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.

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<p>Hi, I just did all of the electronics<br>mods today, all worked great!!</p><p>Thanks so much for the info!</p><p>I'm doing the baffle panel replacement<br>tomorrow, picking up a piece of Baltic Birch plywood. I might try<br>building one with the speaker offset from centre so I can possibly<br>install a 12&rdquo; speaker instead. Will have to check to make sure the<br>magnet doesn't hit the tubes...</p><p>A few custom touches...</p><p>I decided to make the feedback on/off<br>control (Step 9) and headroom mods (Step 8) controllable from the top<br>main control panel. </p><p>So for the feedback loop control I<br>carefully installed the (small) feedback on/off switch right between <br>the volume and tone knobs. </p><p>And in my Junior III the headroom<br>resistor R6 is already 20k, so to get the full range available I<br>replaced the 20k with a 10k and connected it in series with a 50k<br>pot, which I installed in place of the power light. (I will relocate<br>the on/off light to the new baffle I am building... might even make<br>it a whole LED strip for a 'surround glow' on the front panel...<br>cool!)</p><p>This way I can play away with the<br>feedback loop on/off and full range of headroom controls, right from the<br>main panel.</p><p>So far the results are great! The feedback loop control is subtle but noticeable, the headroom control range is huge and very useful.</p><p>Also added new tubes (Northern Electric, best available) and next to try out a few different 10&rdquo;<br>(Jensen C10Q, Ragin' Cajun and Tone Tubby 40/40) and maybe 12&rdquo;<br>speakers, and I've got a killer little amp.</p><p>Thanks SO much for the inspiration and<br>information! </p><p>Regards</p><p>Lou</p>
<p>Can you drill the hole without removing the 2 pcb ?</p>
I suggest that you also add some clear silicone caulking between the two caps and between the caps and pcb. Just a 1/4&quot; bead will do. This will keep the caps from vibrating and possibly loosening the solder joints.
That's a good idea, EVERYONE, DO THAT!
<p>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!</p>
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?
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. <br> <br>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.
Thanks for posting this. I just got a pro jr. If I lower the resistor to get more clean headroom will the volume drop?
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. <br> <br>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. <br> <br>I&acute;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&acute;t know if 12AU7 has lower gain than 12AT7. <br> <br>I will appreciate any help, thanks.
Great info on moding the pro junior ,just a thing ,what watt soldering iron would recomend ? <br>many thanks
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? <br> <br>thanks in advance <br>damien
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.&nbsp; Everyone SHOULD really do this mod. It took my 20 mins from start to finish.<br> <br> I also replaced the R-29 resistor. &nbsp;Of course this&nbsp;doesn't&nbsp;affect the tone or hum but should make the tubes last longer. &nbsp;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. &nbsp;I was able to unhook wire then just refer to those pictures to hook them back up. &nbsp;I also found that taking off the board that holds the tubes helped me to remove the main board.<br> <br> This weekend I'm going to give the negative feedback mod a try. &nbsp;Thanks again for the great article!
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.
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.
Can i wire in a potentiometer for gain instead?
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<br><br>or<br><br>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.<br><br>Changing gain might lower/higher the volume a bit<br><br>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.
Did the purple wire (NFB loop) mod with the switch. Turned out great. Just building a custom cab with a 12&quot; Cannabis Rex (Eminence). Looking forward to seeing how it all sounds together. Great job putting this all together.<br><br>JC
Great! send us pics when done!
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!<br><br>P.S.- Any leads on a good place to buy a cheap or used Pro Junior (besides: Ebay/CraigsList)?
Thanks, glad to see that someone appreciates!<br><br>To get a pro junior, you can always look on forums like tdpri...
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 <br> <br>Dan
When you added the 47uF cap, did you solder the leads of one cap to the other?
Finally! someone made a comment and asked a question!<br>I was sure nobody cared!<br>precisely, wrap the leads over the ones of the cap in place, make sure it the same polarity, and solder.<br> Good Luck and if you have more questions, ASK!<br>let me know how it turns out, seriously.

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Bio: 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 ... More »
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