Mod Your Fender Pro-Jr!




About: I play guitar, drum, bass, banjo, blues harp and more... I make guitar and basses, effect pedals, I mod amps... I listen to lots of prog, jazz and 60s music... My favorite bands/composers: Harmonium, Camel, ...

Hi, in this instructable i will show you ways to mod your pro-jr to change the sound in pretty much every ways. this instuctable also regroup lots of information

I will cover easy to hard mods, how to bias it properly, route wires to reduce, see remove hum, hiss and oscillation. ADD/REMOVE GAIN and a lot more!

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Doing these mods WILL void the amp's warranty, if you don't feel confident enough to mod your amp, get it to a tech.

If you do one or some of these mods, you agree to indemnify and hold harmless to the author(ME) if something goes wrong.(see step 3)


Step 1: Open Amp

Follow steps in pics. YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

I recommend that you do all of the pcb mods at the same time, i did 'em on 3 times and broke a ribbon cables connection, no big deal. i soldered it back in place and it worked fine.
If after the mods the amp sound bad or even doesn't produce any sound(it won't happen if you are careful), you probably broke the ribon cables connection. Solder it back!


Step 2: Make/Change Speaker Baffle

The baffle in there is made of a very bad material, it doesn't sound good. Changing it with a nice one made out of pine will make your clean sound sweeter and warmer, vibrating.

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Step 3: Discharge Capacitors


Here's how to discharge Filter caps Safely.

TO DISCHARGE THE FILTER CAPS: remmelt from (nice website btw)constructed a low tech but very effective tool. he got some nice wire in pretty yellow and a 5 Watt 15K Ohm resistor. In combination with a pair of alligator clips and some shrink tube it looks like the pic. You can use a resistor like the one in the tool and use it like the tool but I HIGHLY recommend that you build yourself one since it won't cost more, it'll be easier to use, a lot safer and a lot prettier. build one.

with both leads of the tool, touch both leads of every Filter Caps(big ones) for about 30 secs until there isn't much electrical current in, 30 seconds should do, you should check with you multimeter, if you don't have one, I don't know why you are doing these mods, GET ONE NOW! its one of the most useful tool you can have in electronic. After a having discharged the caps, Take an alligator clip with one end clamped to the chassis, the other to one lead of the cap. Don't take these off until you're over with the mods.

Discharging Caps is Extremely important, They can hold a deadly voltage long after the amp was unplugged.

Step 4: Access the Pcb

Look at the pic.

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Step 5: Tone Mods

Look at the Pic, Follow the link will explain it more and show you how to adjust bias with a trimpot so you can tweek it. I found it a lot easier the Put a Fixed resistor, see for yourself.

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Step 6: Adjust Bias

The bias in this amp is set way too hot, every one owning a pro jr that can use a soldering iron should do that mod, the mod consistof changing a resistor:  R29, its a 15k resistor, changing it to a 18k-20k lets the tubes cool down and won't affect tone, it will highly increase tube life!

Here i changed mine for 20k, its perfect.

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Step 7: Add a Line Out

I have not added a line out to my amp since I don't need one, but if you do, Here are some useful links, If someone does the mod, please, post a comment! tell us what values you used! The values shown in the pic should be a good start, it may even be perfect.

Step 8: Add/Remove Gain

R6: Controls BreakUp, right now it's set to 56K.
More gain: increase the value, i don't recommend going much higher than 80K
Less gain/more clean headroom: lower the value, don't go lower than 10K

My Pro-jr sounds really warm and clean, it breakups at about 6(volume) my R6 is at 22K
I like it clean to use my effects(fuzz, overdrive, booster...). At 22K I still have natural overdrive, it's not completely clean(good thing) I can get a quite nasty overdrive, don't be scared to lower value, yes you get more clean headroom but you won't say goodbye to overdrive.

Step 9: Negative Feedback Loop

Thats an easy mod. Its the negative feedback loop wire, without it, the amp works just as well but it won't sound the same, more VOX like. Remove the purple wire from the pcb  to the speaker jack, to test if you like, unsolder it from the jack, if you don't like the sound, solder it back, if you like both sounds, you can add a spst(single pole single throw) switch, any on/off(2 lugs) switch will work, solder the purple wire to the 1st or 3rd lug of the switch and connect the 2nd lug of the switch back to the jack where you first unsoldered the wire from, drill a hole in the chasis  (drill it before soldering the switch) next to the speaker jack if you don't want the line out jack mod, if you did the mod, solder the switch next to this one.

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Step 10: Tubes

Google for good tubes combination, you should find LOTS of forums that answered these questions. 

As with speakers, you can't know what you'll like until you tried, so buying a couple of different brand and models and spend a few hours to swap until you are happy is a good idea.

I have 2 EL84, 1 ECC81(12AT7) all JJtubes and an old RCA 12AX7, It doesn't sound harsh as I experienced with the groove tubes. sounds warm, clean, very musical, with TONS of Sparkle.

Step 11: Change Speaker/remove Magnet Cover

The stock speaker in the Pro-Jr. is harsh, I changed mine with a weber sig10s alnico

ceramic vs. AlNiCo.
Ceramic: stronger magnet, more focused, Later Breakup, More balanced.
AlNiCo(aluminum-nickel-cobalt): Warm, early Breakup, compressed at high volumes, more squishy.

How to choose?
Take the one you think will suit your tastes better, a used one would be a good idea, if you don't like, from what you heard from your amp with the speaker, you can find comparisons on forums from people who tried the speaker and another one, to know how they compare.
descriptions and reviews gives a good idea PERIOD. you can't know until you tried.

You can google ceramic vs AlNiCo guitar speaker to get better comparison.

from what i heard, i was sure i'd like Ceramic better over AlNiCo, guess what, i don't.
I have a weber sig10s alnico and it sounds warm and it has tons of sparkle. i love it. 
If someone gets a weber alnico speaker, the magnet cover is too big, it won't fit.
here's how i removed it:

1) get a big flat screw driver and a hammer.
2)find a good/comfortable position.(between you knees)
3)put screwdriver tip over small tab(see pic)
4)Hit screwdriver with the hammer to straighten the tab. Be careful not to slip and CRY BECAUSE YOU DESTROYED YOUR SPEAKER.

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Step 12: HUM!

90% of the hum coming from this amp is due to the bad lead dress!

YOU MUST BE LOGGED IN TO SEE INSTRUCTION IN THE PICS(you cant see all of the text if you're not logged in)

Cut the cable ties holding the two green heater wires together and twist them together like in the picture

while unscrewing the midle screw down the back panel i realized that it was too long, as i was unscrewing it, the hum lowered, check for yourself, if it does the same, check for a smaller one but you probably wont find find one that match so you might want to cut the end or grind it, if it doesn't affect hum, leave the long screw as it is.

Also, all ribbon cables should run smooth without any sharp turns as seen in the pictures

Step 13: Thanks To:


IF YOU FOUND THIS USEFUL, CONSIDER POSTING SOMETHING TO THE DIY COMMUNITY!   Also, if you have questions or if you did a mod, Post a comment! 




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    32 Discussions


    9 months ago

    cool site.I like less hum for sure. Ill try that. But really I bought two tweed pro jrs back when they first came out because they are a tone unto themselves. A great stand alone amp with no effects. Just a guitar, a cable and an amp. really reactive dynamics and beautifully honky gain when you turn them up past 4 they get dirtier not louder. I have often used two together or one in series with a vibro king for an incredibly balanced and warm tone with plenty of gain for larger club volume. All these mods on the internet (a 12" speaker?, switch to 6v6? etc..) may be fun projects but defeat the original intention of that amp. I love the punch and low end of 10" speakers and sound of overheated EL84's. But I blew one of the old Blue frame speakers (I've been through many) and I just ordered a Celestion greenback which is less efficient and darker. I'm interested to hear it. I just may replace the baffle with some birch ply while I'm at it. Thanks!


    Question 1 year ago on Step 6

    Hi, i own a pro junior iii, i live in Ecuador.... I really love the amp but it really runs hot, if I set the bias mod (r29 with a 20k resistor) could be needed to do other adjustments in other circuits or is it just as simple as replacing this resistor, could it damage something in my AMP or cause future issues? Really need your help here it is impossible to find somebody to do something like this in the right way ...thanks


    2 years ago

    I'm writing again a year later after working on the amp. To reiterate, I found your work here very helpful.

    I have an early model (Rev B) actually ended up putting a 47k in R6, and then running across it to a three way switch, allowing two resistors (or none) to run in parallel, reducing the resistance to like 22k and somewhere closer to 15k, which seems to work pretty well. I added an .01 bypass cap on the first preamp's cathode, and a switchable BPCap on the second stage for a late boost. (I had 10uF but i'm thinking about changing the value there). I'm playing around with the first coupling cap as well. right now at .022 (which they used on later versions) but may go up to .047. The thing lacks bass, even with other speaker cabs.

    I also made the NFB switchable, but made it go from stock to more NFB instead of lee. It's not that I want everything to be clean all the time, but I did want to be able to use this thing with effects without everything getting smeary.

    But I have a question: that tone stack seems like an oddball to me--at first glance it seems tweedy, but it takes signal from after that little resistor network and connects back to the volume pot in an odd way. I find the tone stack to be kinda weak: it goes from relatively neutral to bright, with almost no way to roll into a deep sound. Not sure I care to change it, but am just curious about the circuit and how it's supposed to work. Another question: why, do you think, did they go with 250k on both pots instead of 1 M or 500k?

    Wow, didn't mean to write a novel here. Thanks again for your well-written and illustrated work.


    3 years ago

    Thanks for this. Very clear text and pix.


    3 years ago

    Hi, I just did all of the electronics
    mods today, all worked great!!

    Thanks so much for the info!

    I'm doing the baffle panel replacement
    tomorrow, picking up a piece of Baltic Birch plywood. I might try
    building one with the speaker offset from centre so I can possibly
    install a 12” speaker instead. Will have to check to make sure the
    magnet doesn't hit the tubes...

    A few custom touches...

    I decided to make the feedback on/off
    control (Step 9) and headroom mods (Step 8) controllable from the top
    main control panel.

    So for the feedback loop control I
    carefully installed the (small) feedback on/off switch right between
    the volume and tone knobs.

    And in my Junior III the headroom
    resistor R6 is already 20k, so to get the full range available I
    replaced the 20k with a 10k and connected it in series with a 50k
    pot, which I installed in place of the power light. (I will relocate
    the on/off light to the new baffle I am building... might even make
    it a whole LED strip for a 'surround glow' on the front panel...

    This way I can play away with the
    feedback loop on/off and full range of headroom controls, right from the
    main panel.

    So far the results are great! The feedback loop control is subtle but noticeable, the headroom control range is huge and very useful.

    Also added new tubes (Northern Electric, best available) and next to try out a few different 10”
    (Jensen C10Q, Ragin' Cajun and Tone Tubby 40/40) and maybe 12”
    speakers, and I've got a killer little amp.

    Thanks SO much for the inspiration and




    7 years ago on Introduction

    I suggest that you also add some clear silicone caulking between the two caps and between the caps and pcb. Just a 1/4" bead will do. This will keep the caps from vibrating and possibly loosening the solder joints.

    2 replies

    Reply 4 years ago on Introduction

    I really appreciate seeing great modification techniques like this. I just bought one of these Pro Juniors. I play a Modified Epiphone SG double coils, and a ESP ST-203 Strat style and cannot tell you how much the Pro Rocks. So looking forward to trying out some of these awesome mods. Thank you pipicher. You Rock!


    5 years ago



    5 years ago

    Hi, i have a MIM fender pro junior (I'M MEXICAN) and my amp it's way too different all inside, i wanted to change the R6 of 56K to 22K but when i checked the value of the R6 was 22k already so i did get a 10K R6 and it's more clean, but not enough to my personal taste. what would i do for gettin' more clean sound?


    6 years ago on Introduction

    Yeah! I found a solution to more clean headroom. R3 and R10 that have 100 k values I have change them to 56 k and the sound goes cleaner without affecting volume or tone.

    Next step i will try to put a double pot of 100k in series with two resistors of 22 k in these R3 and R10.


    7 years ago on Step 8

    Thanks for posting this. I just got a pro jr. If I lower the resistor to get more clean headroom will the volume drop?

    2 replies

    Reply 7 years ago on Step 8

    Thanks to let me know it's useful no, the volume stays as high.


    Reply 6 years ago on Step 8

    Hi everyone, I write from Spain, this article has been very useful for me, i've made several mods from here.

    I've made the gain mod, changing the R-6 to a 10 k resistor and a linear pot of 50 k in series. But when I turn down my gain pot the volume falls down too and the tone pot almost does no effect. I think the R6 affects to volume and tone, not only gain.

    I´m trying to get more clean headroom to my Pro Jun from '94, i've changed the pre tube from ATX7 to a 12AT7 and it works a little, but I don't know what more can I do. Don´t know if 12AU7 has lower gain than 12AT7.

    I will appreciate any help, thanks.


    6 years ago on Introduction

    Great info on moding the pro junior ,just a thing ,what watt soldering iron would recomend ?
    many thanks


    6 years ago on Introduction

    This a great site matey .just got yesterday a pro junior relic tweede ,just fitted a greenback done the nf mod and some new tubes ,what does the 47 uf cap do ?any is a good idea to replace the caps with orange drops?

    thanks in advance


    6 years ago on Introduction

    Thank you so much for the great post. This is an excellent jumping point for modding these wonderful amps. I have been looking at this page as well as a few others for awhile but just yesterday actually tried a a few mods. I twisted the green wires on lunch yesterday and I cannot express what a difference this made! There is still a bit of hum but I'd say its about 80-90% gone.  Everyone SHOULD really do this mod. It took my 20 mins from start to finish.

    I also replaced the R-29 resistor.  Of course this doesn't affect the tone or hum but should make the tubes last longer.  This was quite a bit more difficult as it involves removing the amp as well pulling out the main board. I found that taking close up pictures of your board before removing it helped a lot.  I was able to unhook wire then just refer to those pictures to hook them back up.  I also found that taking off the board that holds the tubes helped me to remove the main board.

    This weekend I'm going to give the negative feedback mod a try.  Thanks again for the great article!

    Gwyn G

    7 years ago on Introduction

    HI ,this is Gwyn writing.I owned (and repaired) a great many amps in my lifetime. Still have lots of amps Fenders,Marshalls, Vox, Lazy J to name but a few,but I now use a Pro Junior a lot of the time.I think it is a GREAT AMP, light, portable, and with a sweet sound.What do the rest of you think? Regards to everyone,Gwyn Davies.