Questions about Rat II mods (and hello!)

[zarathustra]

Hey guys! I've been using this forum as a resource for quite a while, but now I finally have some questions. I've done a few smaller builds and mods (feedback loop, a few TB mods), and I've decided for my next project I'd like to mod my Rat. It's a Rat II from 1996, with the LM308 chip. I know there are a ton of questions about Rat mods in this forum, and I've read most of them, but these mods aren't nearly as straightforward as, say, a TB mod, so I'd just like to get some feedback before I start tearing this pedal apart.

Okay, so here's what I'd like to get out of my Rat, in order of priority:
1. Get rid of the thin, papery harshness that never seems to disappear
2. Tighten up the distortion, make it thicker and creamier, not so loose and floppy sounding (I suspect this is related to #1)
3. Increase the bass response
4. Make the filter knob so it actually does something... stock, it's like a cold fish, not very usable
5. Maybe increase the gain... I'd love to get crazy gain out of this, but I'd also like to retain the ability to use it as a semi-clean boost. At the very least, I don't want the mods to diminish the gain beyond stock levels.

From what I've read on this forum (and other sites), here's what I have so far: (and here's a pic of my Rat's board)
- Caps: I understand that higher-valued caps will increase bass response, but I don't know which caps in particular I should target. Which caps should I swap for best results? There appear to be 10 caps in there. Aside from bass response, what other caps should I play around with to get the results I'm looking for?
- Diodes: I'll probably just install a couple of sockets so I can switch out diodes until I find a pair I like. From what I've read, Ge diodes will be fuzzier, while LEDs will be crunchier. Maybe one of each would yield a good tone? Any suggestions here?
- Op Amp: I already have the LM308, but I'll probably install a socket for that so I can try out a few others. Any suggestions for op amps that will help me get the sound I'm looking for? I've got a decent selection on-hand.
- Resistors: not sure about these... anything I need to worry about here?
- Gain: if I want to increase the available gain, is that just a matter of installing a higher-valued pot? Or maybe I could just wire a resistor (say, 100k) in series with an SPDT switch?
- Filter knob: not sure how to deal with this one... any ideas?

Thanks for the help! Sorry about the long-winded first post. I'm really enjoying this whole modding thing. I'm a lowly grad student, and I spend all day writing my dissertation, so it's nice to be able to use my hands for something other than typing.

[zarathustra]

Nobody, eh? Alright, I guess I'll just have to start throwing caps and op amps at this thing and hope something works!

[amz-fx]

Hello, and welcome!

See the electrolytic caps at the bottom center of the picture?  One should be 2.2uF and the other 4.7uF. 

Increase the size of either, or both, will give you more bass.  Experiment and post your results! 

http://www.muzique.com/fx/

regards, Jack

[zarathustra]

Cool, I'll give that a shot. Thanks!

Btw, I noticed the Rat e-book in your link. You think that'd help me get the sound I'm looking for? Would it work for my version of the Rat II?

[WGTP]

Welcome.  Have you tried searching?  There have been several discussions this year about the infamous Rat and the 2.2/4.7uf caps.   

[PenPen]

I did a lot of experimenting when building a Rat clone myself. You can search for posts by me containing the word rat, that may help. Otherwise, listen to amz, yes his book is awesome, and those caps will help out a lot. Most of the tone of the Rat is the frequency response from the opamp feedback loop. I wanted a bassier response also, so in mine I changed the feedback loop caps, the resistor before them, and also the cap that bypasses the pot. Upping the value of the bypass cap slightly will lower the gain at higher frequencies, and changing out the caps to ground after that will increase the bass response. Again, if you want to go this route search for my posts on the subject.

Diodes, the normal clipping is two silicon diodes, I used one silicon and one LED for some assemetric clipping, I love the sound of it.

[dosmun]

LED's in the feedback loop are very cool.

Changing the Tone pot to Linear Taper is an improvement.

[Mark Hammer]

There ought to be a 47R resistor in series with a 2.2uf cap.  This pair of components is responsible for providing tons more gain for content above roughly 1.5khz.  If you increase that 47R resistor to 100R, that point of emphasis drops down to roughly 720hz, and also results in comparatively less gain for the top end than existed stock.  In principle, that ought to result in more of a "meaty" and less harsh sound, by shifting the balance in drive a bit downwards.

Alternatively, or maybe in tandem, consider changing the 100pf cap in the feedback loop of the LM308 for something higher in value.  Stock, at maximum gain, that cap produces a rolloff around 16khz.  Double that cap to 200pf (i.e., place a second 100pf in parallel) and that rolloff point drops down to 8khz at max gain, which ought to "round off" the tone a bit.  A cap of 220pf would make that 7.2khz.

Finally, the tone control will likely appear to have even less impact with such changes, so consider changing the .0033 filter/tone cap for .0039 or even .0047.

All these things should warm up the pedal so that it sounds thicker, meater, rounder, and less strident.

[Peter Snowberg]

Welcome to the forum. 

[zarathustra]

Awsome. That's very helpful stuff. Thanks, guys.

I hope to get started this weekend, so I'll let you know how it comes out.

[zarathustra]

I finally managed to finish up this project. I ended up doing a few minor mods--e.g., increased a few cap values, socketed the opamp, added a resistor to the gain pot to give it a little higher "gain floor," etc.--and a few more difficult ones (for me, at least).

- replaced both the 47R and 560R resistors with 1k pots. I wasn't sure if substituting a pot for the 560R resistor would make much of a difference, but it really does. It behaves similarly to the Ruetz mod pot but seems to dial in some unique tonal characteristics. The two pots also seem to interact with one another a bit, which is fun to play around with.

- added a 3-way diode switching array to go between LEDs, MOSFETs, and a Ge/Si pair.

- added MOSFETs and LEDs to the feedback loop (with a switch)... just might be my favorite mod. LEDs in the loop plus the Ge/Si pair to ground sounds amazing.

I'm very, very happy with the results. This pedal now sounds awesome through my VT-22. I wish I could make some clips, but my recording gear is all stashed away, at the moment. Here's a picture of the new switches and knobs:



So now the downside. I've got two bugs. First, as I mentioned in this thread, if I'm using the LEDs (to ground) and crank up the gain, the signal starts to act funny; it kind of fades out right after the pick attack, then quickly fades back in. To my neophyte ears, it sounds like something is "overloading" or can't handle the gain. (Before modding the pedal, I'd get the same results if I tried to push the Rat with another gain pedal.) Any ideas on this one? I just can't figure it out, and I'd really like to crank up the gain on those LEDs.

Second, after soldering everything together, I noticed that the LEDs-to-ground don't light up as brightly as the LEDs in the feedback loop. Is that normal? I seem to recall them being equally bright when I was testing out diode pairs, and the LEDs-to-ground just don't seem to sound as good as I remember from my tests (but that could just be my imagination). Is there any reason why LEDs might not light up all the way or not fully clip? The two pairs might be different brands... maybe that's the problem?

Thanks for all the help, guys. I wouldn't have been able to do these mods without it.