Fender Blues Junior Mods

Fender Blues JuniorI got this pretty Fender Blues Junior amp.

Mods are essentially from the excellent site http://billmaudio.com (see also his old site http://home.comcast.net/~machrone/bluesjunior.htm)

My Fender Blues Junior is from 2003 I think (MIM cream board) so I use pages 2 and 3 of this document : Fender Blues Junior Schematics

Bias mod

Bias is set far too hot by Fender. I changed R52 with a 100k variable resistor (I think a 50k would have done the job, but I had a 100k in my spare box …)

Fender Blues Junior Bias mod



EL84 max Pa is 12W.

I measured 345V on the plates (and 310V for Vg2) of the EL84. A little high as “limiting values” in datasheet are 300V max for anode voltage and 300V for Vg2. So I used a pair of 6P14P-EV (EL84M equivalent, 400V anode max, 14W max for Pa).

Ig2 was ~2mA, so I set Ik to 26mA. So Ia = 24mA ; 24mA x 345V = 8.3W = 70% x 12W

Power supply caps mod

I used F&T 100uF for C25, 47uF for C26, 22uF for C27 and C28, and I replaced C24 with an Orange Drop 716p (0.1uF) and C29 with a high quality polyester (0.01uF – Orange Drop was to big to fit …)

Fender blues junior power supply capacitor caps mods

Other caps mods

I replaced signal with high quality Orange Drop polypropylene or tubular polyester caps (OD because in a Fender amp, I like the chime … and tubular polyester when OD where to big to fit …). I also replaced many ceramic caps with silver mica caps. Be careful to keep C14 (47pF) as a ceramic 1kV because it is used to prevent oscillations …

Fender Blues Junior signal caps mods 1

Tone stack and Cathode Follower mod

I replaced the 0.022uF bass cap in the TS by a 0.1uF orange drop for a better bass response :

Fender Blues Junior Tone Stack mod

As V2A does nothing, I think it’s a good idea to implement a cathode follower to drive the tone stack, as described in http://billmaudio.com/wp/?page_id=141

Billm proposes to use a 12DW7 (a 1/2 12AX7 and a 1/2 12AU7 for the cathode follower). An exciting idea as 12AU7 are good for cathode follower, but I prefered use a 12AX7 as the schematic mimics a Fender Bassman 5F6 stage pre-TS … did you say mojo ? ;-)

Fender Blues Junior Cathode follower mod 1

Fender Blues Junior Cathode follower mod 2


I decided to replace the output transformer with a Mercury Magnetics.

Fender Blues Junior output transformer mod mercury magnetics 1

Note the difference in size :

Fender Blues Junior output transformer mod mercury magnetics 2So I needed to drill the chassis to put in place the new OT … not a big deal !


A Celestion Greenback is my choice to achieve the modifications :

Fender Blues Junior Celestion Greenback

So, from this :

Fender Blues Junior MIMto this, for a very nice sounding amp :

Fender blues junior mods complete 1
Fender blues junior mods complete 2

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91 Responses to Fender Blues Junior Mods

  1. Chris says:

    Hello, I love your site and the great content you’ve been publishing !

    A question about the Blues Junior mods you performed …. regarding the R52 replacement with a variable resistor potentiometer. I have gone over the the BillM bias mods to some length and am perplexed as to why he recommends his method over just replacing R52 with the method you describe ?

    I am looking to complete this modification myself and have been wondering about this implementation difference.

    Many thanks !


  2. t-geek says:

    Hi Chris,

    Glad you appreciate this site !

    If you mean this : http://home.comcast.net/~machrone/bjr/bjbias.htm, please note that Bill is talking about first version of the Fender Blues Junior (hence R31 / 37 instead of R51 / R52, but it does not matter). I think Bill uses 2 trimpots to bias separately each EL84, so no need for a matched pair …

  3. vinic says:

    As I mentioned on the WEM thread, I ordered kits from billm seeing this thread, including the TO20 OT to get more bass response with my baritone Telecaster (using P90s).
    Back to this thread now I did the mods, I was wondering about efficiency of your bass cap mod… Is it really efficient, knowing that 12″ speakers generally have a bandwidth starting around 75Hz? Don’t you waste power into inaudible spectrum part of the amp?

    PS : what’s that software you use to simulate tonestacks?

  4. t-geek says:

    Sorry Vinic, I missed your message !

    Yes slightly more bass w/ this mod.

    For TS, I’m using Duncan Tone Stack Calculator ;-)

  5. Monty Daniel says:

    What would happen if I connected the unused half of the second preamp tube in parrallel. grid to grid, plate to plate, cathode to cathode?

  6. t-geek says:

    First of all, please note that grid, plate and anode are connected together, so you will have to cut the traces on the PCB.

    If you put this triode in parallel with V2-B, you will get a little more gain and lower output impedance of this stage (but I can’t “see” the effect on the sound). You will have to half the cathode resistance from 1.5k to 820Ω to correctly bias the tube.

    If you try and get an interesting sound, please post here ! ;-)

  7. Eric Vaughn says:

    Hello, great site. Why the 47uF for C26 instead of the standard 22uf? (if this is a cream board thing I apologize, I am working on a green board)

    • t-geek says:

      I think I put 47uF for C26 instead of 22uF for better filtering (I’m not sure it represents a big improvement !). Maybe on your green board, it is C17 ? (not sure, please check !)

  8. Eric Vaughn says:

    Thanks for the quick reply. Yes the cap I’m referring to is in C17. I was mainly just curious about the deviation from the other mods I have seen. The pictures you posted have been the biggest help of anything I have found on the tubes. Cheers, Eric

  9. Bergs says:

    On your bias mod. Did you connect the wiper and the other side of the trim pot together on one side?

  10. peter says:

    Is there any chance you’d have a list and values of caps replaced? That would be a great help when following and implementing your mods. Kindly plz advise

    • t-geek says:


      So, for power supply : I use F&T 100uF/450V for C25, 47uF/450V for C26, 22uF/450V for C27 and C28, C24=100nF/630V Orande Drop, C29=10nF/400V polyester
      Signal Caps :
      2.2nF OD716P C2, C8
      100pF Silver Mica C3
      250pF Silver Mica C5
      100nF/400V Polyester C6
      22nF OD 716P C7, C15, C16
      33nF/400V Polyester C10
      10nF OD 716P C11
      100nF Polyester C12, C13
      470pF Silver Mica C17
      47pF Silver Mica C21
      15nF/100V Polyester C22
      3.3nF/100V Polyester C23

      Please note : R25 replaced by a shunt, R26 = 1k, R27=100k

      Hope this helps

      • peter says:

        Kindly thank you for a swift response and helpful info. One more question: if I was to replace/upgrade coupling/signal caps only with OD’s or other “high quality” like SoZo or Jupiter which ones would that be (cream board version)? I’d like to concentrate only on those caps that might be beneficial to overall sound quality.Tone stock; I feel that range of adjustment is sufficient for me.
        I don’t know how to identify coupling/signal caps so any help will be greatly appreciated.

        • t-geek says:

          Be careful : OD are great for Fender amps, I think, but they are « big » capacitor and sometimes they will not fit.
          Signal caps are :
          C2, C3, C5, C6, C7, C8, C10, C11, C15, C16.
          I suggest to change power supply capacitor (easy) : C25, C26, C27, C28.

          Let us know !

  11. Mike says:

    Help! I have a 2010 Blues Jr III, cream board. I changed out my power supply caps to F&T: C25 100uF/450V C26-28 22uF/450V. When I turned my amp on, I have a really loud squeal. I do have guitar sound coming through. This squeal is there no matter if I turn down the volume to crank it. It was working fine up until I did this cap swap. Ive re-flowed the solder joints etc etc. Any idea on what I’ve screwed up? Thanks for your help. Mike

    P.S I modded this amp a few years back with Billm mods without any problems……

    • t-geek says:

      Hi Mike !
      Did you check the right polarity for the power supply caps ?
      Maybe you get oscillation due to phase inverter tube. Try to remove this tube : does the amp still squeal ?
      (see http://billmaudio.com/wp/?page_id=115 )

      You can also check ribbon cables solders

      • Mike says:

        Hey, thanks for the reply. Yes I made sure of the polarity, I even double checked to make sure I didn’t have one backwards. I will pull the tube this afternoon. I also thought it may be one of the ribbon cables…..I may remove all the ribbons and just re wire it…. I’m confused….

        • t-geek says:

          Ribbon cables = Evil

        • Mike says:

          Ok, so I got home from work and went straight to the amp……I turned on the amp, while it was on and squealing I pull V3 and it stopped. I am going replace the ribbon on all tube holders with wire and also do the cathode follower mod as well because it was my intentions to re cap and do the cathode follower mod. Any pointers or wiring diagram for doing the re wire? Do you think the cathode mod made an improvement in responsiveness and removing any dullness to the amp? Thanks a lot for your help!!!!! Very much appreciated, Mike!

          Happy Thanksgiving to all and please remember our troops and their families as well as those that GAVE ALL!!!

          • Mike says:

            Hey, I wanted to update what I’ve done to my Blues Jr. I replaced all the ribbon cables with 22 awg 600v wire. I soldered the wire directly to the socket. First, I did the cathode follower mod….WOW! The amp was lacking presence/responsiveness/mojo etc, it is all there now. That being said, I did another little mod my Blues Jr. The mod: R11 to 56k 1/2w C5 to 500pF 1000v silver mica C10 to 0.1uF 400v C11 to 0.022uF 400v C15 to 0.1uF 400v C16 to 0.1uf 400v….now it has lots more drive and mojo!!!! I saw another mod where R8 to 82k 1/2w R10 to 2.0k 1/2w C4 to 0.22uF 400v by Fritz D Cat on fenderforum.com. Any thoughts on my mod I performed and the other mod mentioned? If I took my mod I did and reversed it back to factory, and did the second mod mentioned, how would the drive/tone sound then? Thanks for the info. BTW, I found the ribbon cable on V5 to the culprit for my problem too. It had two (2) of the wires broken at the solder joint and was still making some connection thus…….I am glad I took those ribbons out!!!!

          • t-geek says:

            Good for ribbon cable replacement ! Your mod changed the way the tone stack reacts, there are many mod you can try there, and listen to the result.
            The mod R10 2k, C4 0.22uF will change how V1-A works : less bass, asymmetric clipping, a little less gain (R8 = 82k) ; you could try it while keeping your tone stack as is (modded)

  12. Gary G. says:

    Hi t-geek. Could you give a bit more info about the output transformer mod? Which of the two original holes did you use. I’m wondering if it makes any difference as far as the Mercury Magnetics OT not touching the speaker frame is concerned. Thanks!

  13. pista says:

    Hi, great site, thanks for your help! I’m thinking of changing the OT to a Hammond 1760H, as Bill M. recommended somewhere… My question is, if it’s ok that the stock OT’s input impedance is 6900 ohms and the 1760H has 6600 ohms..?

    • t-geek says:

      Oh yes, definitely no problem, 6900Ω or 6600Ω is the same in a rock’n’roll world !

      Are you sure about the 6900Ω ? Never measured the genuine OT from a Jr, I would have thought it was 8000Ω … not a big deal anyway …

      • Victor Zart Bonilha says:

        Yes, it is around 6.6k, I’ve measured it a few weeks ago. I’ve replaced the stock OT to a high quality brazilian brand transformer (Smithers SM18). The new OT is 8k, and is way better than stock OT to my ears.

  14. Damon says:

    Hey T-geek . I am wanting to mod the bias so it is adjustable. I see so many different ways of doing this. I do not want to drill holes and such. On the variable pot do you connect the wiper to one of the other arm. Just an explanation of how to get this in the circuit would be very much appreciated. Thx

  15. Mike Warren says:

    Hey, After doing the cathode follower mod and running new wires to the tube sockets, etc. this amp is really nice now. New problem: my reverb has quit working. Its not the tank because I checked it on another amp and it works. I’ve changed the IC and traced the board for a break or bad components. I also checked voltage at all the check points per the schematic and still i cant trace down the problem. Any suggestions? Also, I installed the presence mod from BillM back when I did those mods a few years back. When I turn the knob counter clockwise, I get a whinny noise with some static????? Thanks in advance for any info!!!! Mike.

  16. Nicolas says:

    Hi T-Geek,
    Great site, I will started the Bjr mods but I have two questions
    About the silver mica capacitor, I bought Silver Mica Cornell CD15 in 250pf / 500V and I saw the treble capacitor is in 1kv
    Can I replace it or not ?
    What is the size for the connexion on the output tranformer I bought the same as yours but like yours the connexion are free and I need to put some connectors
    I forget i have a Bjr 3 with a cream board
    Thanks in advance

    • t-geek says:

      Hello Nicolas,
      French ? ;-)
      No problem with your Silver Mica 500V !
      Pas de souci pour 500V dans le “tone stack”, je ne sais pas pourquoi Fender place un condensateur 1kV à cet endroit …

      Je n’ai pas compris ta question sur le transfo de sortie : tes fils sont trop courts ?

      • Nicolas says:

        Merci pour ta réponse
        Oui je suis français désolé pour mon anglais technique approximatif :)
        J’ai trouvé la solution pour mon transfo, je couperai les fils de l’ancien et récupérerait les cosses pour monter le nouveau en lieu et place
        J’attends les dernières pièces (reverb, knob) et j’attaque le chantier
        Encore merci pour toutes ces infos précieuses ;)
        Très bon site !!!!!!

        • t-geek says:

          Cool !

          • Nicolas says:

            Salut T-Geek
            ça y est mon Bjr est au top avec ces modifs ;)
            Je vais m’attaquer au bias et ajouter un footswitch sur la reverb
            Voila quelques photos de mes modifs

            Bientôt la suite ;)
            Encore merci pour le coup de main

          • t-geek says:

            Cool ! Par contre, il me semble que les URL relatives aux photos pointe sur la homepage de hostingpics …

  17. Tim Ryan says:

    On BillM’s youtube video titled ‘Set BJr Bias’ it looks like he removed R51 and R52 and sort of jumped them but can’t tell how he connected the pot. The picture you show you replaced R52 but left in R51. I can’t get any info from BillM on this and don’t want to mess it up. From the schematic R51 and R52 are in series and putting a pot on R51 allows for adjusting bios. If you remove R51 and R52 and put in the pot the way BillM has it in the video, what happens to or at C33?

    • t-geek says:

      Hello Tim,
      Bias on stock Blues Jr is very “hot”, so I keep R51 and I replace R52 with a pot to “cool down” the bias. You could replace both R51 and R52 by a pot (so you could get hotter or colder bias) and place C33 between wiper and ground

  18. Nicolas says:

    Salut T-Geek,
    Désolé pour les photos je n’arrive pas à les mettre :(
    Je vais surement faire la mod sur la V2, et j’aurai une petite question sur ta mod. Je n’ai pas vu la résistance de 100k que préconise Bill M. Je pense que je vais faire la même chose que Bill M en perçant pour ajouter la résistance sur la carte
    Qu’est ce que cette mod ajoute au son ? plus de gain ? meilleur grain ? meilleur sustain ?
    C’est fou que Fender câble les lampes à moitiés :p
    Merci ;)

    • t-geek says:

      Je n’ai pas testé avec / sans cathode follower. Comme le dit Billm, je ne suis pas sûr que la différence soit flagrante (voire que l’on note une réelle différence de sonorité, c’est plus pour “l’amour de l’art”, cela dit c’est avec de petites modifications sonores accumulées que l’on finit par avoir un ampli qui sonne mieux ;-) ). Je ne sais plus où j’ai placé la 100k, mais elle y est sûrement, sinon le cathode follower ne fonctionnerait pas …
      Hiwatt, dans son légendaire DR103, laisse également une triode inutilisée ;-)

      • Nicolas says:

        Ok je te remercie bon bah je vais laisser tomber celle ci. Je vais faire la reverb mod en ajoutant un jack, le footswitch est prêt.
        J’ai changé mes lampes par des JJ Electronics, le son est très bien juste un léger buzz en fond un peu comme quand un halogène est allumé dans la même pièce je n’ai pas encore trouvé pourquoi, ce n’est pas grave mais je ne l’avais pas avec les lampes Groove d’origine.
        Je vais surement ajouter le condo sur la résistance pour éviter les oscillations, est ce que je peux remplacer le céramique 1kv par un silver mica 500V comme pour les autres ?
        merci ;)

        • Nicolas says:

          J’ai bien avancé ce week-end. Installation de la résistance de bias avec un trimpot multitour 1/2w et installation du condensateur anti oscillation sur R30. Le résultat est très satisfaisant.
          J’ai gagné un headroom, en précision des notes, en sustain, vraiment très content ;)
          Encore merci pour ton aide précieuse

  19. John Michael Jones says:

    This is by far the best site when it comes to explaining bjr mods & im so glad I found it. Sad to see billm is no longer doing business & suffering such extreme health issues. What a loss to the bjr community…. Hopefully someone will pick up where billm left off. Hate to see such a legacy & the endless amount of knowledge bill left behind be erased. I’m praying he’s ok.
    Anyway, moving forward…. Quick couple of questions; I just got lucky & scored a mint, all original 2003-ish (cream board) bjr for $150. Having so little invested, I want to go all out modifying this little box of heaven.
    Anyway, my question is, I’m torn on whether or not I should do the cathode following mod… Is it as scary as it looks to do &/or worth the effort tone wise? Also, if I decide against doing it, can I still follow all your cap & resistor swaps as mentioned above without blowing up the house? Lol.
    Thanks for being so kind & helpful! LOVE THIS SITE!

    • t-geek says:

      Thank you very much for your comment, JM !
      Very sorry to read that Billm has health issues
      Cathode follower is kind of “state of the art” … I’m not sure this mod makes a major difference to the tone, so feel free not to implement it ;-) (In fact, I *did not* compare tone without CF and with CF …)
      It is possible to do resistors and caps mods while not implementing CF mod !

  20. John Michael Jones says:

    AWESOME! I’m going to go nuts on this thing! Haven’t been this excited about doing mods in a long time! Finding your site & looking/reading your advice has got my blood pumping! Do you or will you be selling any kits, upgrades for the blues jr. anytime soon? I ask because there’s one mod on billm site that I’d love to get my hands on & that’s his clean boost module. Does anyone have this mod? If so, any feedback? Pros vs cons etc? Thanks guys! LOVE THIS SITE!

    • t-geek says:

      Thanks !
      No, I don’t sell kits … maybe a good idea for expand my business ? ;-)

      • John Michael Jones says:

        YES! I’m telling you brother, people got no where to go now that billm is gone World wide market too! I’ve been in tons of forums, groups, blogs etc… people are screaming for direction…

  21. John Michael Jones says:

    Oh, one other question…. is it ok to use 500v on the 3 22uf filter caps & a 450v on the 100uf main filter cap? I can’t seem to locate 450v in the 22uf values.

  22. John Michael Jones says:

    Almost got all the parts ordered but I got a few more questions if you don’t mind, I noticed you said these orange drops are <> …. Could I substitute 400v OD where you have 600v to save space or are you using the 600v for any tonal character purpose? Also, what exactly did you use in C22, C23 & C29? Are they Vishay, Mallory 150′s type etc?

    • t-geek says:

      OD 716p are polypropylene film where Mallory are polyester film … I have the impression OD 716p sound a little brighter than polyester caps, but it is a minor “nuance”. What is important is to have good capacitors, and a mix OD 716p and Mallory 150 (or equivalent : Roederstein, Vishay …) is very interesting (Mallory, Roedestein … Some people are able to hear differences but to my ear, it’s the same ;-) )
      Don’t worry for 400V Vs 600V in this amp, 400V caps are OK.
      I think I put Mallory or equivalent when not enough space for Orange Drop (compromise ;-) )

  23. John Michael Jones says:

    orange drops are big …. i meant to write. Sorry for the typo.

  24. John Michael Jones says:

    ToneGeek, when switching out R26 & R27, what type of resistors did you use? The carbon film, metal film, etc… does it matter if the resistors are 1/2w or 1w?

    • t-geek says:

      Hi !
      I quite always use carbon film 1W resistors for audio (and 2W or more in power supply). 1/2W could be OK for negative feedback …

      • John Michael Jones says:

        Thank you brother. A buddy suggested I use carbon comp but im a bit nervous about using any NOS products. Given shelf life span & the history of inconsistencies in values etc… even though he swears by them, id rather not take that chance. Truth is, as with everything else in life, it’s all subjective.. I’m going to follow your mod to the T… I’ll let you know how everything turns out! Oh, I wanted to ask, how did you like the MM output transformer? Make a difference to put forth the extra $?

        • t-geek says:

          Carbon comp are great (maybe a little noisy, but not too much) ; you should check value with a MM. I quite always use modern carbon film anyway …
          Yes, the MM transformer is great. Maybe another “good” OT would be OK for the job (Hammond, Heyboer …)

          • John Michael Jones says:

            I noticed in C2 & C8 you said you used 2.2nf (which is .0022uf) & I was wondering if maybe that was a typo cause aren’t C2 & C8 22nf (.022uf) opposed to 2.2nf .0022? Or does it really not matter? If not, was there any significant changes by utilizing the .0022 opposed to the .022? Sorry to keep pounding you with questions brother. I just want to be sure I get this right.
            Still waiting for the MM OT, CRex & a couple of other parts. I’m dying tho dive into this amp & get this project done! Super stoked!

          • t-geek says:

            Hi John,

            It *does* matter and yes, C2 and C8 are 2.2nF (see the Fender schematic, page 2) ;-)

  25. John Michael Jones says:

    You’re the man! Good cause that’s exactly what I got! Thanks brother.

  26. John Michael Jones says:

    Hey Tone, got a bunch of the mods done today & so far so good. In my excitement I got a bit ahead of myself & accidently cut out C9. Not a big issue but was wondering if I could replace that cap with a better quality cap or does it not matter?
    Also, weirdest thing, when I cut out & tested my old C2 & C8 they read 279pf @ 48% v.loss & 496pf @ 52% v.loss respectively…. absolutely crazy but does that mean they were bad? Might explain why this thing sounded like a wet dish rag hitting a wall at 250mph! Ha!
    Anyway, after some careful consideration & countless hours of research, I decided to go up to .0047uf in C2 & C8 for a little more umph. Billm said he “always uses” .015uf in c2 & .022uf in c8 for more bottom end. But a few people complained that was too much so instead of going all-in i decided to use .0047uf. That would still give ‘more’ without running the risk of overloading the next section.
    I’ll let you know how it goes once I get er done! I can’t thank you enough for all of your help brother!

    • t-geek says:

      I don’t think you need a very good quality capacitor for C9 …
      C2, C8 : wow, weird values !
      Let me know if you like 4.7nF for C2 and C8 !

  27. TD says:

    If I replace R52 with a 50k pot and use a new set of tubes, what is the measurement I take to set bias? Can I measure current through a 10hm resistor between valve cathodes and ground? Does it matter which tube I measure off of?

    • t-geek says:

      Yes, you could place a 1Ω 1% resistor between EL84 cathodes and ground ; if you buy a matched pair of EL84, one common 1Ω resistor between the two EL84 cathodes and ground is enough. Let’s say EL84 anode voltage is ~350V, you have to set your 50k pot to get ~50mV across your 1Ω resistor (= 25mA per EL84)

      • TD says:

        Great! I’ll see if I can find the net for the cathode to find a good place to jump a resistor. Looking at pin 3, right? Is there a particularly good place on the board to make the jump to ground?

        Also, thank you so much for not only posting this valuable guide but continuing to provide feedback in the comments. Truly appreciated

  28. Tristan says:

    Hi T-Geek,
    I came across your website while researching mods for my Blues Jr. Great site! For some reason my amp has been thin sounding and sort of shrill. I’ve tried a bunch of different speakers (I think the Swamp Thang is the winner) and tubes but the issue is still there.

    I’ve swapped the filter caps thinking that might have been the issue, it wasn’t. I have the cream board version, like yours, so now I’m looking at the signal caps specifically the bass (C6) and mids (C7) caps. I’ll try 0.1uf for C6 and I’m thinking 0.015uf for the mids (I’ve already done the jumper wiper on the mid pot which I really like). Is 0.015uf for the mid cap adding mids or reducing them?

    Is it worth removing the bright cap C3 or changing its value?
    Is it worth changing the value of the other signal caps?
    Do OD’s make a difference compared to the stock caps of the same value?
    Could the issue be the fixed presence control values of C13 and R25?

    Thank you for your help!

    • t-geek says:

      Has your Blues Jr always sounded thin ? Some find this amp a little too bright, but he has a full sound anyway … did you compare with another Blues Jr ?
      ODs tend to colore the sound on the bright side, so prefer Mallory 150 for instance, but these are nuances …

      Yes you should definitely test with / without C3 (bright cap) … or replace C2 with a 22nF cap instead of 2nF

      • Tristan says:

        Thank you for your response! I replaced C6 with 0.1uf and C7 with 0.015uf and it made a huge difference! But now I think I have too much bass (I may have damaged the stock speaker by maxing the bass and turning up the volume because I’m getting some crackling/static/farting like noise when I play lightly LOL!),not enough mids and piercing treble. I may go back to the stock value for C7 and then try a slightly smaller cap value for C6 (maybe a 0.068uf). I didn’t try anything with C3 or change the value of C2.

        I never recalled the BJr sounding thin/sterile but about a year or so ago I noticed that the sound at changed somehow. I took it to one tech and he noticed the bass was not a prominent as others he had seen. I’ve never compared it side-by-side to another BJr but I have tested some in a few different stores and they all sounded a little different.

        I think I am on the right track by changing the bass cap to a larger value. Definitely helps eliminate the boxy sound.

  29. Adrien says:

    Hi everybody,
    I am looking for connection between green pcb and cream. All forum are for cream and i have the 1995 green one. So it’s easy to change the big capa but not for C24, C1,C4, C5, C6, C7, R52 (I find it’s R31 on green) and others are not the same on green. I try Billmaudio web site and many others, I find nothing too. Someone can help me? I ‘m french, sorry for my english!

  30. Vince says:


    Have you experimented with changing the mid tone cap at all? Like you, I’ve changed my bass cap from 0.022uF to 0.1uF. However, I also changed the mid cap from 0.022uF to 0.015uF, per BillM’s specs. Now, I find the mids to be muddy (too many 150-400Hz, not enough 700-1,000Hz).

    Would it make sense to put the 0.022uF mid cap back in? Or maybe go all the way up to the 0.047uF that many Fender amp’s use?


    • t-geek says:

      Hi Vince,
      I keep the 22nF for mids. Here is a simulation with 3 different caps (15nF, 22nF, 47nF) : you could test 47nF !
      Let us know which one you prefer !

  31. Nathan says:

    Hey t-geek, great site! I’m preparing mentally to do some mods and your site has been super helpful.

    Question! There’s a hum that happens as soon as I flip the switch. I can actually feel the vibration coming from the PT and when the amp warms up it makes its way through the speaker. Is something else causing this (filter caps perhaps?) or is this a bad PT or is this normal and the filter caps aren’t…filtering? Help?

    Side note: one power tube is hotter than the other. Even when I switch sockets it’s still hotter. Obviously not good but could it have to do with it? I’ve tried pulling preamp tubes and switching them out. Didn’t help.

    • t-geek says:

      It seems : 1/ Your PT vibrates. This is often the case, it is difficult to say whether it is defective or not. Try to check the nuts and bolts. 2/ Maybe your EL84 pair is not well matched : you could install the variable bias and give a try to another EL84 pair …

      Let us know !

      • Nathan says:

        Ok will do!

        Am I on the right track with my thinking on the filter caps? Is this the kind of thing they should be “filtering” out?

        Also, do you recommend a specific pot for the bias?

        1 million questions later, thanks!

        • t-geek says:

          Hi Nathan,
          I can’t say if filter caps are bad. Begin with a new EL84 pair and bias mod (variable resistor), imho. Of course, you could also replace filter caps with better quality caps ;-)
          Bias pot is a 50k to 100k variable resistor …

        • Ryan says:

          I have a Blues Jr. that seems to have some excess vibration coming from the transformers. You can put your hand on the top of the amp and feel it. The amp works, and sounds ok.
          I inspected the 47uf 450v Cap and noticed it was leaking.

          Im going to replace all the power caps, and possibly do the tone stack mod. Would like to know if it lives up to all the praise!

  32. Ian says:

    Why do you shunt R25?

  33. Geoffroy says:

    Hi, sorry for my English I’m French ^_^
    First, bravo!! It’s a really great site, TGeek
    Thx a lot!!!

    I made lots of mods on my blues Jr thanks to your site.
    I decided to convert my combo into head
    I swapped the tubes pcb and I relocated them with “PTP” wiring
    Some pics ;)


    The finished work

    Just a question or your opinion on this?
    Some people told me to change only V3 anodes resistors with carbon comp to add some “grain” (don’t know how to translate in English).
    I love BJr overdriven!
    What do you think of this mod and can you help me to find the right resistors to replace?
    I got a cream board like yours
    I guess it’s R30 100k for second anode but I’m not sure for the first anode
    R28 91k and R29 9,1k seem to be in series.
    Should I swap R28 and R29?


    • t-geek says:

      Very nice job !!! Don’t worry, I’m french too ;-)
      R28, R29 and R30 are anode resistors for the phase inverter … Some say carbon comb are good as anode resistors, you could try but I’m not sure it will be an easy task to find 91k and 9.1k Imho, if you replace R28 with a 100k resistor and R29 with a shunt (a wire), it will work …

  34. Geoffroy says:

    Thx a lot for your help !!

    I found some 91k carbon comp resistors but the 9,1k could be harder to find for sure
    Banzai music have some, but there is always some back orders with them.
    Maybe I will try with a shunt for R29 , thx for your advice !

    I’ve got two Bjr.
    my Bjr head with an Heyboer OT and a combo with a Mercury Magnetic OT.
    I want to sell the combo but I don’t know which Output Transformer I have to keep?
    Do you think they’re equals or there is one who’s better to keep?

  35. John says:

    This may be a dumb question… I want to add a trim pot for bias at r52. I don’t know where to measure the voltage to adjust the bias. Do you have a pic or webpage that shows where to land the probes from my meter?

  36. Pat says:

    I keep hitting this page for great info on Blues Jr’s. Just wanted to give a BillM update–he passed away sometime last year and his son supposedly took over but don’t order anything from the BillM site. You won’t get it. I read on Facebook that people have even sent their amps in for mods and never got them back. I ordered an aux out jack and instructions but never got it. I DID receive a reimbursement on PayPal. I figured out the aux out mod myself and it works great. Anyways, great info here–helped with my re-cap mods, thanks.

    • David A Menke says:

      Bill’s son took over and the orders and mods are going out on time. For awhile, the site was closed after Bill M passed away.

  37. Chris johnson says:

    I was wondering where I could get a bias pot to install in my blues jr iv, and maybe some instructions on how to?

    • t-geek says:

      Bias pot : electronic components store ;-)
      Sorry, I don’t know how to implement this mod in a Blues Jr IV (not so different from previous version, I suppose)

  38. J ai remplace les capas sur le chemin du signal par des Orange Drop polypropylene ou par des polyester (des OD, car dans ce genre d ampli Fender, j aime bien le chime de ces condensateurs question de gout, tout est discutable. Les polyester parfois pour adoucir un poil le son, parfois tout simplement car les OD auraient ete trop gros a cet endroit precis ??

  39. Julien David says:

    One question :) !
    For C29, there is the 5% tolerance and the 10% tolerance. Which should you choose?
    Thank you for your work and your response!

    Have a nice day :)

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