I recently got a PJ and while it sounded good, I wanted it quieter. I also wanted to have a little more control over the gain in the amp so I had a look at the schematic and popped open the amp to see how it was laid out. I fixed both the issues in my amp for $3 in parts and no PCB soldering. The whole mod process took me about 15 minutes including waiting for the soldering iron to get nice & hot.
1. Noise:
Turns our Fender in their infinite wisdom of R&D decided the PJ doesn't need the green heater wires twisted together and routed separated from signal wires in the chassis like every other amp on the planet. In fact, they're run un-twisted, in parallel and zip tied tightly to both power and signal wires. Duh, Fender. I started by cutting the zip ties and moving the rest of the wires away and laying those wires close to the PCB and front of the chassis. Then, holding the heater wires apart (still attached on both ends) I started twisting the pair together at both ends working my way to the center. Then I routed them first from the lower tube socket PCB along the rear edge of the chassis, crossing other wires at right angles and keeping them toward the rear panel (or where it will be when buttoned up) to the power transformer. I then pushed the other wires together and tucked toward the top of the chassis but didn't zip tie anything together. I powered the amp up and it's not dead silent, but at least 90% better than it was stock.
2. Gain
The purple wire soldered to the speaker jack is a negative feedback to give the amp more gritty volume. After discharging the caps I simply desoldered the NFB wire from the speaker jack, then drilled a 1/4" hole next to the speaker jack in the bottom of the chassis for a small DPDT toggle switch. I then resoldered the purple NFB wire to one end-pole of the switch and found a small piece of almost-matching purple wire from my junk bin and soldered the center lug on the switch back to the speaker jack where the original NFB wire came from. Now I have my choice of Blackface NFB type gain or tweed style gain with the flick of a switch that's out of sight behind the back panel. As a little addition to control a little more noise, I made sure the NFB wire crossed the red & white wires running between the 2 PCBs at a right angle.
Works perfect, has nearly no noise and I have an option of how snotty I need the amp for different gigs. Cost was only the switch, some solder , a piece of scrap wire and about 15 minutes.
I also use a 60s era GE 5751 in V1 & a 1962 Wurlitzer/RCA 12AX7 in V2. EL84s are not matched Ruskie something or another. I'll be trying out some late 50s Dutch Gold Star Motorola 6BQ5s in there that came in a radio chassis I salvaged on a recent trash day.
I find when I'm messing around in tube amps that wiring placement makes a HUGE difference in the noise floor. Even PCB amps can usually benefit from some decent routing of the off-board wiring, especially the heater wires, NFB (if present) and signal wires. Originally, the Pro Jr. had ALL the wires zip tied together in one big bunch. AC, NFB, speaker, signal, heaters - EVERYthing. Way to go Fender.
I haven't done a lot of messing around with the amp since I closed it up because we have relatives here for the weekend and I don't think they'd appreciate me plinking for hours going back & forth with different tubes, settings and other junk looking for the 'Holy Grail of Junior Tone'. I did plug it in, let it get hot the other night and play around with it a little (about 3 minutes) and found that between the heater wire mod (quieter) and the NFB switch (different gain voices) that it's surprisingly versatile for doing so little work on it compared to what it was stock.
A picture's worth a thousand word (but that never stopped me from blabbing). Here ya go!
http://s233.photobucket.com/albums/e...na/amps/projr/
Cheers,
- JJ
Cheers,
- JJ