Hi All. Here is my Princeton Reverb build. Pretty much stock, but added a switcahble negative feedback loop in lieu of the ground switch. Soursoud PT and OT, Mojotone RT. Laser cut/engraged faceplate from purpleheart, with an epoxy resin fill. Cabinet by guitar cabinets direct. Vintage RCA 12AX7s and 6V6s, and Eminence GA-SC64 12" speaker. Knobs by knobheadpdx on Reverb. Thanks again for the great PCBs, Andrea!
Thanks Bret, will give it a try!
Hi Ronald-
I re-routed the green wire that normaly comes off J4 from the output jack to the NFB pad on the PCB over to a SPDT switch where the ground switch would normally be.
The wire from the jack goes to common. One side of that switch goes directly to the PCB, while the other half has a resistor in its path back to the PCB. They can share a common wire backājust connect the two halves of the switch in a V configuration to the return wire.
Be thoughtful about how you run all the return wire. Being that close to the power supply switch and fuse, I had concerns about it picking up and injecting more noise into the signal. Even though they are AC current and not as susceptible to picking up hum from the power supply, you should still keep it low and back against the chassis and to the PCB.
I can't remember what resistor value I put on the switch, to be honest. I would say set it up initially so you can swap out values. I want to say start out around 6k and adjust from there. This is where it is especially helpful to have a +/=1/2w resistor decade box.
You will also want to measure total resistance between the output jack and the other side of the connecting R14 2k7 resistor on the PCB to see what the total series resistance is with the switch, if that makes sense. It will definitely give you an earlier breakup, but it's not extreme, so YMMV. Best of luck!
Hi Bret,
How did you do the switcahble negative feedback loop? I finished my build but am looking for a bit earlier brake up.
Will this mod help and if so how do I connect the switch?
Thanks, Ronald