vol. low. Replaced all tubes with new GTs and no difference. I have listened to your sound samples and there should definately be distortion, but I cant get any without using a pedal. Any ideas what may be going on here…thanks Ron

    • bill says:

      Even single coils on a Strat or Tele should drive the BJr into distortion if the guitar volume is wide open. Have you tried it with a different guitar, different cord?

      If it’s a new Blues Junior, there may be some “infant” problem that’s stealing or reducing the signal. You should get it repaired under warranty before trying anything else. If it’s used, has it been modded? Someone may have done something to drastically reduce the gain.

      • Tridyed says:

        I work at one of the mainstream stores that sell them and I can’t tell you how many times UPS has rattled the tubes loose or broke them in shipping. If you purchased it locally take it to the store and have them look it over and check the tubes. You would not be the 1st person to fall to the UPS shipping torture.

        We have some amazing packing we do with a used items to try and offset how UPS handles things. We pack for the worst possible thing and it has payed off many times. Fender does pack well but UPS can be pretty aggressive in their methods to get things shipped.

        Hope it helps.

  19. Tridyed says:

    Well it’s 2:30 am and the eyes are a bit bleary…. just finished up the mods to my amp that bill sent to me. As an amateur that has done this kind of work but it’s been a few years, It took me about 6 hours to get it all done and setup and tested. I will say I was a bit nervous to throw the switch while thinking about all the wiring and crap I took apart. But in the end it all turned on 1st time and it all worked right out of the gate.

    This is the setup I ordered:

    Basic mods for the board (orange drop tone stack, bias adjust, filter caps, and the stuff that is included in that package)
    The presence control
    The new input jack
    Audio taper master volume pot
    The TO22 output transformer
    The ext speaker jack
    Standby switch
    I did the Twin stack mod while I was in there.

    After watching the video Bill had for setting the bias I must say he was dead on for the way it went down. I have it set where he suggested and along with the other mods I feel like I have a brand new amp. Amp has a bit less noise and hiss, not sure if that’s the bias set up better or a combination of all the mods.

    It’s almost 3am so I can’t let the beast loose till tomorrow at high volume but just messing around here in the studio tonight I would say its has the feel of somewhere between an old Fender Bassman over to the new HotRod Deluxe and with a few tweaks and a move of the presence knob I can get pretty close to my old Plex I had quite a few years ago.

    The bass feels more like I am playing through a 4×12 and a much larger head then I actually have in the combo Blues Jr. Then I hooked up the 2×12 Mesa Cab as an external speaker and the volume change was HUGE. I can say with out a doubt I will able to play live gigs easily with this monster now. I have the C-Rex speaker in the Combo amp, it was really a good choice for what I was after. This is a really loud speaker and has very smooth highs that are not the typical fender shrill that I hate.

    Mind you I didn’t even do all the mods because I was looking for a certain sound and I am able to get what I was after in no time at all. The presence knob really can add the twinkle or pull it back to go from Fender to Plexi tones very easily.

    Kudos Bill I must say this really rocks. I can’t wait till tomorrow to really let this thing loose for a few minutes. Thanks for all the help and advise on which mods to grab to get what I was after.

    The instructions where very helpful and spot on.

  20. BufordBluz says:

    Hi Bill,

    I have finally got my BJ III upgraded with your Mods and what a difference! Like night and day and worth every penny. I went whole hog as they say but am loving it. I finally have the amp I want. My changes include your basic mod kit, audio taper pots, standby switch, input jack and best of all, the mini transformer. I also replaced the stock cab with a Mojo Tone Tweed cab which is a little larger than stock and made of Pine and Baltic Birch. I installed a Cannabis Rex speaker and it performs exactly the way you described it in your speaker section. This amp sounds unbelievable now. Beautiful, rich bass, no more thin ice pick highs, fat, lush, clean, and with great control of tone and volume at any level…and when you crank it, stand back ! This thing sings, growls and howls ! Wow ! My Strat and Les Paul sound like new guitars now. I am VERY pleased. Thanks so much for putting out such a great product and as a customer I appreciate all your hard work and great customer service.


  21. trav357 says:

    Greetings Bill,

    This may be a silly question. If a Blues Jr. benefits from power supply stiffening by doubling the first capacitor, Why would you recommend
    the opposite for a Princeton Reverb Reissue (change from 100u to 47u)????


    • bill says:

      The Princeton Reverb’s first cap is a 22uF. I recommend doubling it to a 47uF. You can’t go higher than that with the tube rectifier because it could possibly arc internally.

  22. trav357 says:

    Thanks for the reply!! My mistake. I thought i read in one of the forums that it was 100 uF.
    Thanks again!!

Leave a Reply

You must be logged in to post a comment.