Pro Co Rat Bass mods as my first undertaking...

[username]

Maybe someone with some electronics know how can help me. I've searched around for quite some time to find some detail on mods to the Rat to better suit bass guitar and haven't found out much. I've switched the IC on mine to a National LM308N. I like the tone better, but I have a huge reduction in volume. I now have to have the volume cranked almost full blast for a balanced volume between effect on/off. Is this a brand issue with my IC chip? Are there better LM308N chips out there than National? Also, I really want to find a way to allow more bass frequencies through the mix, post effect. I've seen something posted about changing a couple of cap values, but I am an absolute layman when it comes to electronics and have no idea what I am looking at. After doing some research, I'm starting to better understand what I am looking at. I saw a mod that replaced the (post effect?) cap with a 1k trim pot, supposedly to adjust the frequency band that is dumped to ground. Is this what I need to dial in more bass frequency, post effect? I have a picture of the layout of my pedal. Can someone explain to me in simple terms what I can do the help beef up the sound of my Rat? I have zero electronics background, so go easy on me, please. This IC chip swap was my first electronics undertaking. Also, would adding a clean blend be as simple as running two wires, one wire directly from the input and one wire at the end of the signal path to a blend pot running to the output, or is it more complicated than that?
Cheers!



If anyone is curious how the LM308N chip sounds on bass, check this video out. I use it a lot. Also a good way to plug my band.
http://www.youtube.com/watch?v=J45ypxlD8PI
Lots of Rat at about 40:17

[GGBB]

You shouldn't get a huge drop in volume from the op-amp change - something probably happened to the LM308 or the circuit when you did the mod.  Check your soldering.

The most common RAT mod for enhancing bass is probably the Ruetz Rat mod (http://www.diyguitaramp.com/rat.html).  This has the option of using a 1K trimpot for adjusting the bass response, but the basic version of the mod can be accomplished with a simple snip using wire cutters.  The 47-ohm resistor mentioned in the article will be labeled R6 on your RAT and is the one right beside C7 below the distortion pot.

Other bass mods would including raising the values of the C3 input cap (let more bass in) and/or the C13 output cap (let more bass out), but from what I've read those don't accomplish much in the RAT.

[username]

Thanks! I was going to try the Reutz mod out and add a trim pot on the 47 ohm resistor and the 560 ohm resistor. So, if I simply remove the 47 ohm resistor, it should let the whole frequency band through? I can always replace it with a pot I suppose. I put an IC cradle in today and popped the LM308 I have back into that and the volume drop disappeared. I must have had a bad solder point. I'm waiting for a metal can style LM308 to come in to give that a try next. What should I raise the value to with the C3 and C13 caps? I may as well give it a try. If it doesn't do much it won't really hurt will it? I wanna try and add the extra clipping circuits as well. The MOSFET and LED circuits. I still can't find any info on a adding a clean blend. I'm not even sure if the stock RAT housing will hold all of this stuff I want to add. I hope so.

[username]

Ok, so I pulled out the 47 ohm resistor and got a significant bass boost... However, I've now lost all of my upper mids and highs and no longer have that bitey RAT sound anymore. Would replacing that resistor with the pot, a'la Reutz mod give me a happy medium between the two? I don't want to lose my clanky tone, but I want to add a bit more bass. Maybe the output cap swap or the clean blend is what I'm looking for?

[username]

For the video I posted in the OP, I was using a friends Vintage RI RAT from around '92 I think. I loved the tone over mine. What's the main difference between that and my late 90's/Early 00's RAT 2 aside from the LM308 chipset? Even after swapping the IC, his still sounds a little bit better.

[GGBB]

Technically what the Ruetz mod does is it affects the frequency range of the gain section.  It doesn't really "let the whole frequency band through."  The Rat processes the whole audible frequency range because the input and output sections don't filter out anything audible.  The Ruetz Rat page explains it far better than I can: the R6+C7/R7+C6 network is a high pass filter in the feedback loop of the op-amp, and by modifying this you change the gain characteristics and which frequencies get amplified and distorted.  This section affects the tonal balance of the Rat more than anything else. The 47ohm+2.2uF (R6+C7) piece filters out more bass than the 560ohm+4.7uF, so by removing it you have more bass frequencies being amplified, but because of the larger resistor, the whole shebang is amplified less.  Changing the 47ohm to a higher values, such as with a 1K trimpot, will reduce its impact rather than remove it entirely, by lowering the gain applied to the higher frequencies.

As far as input and output cap mods go, I really wouldn't bother.  A stock Rat is set up to let everything in and everything out already.  You will get far more mileage out of the Ruetz mod.  Feel free to experiment with the values for R6, C7, R7, C6.  As a general rule of thumb - higher cap == more bass, higher resistance == less gain for that cap's range, and these two things interact with each other so that for example a bigger cap along with a bigger resistor will have more extended bass but at lower gain.  Another variation on the Ruetz mod is to replace both resistors with a single 500ohm (or 1K) trimpot - lugs 1 & 3 go to each of the caps, and lug 2 connects to the other side of where the resistors were (which is connected to lug 3 of the distortion pot).  This gives you the ability to raise gain for one cap while simultaneously lowering gain for the other cap, in either direction.

The only other mods to consider would be filter mods, which won't give you more bass, they will just filter out more treble giving you more apparent bass - they really just warm up or dull the sound.  Try a 4.7 or higher at C11, or a .022 or .033 at C4.

[GGBB]

For the video I posted in the OP, I was using a friends Vintage RI RAT from around '92 I think. I loved the tone over mine. What's the main difference between that and my late 90's/Early 00's RAT 2 aside from the LM308 chipset? Even after swapping the IC, his still sounds a little bit better.

I have both and honestly hear very little difference, but would agree that I like my VR a tiny bit better, primarily at lower distortion settings.  In my case my RAT2 is older (1992) than my Vintage RAT (2000).  There are technical differences between the two circuits which are entirely related to the LED indicator, but IMO they don't truly affect the sound.  The only thing that may be a factor as far as the circuit is concerned is the fact that the RAT2 has lower output impedance as a result of the LED addition and therefore will interact differently with whatever is connected after the pedal.  I personally suspect it mainly comes down to the fact that no two Rats (or any pedal) are the same because of component tolerances - it may just be that having the distortion and filter controls in slightly different places are required to have "equal" settings.

If you are interested in the technical details, I put this comparison together a while back:

http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/Multi-RAT.png

Note that the C5 and C7 labels are mixed up (I really should fix this).

The only other thing I will mention is that some RATs have factory mistakes - my friend's Vintage RAT had a 30pF cap instead of 100pF for C5, and had BAT41 Schottky diodes instead of 1N4148s.  It sounded noticeably different!

[username]

Well, I want to retain the high end of the spectrum, and completely removing the r6 resistor seems like it killed the tone of the pedal, if only by boosting the bass so much that it overwhelms the highs. I'm really a beginner with electronics, so I'm not sure the exact mechanism that is going on. All I need is a tiny bit of a bass boost to make the switch from clean to distorted less apparent in the lower end of the spectrum. It sounds like using a single trim pot for both resistors to dial in a little less resistance may be the way to go, or would leaving the r7 resistor in place and adding a trim pot to r6 to dial in a little less resistance on that specific one be the way to go? If I do blend the two in the with the same pot, it sounds like you are suggesting to run the positives from each path into lugs 1 and 3 and then run two wires off of lug 2 into the negatives of each path. Am I correct? The positives and negatives of each resistor are flipped on each path, so one is running one direction and the other the opposite, so I'm not running lug 1 and 3 on the same side, but criss-crossing them right? Also, how did your friends VR Rat sound with different caps and diodes? Better or worse?

:EDIT:

I took the 560 ohm resistor out and put the 47 ohm back in and it sounded pretty damn cool! Really really bitey and scratchy. After putting the 560 back in to compare, I think I may like it more. I really need to install two pots I think, so I can dial each one exactly to where I want it. I should be able to phase each from all to nothing right?

[GGBB]

If I do blend the two in the with the same pot, it sounds like you are suggesting to run the positives from each path into lugs 1 and 3 and then run two wires off of lug 2 into the negatives of each path. Am I correct? The positives and negatives of each resistor are flipped on each path, so one is running one direction and the other the opposite, so I'm not running lug 1 and 3 on the same side, but criss-crossing them right?

No - one end of each of the two resistors are connected to each other (as well as to lug 3 of the distortion pot and C5 and pin 2 of the op-amp).  The other ends of the resistors are connected to their respective capacitors, which in turn are connected to ground.  Trimpot lugs 1 & 3 get connected to the caps, and lug 2 gets to the other side of the resistor pair.  Lug 2 only needs one wire to either of the two resistor top positions since on the PCB they are connected together.  Here's a picture:

I took the 560 ohm resistor out and put the 47 ohm back in and it sounded pretty damn cool! Really really bitey and scratchy. After putting the 560 back in to compare, I think I may like it more. I really need to install two pots I think, so I can dial each one exactly to where I want it. I should be able to phase each from all to nothing right?

Basically yes, but you will have to experiment to find what trimpot size you will need for each resistor.  1K should work fine for the 46ohm side, but I'm not sure it will be big enough to have an huge effect on the 560ohm side.  My guess would be 20-25K for that side.

Also, how did your friends VR Rat sound with different caps and diodes? Better or worse?

From a RAT sound perspective it was worse - kind of screechy overall and bright.  Interesting in some ways - very nasty in-your-face kind of distortion that some people might like, just not really the true RAT sound.  I actually owned the RAT at the time - my friend sold it to me because he didn't like it.  When I figured out that it was a mutant and fixed it, it sounded better so he bought it back (I had another VR so I didn't need it anyway.)

[GGBB]

Basically yes, but you will have to experiment to find what trimpot size you will need for each resistor.  1K should work fine for the 46ohm side, but I'm not sure it will be big enough to have an huge effect on the 560ohm side.  My guess would be 20-25K for that side.

Never mind that - I wasn't thinking straight. 1K should be fine for the 560ohm.

[username]

Cool. I'm basically wanting to add the Reutz, Metal Can, and Overdrive circuit from this pic. http://www.beavisaudio.com/Projects/FKR/MightierMouse.gif
Hopefully, I have enough room in the RAT2 housing for the overdrive circuit... Haven't quite figured out what to do about that yet. The OD circuit is basically what the Keeley mod does, right?  Where would I solder into the RAT PCB with the OD circuit? Do I have to actually CREATE a space for it? The pic is from the BYOC RAT clone.

[GGBB]

I've never seen a schematic for the actual Keeley mod so I don't know for certain what it involves.  But based on the description in the Keeley Rat Mod manual, it sounds to me like what he does is he adds two alternatives to the existing diode clipping setup.  So that's not the same as the OD mod.

For the OD mod connections, just use lugs 1 & 3 of the distortion pot - which side is which doesn't matter.

The toughest thing about modding a RAT2 is drilling through that thick steel chassis.  There's not much room on the front face of the RAT2 for adding anything so the back is the easiest place to add a switch.  Google for Keeley RAT pics for ideas.

[username]

From what I've been reading from an old dead thread on here, the positive side of the OD circuit attaches at the side that the existing diodes meet at and the negative goes to ground. What I'm not clear on is if solder wires on top of the existing connections, or take the diodes out completely. I may just replace the stock diodes with LEDs, but I'm not sure which way to face them.  I'd like to have a toggle between the Ge diodes, LEDs, and the MOSFET setup, but I'm still having a little trouble wrapping my head around the setup of the MOSFET array in that pic above.

[GGBB]

You are mixing up two different mods.  Follow the Beavis instructions here:

http://www.beavisaudio.com/Projects/FKR/BAR_FKR_Mark_2_Build_Documentation.pdf

Note the 3 different mods listed on the first page and compare to the schematic on the second page.  The clipping diode mod is #1 and "attaches at the side that the existing diodes meet at and the negative goes to ground."  The OD mod is #2.  This is labeled as "Feedback Clipping" on the schematic diagram and attaches to lugs 1 & 3 of the Distortion/Drive pot.

For the first mod, if you are only doing 1 switch and three clipping modes, you need to have two LEDs in the stock RAT circuit.  You can connect the two LEDs either to the RAT PCB or to the center lug on the switch.  Otherwise when you switch the other clipping options in they won't work.  That's because an on-off-on switch in the middle position doesn't connect anything.  LEDs clip at a higher signal level (larger forward voltage), so they can be left in the circuit and the other clippers running in parallel will clip first so that the signal never reaches the LED's clipping threshold.  If you try to "switch in" LED clippers parallel to Si or GE or Mosfet clippers, these will still clip first and the sound will not change.  The Beavis pedal does not use any diodes in the off position of the main switch so it works differently - all the clipping options are connected to switches.

[username]

Ok GGBB, another question for ya.   I did the blend pot for the 47 and 560 resistors, but it doesn't blend. It's all or nothing. One direction is all 47, middle is both, other is all 560. I'm not sure how two separate pots would work, but I don't want to route holes in my housing if they won't work properly. I think I just want to set it on a DIP switch and toggle between the three settings. How would I set that up with an on-off-on switch, or would I need a dial selector?

BTW, I'm completely sold on LED clipping after replacing the stock Si diodes! The only drawback is feedback. We have a song where I put my whammy in up octave and open my wah all the way up and strike some harmonics. If I do that now, the squeal overwhelms the notes. Not sure what I'm going to do about that...

[GGBB]

Ok GGBB, another question for ya.   I did the blend pot for the 47 and 560 resistors, but it doesn't blend. It's all or nothing. One direction is all 47, middle is both, other is all 560. I'm not sure how two separate pots would work, but I don't want to route holes in my housing if they won't work properly.

What value pot did you use?  Was it linear or audio?  You could also just try a 1K pot in place of the 560ohm resistor and leave the 47ohm as is.

I think I just want to set it on a DIP switch and toggle between the three settings. How would I set that up with an on-off-on switch, or would I need a dial selector?

You'll need an on-on-on switch - either SPDT (if these are made) or DPDT (which are only partly center on - check the wiring diagram).  These are a bit harder to find than the on-off-on or on-on varieties.  basically you pull the two resistors at the end where they connect together (and leave the other ends in the PCB) and connect them to the switch's outside terminals.  The middle of the switch goes back to the PCB.  You could also just use two separate on/off switches.

BTW, I'm completely sold on LED clipping after replacing the stock Si diodes!

That's the Turbo RAT sound.

The only drawback is feedback. We have a song where I put my whammy in up octave and open my wah all the way up and strike some harmonics. If I do that now, the squeal overwhelms the notes. Not sure what I'm going to do about that...

Move away from the amp?

[username]

What value pot did you use?  Was it linear or audio?  You could also just try a 1K pot in place of the 560ohm resistor and leave the 47ohm as is.

I used a 1k linear pot. Wrong one, I'm guessing?

Move away from the amp?

:|

[GGBB]

I used a 1k linear pot. Wrong one, I'm guessing?

Not wrong, but not ideal.  The problem with mucking around in there is that if you change the values you change the gain of the op-amp, which changes the nature of the distortion - it's not just a matter of more or less bass.  That's why it sounds so different if you just clip one side out or another.  With 1K in there, at the position where the balance between the two filters is the same as the original setup (mathematically that would be 1000*47/(47+560)=77.4 on one side and 1000*560/(560+47)=922.6 on the other), you have almost double the amount of resistance on each side so you get more bass but less gain.  Ideally, what you want is to keep the overall resistance the same but have a pot to adjust the balance.  So a 500ohm would work better - you could have it tied to the existing 47 on the one side, and substitute a 60ohm (56) for the 560 on the other side.  With that set up, one side of the pot is the original sound, and adjusting the pot gives you the ability to enhance the bass by bringing up the level of the 560 side while lowering the level of the brighter 47 side.  But that may be too much adjustment for your tastes so you might want to experiment with even smaller values like 250 or 100 along with a suitably higher R value in place of the 560 (you generally never want to go less than 47 on that side since it would make things even brighter).  The smaller the pot, the better the feel of the adjustment it will make, or if the pot is sensitive at one end only, use an audio taper.  This ends up being a lot of fiddling to get it the way you like it, so it is a lot easier to just replace either the 47 or the 560 with a 1K pot and adjust bass as you like it.

[username]

So, I ended up just replacing both resistors with 1k trim pots. It doesn't transition through the range very smoothly either. Should I be using audio trim pots instead of linear trims? By the way, thank you very much for your help so far! I've learned quite a bit from this thread, but I'm still extremely green.   As a complete n00b with electronics, is there a handy resource that you can point me to so I can learn the pathway of this, or any, distortion circuit, so I can better understand how it runs, why it runs the way it does, and what each component does along the way? If I could learn the total operation, I could probably figure most of this out myself. I'm going to run out of space in the RAT's box. I want to add a three way selector to the clipping section to switch between Silicon, Germanium, and LED. Of course, I don't know what all this entails. I also want route those board mounted pots to external trim pots on the outside of the box. Will all of this even be possible, given the enclosure?

[username]

This is what my board looks like now.



Since I have swapped the Si diodes for LEDs, should I add a resistor to R16 now and what would that accomplish if anything?