Picture of Mod your Fender Pro-Jr!
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!


Can you drill the hole without removing the 2 pcb ?

r3son8tr4 years ago
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.
plpicher (author)  r3son8tr4 years ago
That's a good idea, EVERYONE, DO THAT!

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!

esosa41 year ago
esosa41 year 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?
mbañón2 years ago
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.
jgorman14 years ago
Thanks for posting this. I just got a pro jr. If I lower the resistor to get more clean headroom will the volume drop?
plpicher (author)  jgorman14 years ago
Thanks to let me know it's useful no, the volume stays as high.
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.
gibsondog2 years ago
Great info on moding the pro junior ,just a thing ,what watt soldering iron would recomend ?
many thanks
gibsondog2 years ago
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
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 G3 years ago
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.
Gwyn G3 years ago
Hi there.I have just joined up.I am an old amp repair man and builder,with over 50 years fixing electronic equipment,I also play guitar and ukelele,I live in North Wales in the U.K.this looks like a great site! Take care you all,Regards,Gwyn Davies.
drewfunk3 years ago
Can i wire in a potentiometer for gain instead?
plpicher (author)  drewfunk3 years ago
Of course! You could throw in a 100K linear pot wired as a variable resistor but at max Gain it might sound a bit muddy, and at minimum gain it might sound brittle, full CW and full CCW might not sound very nice


You could put in a 70k linear pot in series with a 10k resistor which would make all positions on the pot useful. I'd go with that option.

Changing gain might lower/higher the volume a bit

please, Tell us how it turns out!
Awesome, i was thinking a 100k pot would be to low/high when maxed either direction, but your idea sounds perfect! I'm picking up a Pro Jr this week, cant wait.
jcox144 years ago
Did the purple wire (NFB loop) mod with the switch. Turned out great. Just building a custom cab with a 12" Cannabis Rex (Eminence). Looking forward to seeing how it all sounds together. Great job putting this all together.

plpicher (author)  jcox144 years ago
Great! send us pics when done!
tdoakes4 years ago
Really like this Instructable and appreciate all the work you put in and all the photographs. I don't have a Pro Junior (yet!), but I'm interested in getting one and moding it out and this will be a welcome resource when I finally do. Thanks for posting it!

P.S.- Any leads on a good place to buy a cheap or used Pro Junior (besides: Ebay/CraigsList)?
plpicher (author)  tdoakes4 years ago
Thanks, glad to see that someone appreciates!

To get a pro junior, you can always look on forums like tdpri...
danorama4 years ago
Great instructable! I'm picking up a '94 Tweed Pro Junior with Alnico speaker today. I expect I'll be doing some of these mods shortly

lucian10544 years ago
When you added the 47uF cap, did you solder the leads of one cap to the other?
plpicher (author)  lucian10544 years ago
Finally! someone made a comment and asked a question!
I was sure nobody cared!
precisely, wrap the leads over the ones of the cap in place, make sure it the same polarity, and solder.
Good Luck and if you have more questions, ASK!
let me know how it turns out, seriously.
cap solder zoom.jpg