RAT Mod questions?

[aarondavis]

2 main questions.  I've got a Keeley RAT that I love but am thinking of modding it a bit further?

1. Is there a way to reduce the hard clipping in the circuit?  To reduce the Fuzzyness?  Would a simple OP amp change work?  I've got the "holy grail" chip LM308, but I have a socket to be able to swap chips. 

2.  Also, is there a cap I can change that could scoop the mids just a bit. 

Basically, I'm trying to make it be a meaner (in EQ anyway), heavier contrast to my Lovpedal Eternity for live use. 

Thanks for your help!

[slideman82]

Have you bought a Keely Rat and it didn't came with a LM308? Well, this op amp will give you that Marshall kind of sound. If you want it crunchier (what a word!!!!! It doesn't exist in Spanish!) just replace the Si diodes (1n4148 I think) with red leds. You'll get more output at full gain, and better sound. If you want to go far, remove the tone cap to ground and see what it happens (I've never done this, just come to my mind right now!).

Try it, then let me know. And obviously, be careful with the PCB tracks.

[aarondavis]

Sorry, I wasn't clear in my first post.  I have the LM308 in my Keeley RAT already.  But at some point I had put in a socket so I could swap chips if I wanted to. 

Where does the hard clipping come from?  The pedal has enough crunch (how I think of it), I could just deal a without the fuzzyness.

What about the EQ of the pedal, maybe even an internal trim pot that could control Mid frequencies?   Or just a cap (or two) change to scoop the mids (just a bit). 

[c101aviojet]

You may want to have a look at this: http://www.beavisaudio.com/Projects/FKR/images/MightierMouse.gif at beavisaudio.com (search for FKR rat).
Replacing the Si diodes with leds will give a softer distortion; also you could make a combination of several, which tends to reduce distortion too.
This one is also a great resoruce, you may be able to help yourself by reading the articles available here.

Slideman, in Spanish you could just say "más crujiente", or just "crujientero"... 

[aarondavis]

You may want to have a look at this: http://www.beavisaudio.com/Projects/FKR/images/MightierMouse.gif at beavisaudio.com (search for FKR rat).
Replacing the Si diodes with leds will give a softer distortion; also you could make a combination of several, which tends to reduce distortion too.
This one is also a great resoruce, you may be able to help yourself by reading the articles available here.

Slideman, in Spanish you could just say "más crujiente", or just "crujientero"... 

Thanks for the info.  I'll check it out.  BTW, when I speak of hard clipping, I mean akin to *FUZZ*.  Don't want a softer distortion, actually want to be a little tighter.  All in all, after trying many a boutique (expensive ones too) the RAT just sounds "right".  Even the modded Keeley one could take a few little tweaks though. 

[aarondavis]

Someone posted a link to an ebook about the RAT and Big Muff.... did it get removed?  I really wanted to check this out. 

[96ecss]

Here ya go.

http://www.muzique.com/ebook.htm

Dave

[c101aviojet]

Do you have an opinion about that book? I'm wondering if it's worth to get a copy.

[Dingleberry Electronics]

Hi.

Why not try different op-amps.
Some people like the NE5534 or TL071 or even LM741 over the LM308 in a rat.
Try swapping to one of those that you like best.

[96ecss]

Do you have an opinion about that book? I'm wondering if it's worth to get a copy.

I haven't read the book yet. I don't have a Rat or BMP so I haven't purchased it. I trust Jack Orman completely though. For $7.00 it's worth getting just for the knowledge you'll gain from it.

Dave

[MR COFFEE]

Search for the thread.

TL070 (externally compensated single Tl072 type op amp) can be set for low slew rate like the 308 with much less noise. Small Bear has them.

[MetalGod]

You could try a small trimpot between the clipping diodes and the point where they connect in the circuit now - this will allow some variation in how hard the signal is clipped.  Can't remember exactly where I saw this idea, but it was most likely RG or Jack Orman's website.

[Gus]

What is a Keeley RAT?

EDIT found this http://www.robertkeeley.com/product.php?id=7

Funny to read double the caps for more bass.  Now I guess the people buying this don't play live with openback speakers or record.
I also saw a small little toggle how long will that last on stage on the road?

[DougH]

Search for the thread.

TL070 (externally compensated single Tl072 type op amp) can be set for low slew rate like the 308 with much less noise. Small Bear has them.

Bingo. The TL070 sounds *much* better than the LM308 for this circuit IMO. The LM308 is really an antiquated piece of junk in comparison. The TL070 with an adjusted comp cap will sound similar with much smoother breakup and low noise.

I'm surprised so many commercial outfits are still stuck on the LM308. Well, maybe not...

[c101aviojet]

Bingo. The TL070 sounds *much* better than the LM308 for this circuit IMO. The LM308 is really an antiquated piece of junk in comparison. The TL070 with an adjusted comp cap will sound similar with much smoother breakup and low noise.

I'm surprised so many commercial outfits are still stuck on the LM308. Well, maybe not...

I want to try the TL70, TL71, NE5534 and OP07DP in my BYOC rat, but it seems the thing squeals like a cat on flames with anything but the LM308. I don't know what do you mean by "compensate", I'm quite ignorant, and I wonder if there's a way to make them work. I'd like to make a tone stack in my Rat and be able to go from "clean" to "stock" and everywhere between.

[Gus]

Was that you Doug that was playing with slowing down the 70 in the rat circuit or Ed or Tim or?  How many years ago was that on a forum?

[DougH]

I don't know what do you mean by "compensate",

Look up the datasheet on the LM308. Find the compensation pins. Find the trace on your PCB layout where that cap is. It is probably around 30pf IIRC. The TL070 will need more capacitance there to get the same effect.

But before you do anything, double-check the pinouts on the datasheets of all the op amps you have been trying. I don't believe they are all going to match but I don't remember. You can't just blindly sub op amps, esp if they are externally compensated. I don't know if the compensation pins get used for something else in an internally compensated op amp in a single DIP package or not. You will have to do some research.

Was that you Doug that was playing with slowing down the 70

Yeah, I built a booster based on the Rat op amp stage called the "Thor". That was a long time ago. I used a TL070 based on MRCOFFEE's suggestion. The LM308 is a real piece of crap. I'm surprised it's still available.

[sshrugg]

I'm messing with a rat right now, and I've found the diodes/LEDs you use in the clipping section make a lot of difference.  I haven't used other op-amps.

Dano's FKR pedal is killer...it let's you tweak diode type, symetrical/asymetrical and all that right there in the pedal controls.  I like his style of designing.  Rather than breadboarding different options and deciding on ONE, he tries it out, and just throws everything he can think of into the box : )

DPDT switches are very cool.  I think I'm going to add a lot of diode switching options to my tube screamer project as well.  Then I'll have a hard diode clipping circuit (Rat) with options, and a soft clipping circuit (TS) as well.  Then...cascade them?  Hah!  You won't hear the guitar through all that nastiness!

Check out this link: Cook Your Own Distortion.  It does a pretty nice job of explaining what differences diodes and opamps make.  You have a ton of options.  Enjoy your tweaking!