# Rat mods



## hollowbody (Jan 15, 2008)

Hey everyone, I'm looking for some options in terms of Rat mods. I'm sort of comfortable with a soldering iron, but would like some schematics as well as help in where to look for parts, etc.

Some of the mod I'm interested in are adding a switch to switch between the different resistor/cap values in the gain section, etc.

I'm a noob when it comes to this, so bear with me.


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## Stratin2traynor (Sep 27, 2006)

I tried to get a link for you from the B.Y.O.C. Forum. There is a lot of talk about modding Rats. Check it out. I get the standard error message from the site. They must of changed something. Anyways, a few googles on your end should yield a number of sites that talk about modding the Rat ad nauseum. If I happen to latch on to one I'll post the link for you.


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## hollowbody (Jan 15, 2008)

Thanks! 

I saw a couple links about the Reutz mod, which seems like something I would be interested in doing, since I prefer the lower gain stuff anyway, but I don't necessarily wanna snip that resistor out entirely, which is why i want to add a switch. I've seen some of the guys on this forum putting up pics of rats with all kinds of switches, knobs and other doo-dads on them, so hopefully one of those dudes will point me in the right direction.


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## Ripper (Jul 1, 2006)

hollowbody said:


> Thanks!
> 
> I saw a couple links about the Reutz mod, which seems like something I would be interested in doing, since I prefer the lower gain stuff anyway, but I don't necessarily wanna snip that resistor out entirely, which is why i want to add a switch. I've seen some of the guys on this forum putting up pics of rats with all kinds of switches, knobs and other doo-dads on them, so hopefully one of those dudes will point me in the right direction.


I have the Reutz mod done on mine with a switch and it is great. Use a regular mini switch, lift one side of the resistor from the board and connect it to one side of the switch with a wire. Run a second wire from the other side of the switch to the spot on the board where you lifted the resistor from and viola! You can either have the Ruetz or have it stock.

My rat came with some allum mods which I tweaked to my own taste. It has a switch for the diodes, one side puts leds into the circuit which really really kicks up the gain, the center position removes all diodes from the circuit turning it into a fairly clean boost and instead 1n4002's in the third position I put 1n914's back in to give it the original rat sound. The chip was changed out to a LM308 like the originals and I tweaked a couple of caps to alter the tone stack a little. I then stuck in a super bright LED to change the appearance a little when it's turned on.


Any of the BYOC mods will work with a regular rat. YOu can find tons of schematics etc on the web. You just need to know exactly which board you have in your pedal


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## mhammer (Nov 30, 2007)

There are a ton of mods discussed on the DIYStompbox forum, and some fairly useful ones discussed over at Jack Orman's AMZ site: www.muzique.com.

The key things in the Rat are the use of a deliberately limited op-amp - the LM308 - and separate gain amounts for the lows and highs. One of the more interesting mods for a Rat is to change the degree of what I like to call "treble advantage", or change the frequency location of the fork in the road as regards where the higher gain is applied.

Another interesting mod is to use a dual-ganged pot for the filter control to achieve a steeper rolloff at the designated frequency.


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## hollowbody (Jan 15, 2008)

Ripper said:


> Use a regular mini switch, lift one side of the resistor from the board and connect it to one side of the switch with a wire. Run a second wire from the other side of the switch to the spot on the board where you lifted the resistor from and viola! You can either have the Ruetz or have it stock.


Brilliant! That's a pretty good starting point, I think I'll try that before I fiddle with changing caps, etc.

And yeah, I checked out the BYOC forum today and there's a lot of interesting options for toggle switches and stuff that I will have to look at once I have a better idea of what I'm doing.

thanks!


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## hollowbody (Jan 15, 2008)

mhammer said:


> The key things in the Rat are the use of a deliberately limited op-amp - the LM308 - and separate gain amounts for the lows and highs. One of the more interesting mods for a Rat is to change the degree of what I like to call "treble advantage", or change the frequency location of the fork in the road as regards where the higher gain is applied.


Gotcha, so how exactly would I go about changing the frequency? Is it as easy as just substituting a higher value cap after the resistor? Which resistor though, the 560 ohm or the 47 ohm?

Also, do you have suggestions for values?

I like my tone fairly snappy. I had a Big Muff for a while and found it too 'dark' and low-middy. So I'd be looking for something with more top-end.


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## mhammer (Nov 30, 2007)

You'll need a bit of math, but nothing more complicated than what your Windows calculator can do in "scientific" mode.

There are two parallel "legs" from the inverting pin of the 308 to ground. One is a 560R resistor and 4u7 cap, and the other is a 47R resistor and 2u2 cap. Following the formula F = 1 / [2 * pi * R * C] , where R is in megohms and C is in microfarads, we see that the 560R/4u7 pair provides a given amount of gain for content above 60hz. The 47R/2u2 pair provides even more gain for content above 1539hz.

The gain of that stage is determined by the combined feedback resistor (which can go as high as 100k, depending on Distortion pot setting) and ground-leg resistor, divided by the ground-leg resistor. So, for content from 60hz on up, the maximum gain is (100k+560R)/560R = 180. For the content from roughly 1.5khz and up, however, that maximum gain is calculated as 2128. Yep, over *two thousand*.

If either of those small resistor values are changed, the maximum gain as well as the point where that is introduced, changes. So, for example, making the 560R value 470R instead, changes the maximum gain for everything below 1539hz to x213, and the bass rolloff begins around 72hz instead of 60hz. Since guitar has little content in that range, you'd notice the extra gain more than the diminished bass. If the capacitor values are changed, the same gain applies as before but where it gets introduced changes. E.g., if a person swapped out the 2u2 cap for a 3u3 (the next common value up from 2u2), the low-end rolloff point where the extra gain kicks in would move from roughly 1.5khz to roughly 1khz, for a more aggressive sound.

If a person had a 500R pot, set up so that the wiper went to the inverting pin of the 308, and the two outside lugs went to a fixed resistor and then cap to ground, a person could vary the two ground legs of the gain stage in a complementary manner. As the resistance gets more "even" between the two legs, the mid/treble advantage is lost, and conversely, as it gets more imbalanced the advantage is introduced.

Since 500R pots are often difficult to get, and since you really want more adjustment on one side than on the other, you can feel free to use a 1k pot and stick fixed resistors between the wiper and each of the outside lugs to set the maximum resistance obtainable on each side. For instance, with a 220R resistor to one side of the wiper, and a 680R resistor to the other. The highest possible resistance obtainable on the smaller side will be 180R, and the highest possible resistance obtainable on the higher side will be 404R. Armed with that knowledge, and the formulas above, you can pick what fixed resistor values you'd want to add to what the pot provides. For instance, a 220R on the bass side and 22R on the treble side would permit some absolutely insane variations.


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