Rat Circuit Mods...?

[zer0_c00l]

Hey, I have been making a Rat out of the tonepad circuit but it is not sounding exactly as i think it should so i tried some mods:

first i changed some resistors in order to mod it to the turbo rat
then i made a switch for making selectable the diode or LED clipping

but the distorsion sound a bit muddy, like having too much bass, i'm playing with my guitar with the bridge pickup but it sounds like im playing with the neck pickup

so I tried the kinda ruetz rat mod, but i inserted a 1K resistor instead of just cutting the 47 ohm resistor... i could now add some treble but the excessive bass was still there....
So I did the same thing as the ruetz mod to the 560 ohm resistor, subsituted it for a 2K pot (as I could'nt find any 1K one) and it made me able to control the treble/mids a bit more, but the bass was still there.. and i can't find a way to cut it off...
I'm thinking of trying some 1 or 10 uf caps with pots to ground parallel to the others to see if it makes difference.

I also note that, with a tube screamer behind, the rat sounds more full and more gainy, but not the same as pushing up Rat's own distortion (as this, i don't know how, also controls some frequencies and the more i push up the gain the more the treble disappears) so i tried to substitute the first 0.022 cap that appears with a 0.01 one but it didn't made difference, so i'm thinking of jumping the 1k resistor that comes after so it can, suposedly, add some more pre-gain.

If some has reccomendations on how to control the frequencies and the pre-gain of the RAT I would be really thankful.

Cumpz

[FlyingZ]

Play a clean Lydian track like an Aadd9 vamp or something. The stock rat should do an almost perfect Steve Vai tone, awesome single notes but poor chords without guitar volume roll off. If you are looking for a brighter tone I don't think this is the pedal to mod.

[grolschie]

Interesting. My Rat2 has always been almost unusably bright, for my taste, below 12 o'clock on the filter knob (using a strat-copy with a Laney 50w valve amp). I mostly run the filter at about 2-3 o'clock with this setup.

[Guitar_Ninja]

You could try switching out the opamp for another one of the same type. I built a Rat last year, and with one LM308 it sounded fairly bright, more overdriven, while with the other it sounded bassier and fuzzier.

[FlyingZ]

It can get bright but it's not a good sound IMO

[zer0_c00l]

well the thing is that the more i crank up the game, more bassier the distortion is, in the max i could use the gain at 3/4 but it is very bassy, my bridge pickup souds like a neck pickup =O what i want to do do is only to roll off some basses at the pre-gain and I don't know how to do it =O just because wih a tube screamer behind getting up some pre-gain i can have a distortion very close to what i thought about and the basses are not present


maybe youre right saying that this is not supposed to sound bright =O

[grolschie]

Have you double checked your resistor and caps values? Especially the 560ohm and 47ohm resistors and the 4.7uf and the 2.2uf caps next to them.

What resistors did you change to make it a Turbo Rat? I thought the only difference between a Rat and a Turbo Rat are the clippers? i.e. si diodes vs LEDs.

[zer0_c00l]

those resistors you refer i replaced with pots and tried diferent caps to see if i can kill some bass but not many diferençe though...

For the mod I changed the 1M resistor after the first cap to 220K, and the last 1M one before the FET to 100K, inserted a 10K between pin 1 and 3 of the volume pot and changed the last 1uF cap to 10uF, seen this in a tutorial =O

I'm thinking of building a booster to put behind of the RAT...

[FlyingZ]

This is my take on the intended Rat sound for your reference. You need some imagination because I just threw my headset in front of the speaker with no warm up bla, bla....

http://www.aronnelson.com/gallery/main.php/v/FlyingZ/Line-In+2_Mic+2+2009-12-2+11-57-48.mp3.html

[WGTP]

http://www.diystompboxes.com/pedals/ratmod.gif

Here are some cool mods for the Rat from Tonda.  Searching should reveal lots of info on the ever popular Rat.

In my experience, the Rat gets pretty woolly sounding as the op amp is maxed out with the gain turned up.  This is part of the character of the Rat.  It is too much for some folks.  Also the poor performing (by modern standards) LM308 op amp seems to be part of the character.  Other op amps will sound slightly different.  The TL070 and NE5534 are typically suggested to sweeten/clean things up a bit.  Since the original LM308 op amp has a compensation cap (30pf) it is a little different than the other standard chips like the TL071 RC4558 (dual) etc.  Changing the 30pf cap should also affect the sound.

Reducing the bass content helps this some.  Using the above schematic as reference, reducing the .022uf Input cap to .015uf or .01uf will roll off the bass 1/2 to 1 octave sooner and de-mush things a bit.  (Changing the 1M resistor to 220k lowers the impedance by a factor of almost 5 which also moves your bass roll off up by that same factor.)   You may want to increase the bass on you amp to compensate if it is too much.  

The 560/4.7uf "leg" coming of the feedback loop to ground adds in some additional gain and bass to the 47/2.2uf "leg" and reducing the 4.7uf cap (or eliminating it) will also effect the lows.  While the Rutz mod uses the 560/4.7uf "leg", I prefer to use the 47/2.2uf "leg" myself.  Various circuits to pan between the 2 "legs" or change caps, etc. are cool.

If it is till too woolly, and a different op amp doesn't work, you are probably at the limits of this design.

Hope this helps.  Good luck.  

[Mark Hammer]

The tone of the Rat is impacted on at 4 major points.

1) The feedback capacitor used with the LM308.  As the feedback resistance is increased (for higher gain) the cap cuts more treble, following the formula: rolloff freq = 1 / [2 * pi * R * C].  A smaller cap value will rolloff less top end at max gain.  With the stock 150k gain pot, rolloff will be 6db/oct at just over 10khz.  Not a huge impact, but something to play with.

2) The RC networks from the non-inverting pin of the LM308 (pin 3) to ground.  Their respective rolloffs are also given by freq = 1 / [2 * pi * R * C].   Note that each RC network not only has a rolloff associated with it, but also has a certain implied "gain stagger" such that the treble content has a different amount of gain applied to it via the 47R/2u2 pair, relative to the overall signal, whose gain is partially determined by the 560R/4u7 pair.  The degree of gain stagger also determines the tone of the unit.

3) The 30pf compensating cap between pins 1 and 8.  That cap determines the gain-bandwidth product and is partly responsible for the unique quality of the Rat.  Basically, the Rat asks for more gain from the chip than it can possibly deliver in that frequency range.

4) The max filter rolloff frequency, as determined by the cap to ground, 1k5 fixed resistor and filter pot.  Stock, use of a 1k5 fixed resistor and 3300pf cap means that at least treble cut, the rolloff commences at just over 32khz, and at most treble cut the filtering starts at 475hz.  It's only 6db/octave, so you'll still hear some treble in the background even at max cut.

There is a ton of stuff to play with there.  Points 2-4 represent more "creative intervention" points than #1.  If you want something that moves more towards a "heavy TS", then you probably want to change the 4u7 cap for 1uf, to get yourself a rolloff below 280hz, and perhaps decrease the 30pf cap for a 27pf one to give yourself a little more gain-bandwidth. 

[trjones1]

I had this idea as I was falling asleep last night.  I think it would be a useful addition to a Rat to use a on-on-on switch to switch between the 47R/2u2 pair, the 560R/4u7 pair and both together.  I've never tried it, so I don't know if switching between the two pairs would be a usable difference in tone, but it's worth a shot.

[boogietube]

You should just go ahead and make this:

http://www.beavisaudio.com/projects/FKR/

It's a rat with all of the mods.....

Search his site and there's all of the info.