Modifying Princeton 112 plus - Make Dirty Channel Versatile Clean Channel

[DrGonz78]

Yet another question about modifying a Fender Princeton 112 plus... I got to thinking that the clean channel is not very versatile since it has no mid control and only has bass & treble. So I got to thinking about removing the clipping circuit from the dirty channel and using it as a very versatile clean channel. Could this even work? Now there are six components that run in parallel from the U2b TL072 opamp between pin 6 & 7. R25, R26, LD1, LD2, CR1, CR2 are all in the the clipping circuit and just removing them makes me feel that the expected impedance of the signal would be greatly effected through the channel.

I am sure that the LD1 & LD2 are the clipping diodes but what is the function of CR1 & CR2(more clipping is my guess). If I remove these from circuit do I still need the resistors in place? I know modifying a circuit board design is not always a workable idea. I have brainstormed this idea with a friend who owns the amp and we crunched some numbers, but we were way off. Calculating it now the 470k (R25) and 220k (R26) in the parallel circuit would equal 150k (approx.) of overall resistance. I am really not sure what I should do to modify the circuit to remove the clipping diodes and still have the right impedance. Can anyone help steer me in the right direction here or give any advice as to if this idea will even work?

http://support.fender.com/schematics..._schematic.pdf

[Enzo]

The stage may clip, but it is more than just that. You have the amp? Play through it and watch those LEDs for when they light up. When they stay dark, they are not clipping much of anything. But it is still an amplifying stage, those resistors set the gain of the stage, you can't just lop them off.

Look at the circuit. U1a is common to both paths. The clean channel is the tone stack and U1b. The dirt channel, even without the clipping is two stages of U2, all that tone shaping from R18 through R21, the contour filter, and the different tone controls. You may like the alternate tone controls, the shaping could be, well, shaped or eliminated. If you ditch the clippers, I don;t know, but they would no longer be there to limit signal amplitude, it might make it overly gainy, then again maybe not. Lift your diodes and see.

I wouldn;t worry about impedance.


You also could replace R10 with a pot and have yourself a mids control on the clean.

The little signal diodes CR1,2 I am sure are just 1N4448, with a roughly half a volt drop. The LEDs have a higher voltage drop, and different colors have different voltages. This is not my area, but if you put blue ones in there it might allow a larger signal before clipping.

[mozwell]

DrGonz
I woudl try breaking the connection between U2B pin 7 & the leds / R25, add a switch, and a resistor as shown. You may need to tweak the 10k resistor to get a suitable volume level, i woudl guess that 10k is a reasonable starting point.
Thsi way you can still select "dirty" by using the switch, the switch is manual however, so sort of defeats the purpose of having channel switching & changing the dirty preamp to a clean preamp

CR1, CR2 are normal clipping diodes, the same as LD1, LD2, with R26 in there, they are trying to have some sort of "dual slope" clipping, i guess to make it sound more squashy & compressed, and more valve like..... If you like the overdrive sound, then adding the switch still gives you the option of having that channel either clean / dirty with the added switch......

[DrGonz78]

Thanks guys for the great replies on this matter. I have yet to even pull the chassis out at this point and I am really just brainstorming the idea first. You guys really have given some cool ideas to try and I will let ya'll know how it turns out.

I wouldn't worry about impedance.

Well My friend was interested a lot about impedance and was curious why we are not concerned so much in this case? Probably cuz there are so many other factors to worry about first, right? I guess we are both curious to learn more on this matter too.

You also could replace R10 with a pot and have yourself a mids control on the clean.

This is a great idea too!! Might have to include it on the list of things to add to this amp. Thanks Enzo!!

I woudl try breaking the connection between U2B pin 7 & the leds / R25, add a switch, and a resistor as shown. You may need to tweak the 10k resistor to get a suitable volume level, i woudl guess that 10k is a reasonable starting point.

I really like this idea here Mozwell. Not important to be able switch with a pedal in this mode and manual switching will be perfect. This should be very interesting where I end up on this project. Keep ya'll posted! Thanks again!

[DrGonz78]

Well I am in the testing phase now of this mod. I have bypassed the clipping circuit and opted to put 147k resistor in place where the circuit used to be, as a 150k resistance was already there to begin with... So yes channel 2 is now too gainy as Enzo had said it might be and the tone of the circuit needs to be reworked a bit. Tone sculpting between R18-R21 would be my first bet to help work out the channels overly high end tone. Also, how (or should) could I use negative feedback to reduce the gain on the channel. Any ideas?

Also, later I might work on other details to fine tune the channel to my liking. Also, gonna change the clean channel volume pot from the 50k30c to a 50k15A pot. Thanks for the great feedback thus far! Also eventually gonna try out the mid pot addition on the clean channel as Enzo had said... Probably not gonna have clipping circuit in the amp anymore as once I am done modifying the circuit it might not be usable as the original circuit it once was.

[DrGonz78]

I think this was illustrated by Steve A. It shows the constructs that make up the dirty channel on this amp. I used this document to alter the tone of the second channel. Especially since I took out the clipper diodes which is where the distortion is created. So just a really loud 2nd clean channel which needed a huge gain reduction to find the voice of the tone. After my mod on this amp it was played on stage for countless hours without fail. Great mod for a jazz guitarist.

[glebert]

I've had some good luck with a different Fender solid state by reducing the gain a lot before it hit the clipping diodes. I wonder if you could do that with this circuit by reducing R25 and R26 by a factor of two or so. I like this approach because I'm lazy and I can reduce the resistor values by putting another resistor in parallel from the top side of the board just by looping around the mounted resistors legs. If I don't like it I can just cut the resistors off and I don't have to remove the board to do either.

edit: guess the point of the OP was to make it totally clean, I personally like the gain channel to be clean when the gain is all the way down and then get dirtier as you turn it up.