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Need info on RAT op-amp.


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9/3/2004 2:16 PM
Humbucker Need info on RAT op-amp.
I have a 1991 re-issue RAT and I've got a couple of questions for some of you techs out there.  
 
I read at Keeley's website that he does mods to the RAT pedal, one of which is to put in the proper op-amp, I think he described it as being the big metal can version or something similar. In my pedal, I have an 8-leg op-amp labelled "LM 308N" it looks just like a TL072 etc...  
 
Also, I noticed at Small Bear Electronics website that he has the "RAT op-amp for RAT fanatics" it goes for $4.00 which is way more than the other varieties of op-amps, so I'm thinking that this must be the op-amp that Keeley was refering to. Am I right?  
 
Something else that I've been wondering is changing the tone control because I find the stock tone control circuit pretty radical, kind of sounds like a wah-wah pedal. I'd like to make the tone control more useable.  
 
I like the general flavor of the distortion because it has good harmonics but the thing sounds a little bit too nasally for my taste. So instead of getting rid of the pedal I think it would be a good candidate for a mod or two.  
 
I'd really appreciate some help here, thanks!
 
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9/3/2004 7:00 PM
JS

LM 308N is the correct op-amp.  
Iīve got an unit from the early eighties with the same op-amp.  
 
MODS:  
 
Make the connection of the 2,2(4,7)uF to ground (that one that leads to a 47 ohm resistor) switchable (gives a better volume boost). Replace output trannie with a 2N5488 if itīs a BF245.  
 
Replace bias resisters for op-amp and trannie with 1M trimpots for finetuning of bias-points (decreasing this values will mellow the sound, around 800k for op-amp and 150k for trannie gives a good punchy sound).  
 
Use a Voodoo Lab Pedal-Power or something similar  
(10V DC, 150mA at least),īcause batteries are to weak for powering a RAT (increases punch, crunch and clarity).  
 
You can play with the RC-Network that goes to the feedback-loop. Replace it with 1uF,100 ohm and replace the 3,3nF cap of the tonestack with a 10nF.  
(reduces the fuzzy topend screech).
 
9/19/2004 7:37 PM
Lp
Any ideas LM308AN
Hi, I have changed my rat's IC with LM308AN and the sound radically changed(in positive way). Now it sounds more organic and smooth. But it is too tube like now. Is there any difference between LM308AN and LM308N , Does it change any thing with the sign A?
 
9/20/2004 12:39 AM
Humbucker
I have the LM308N in my stock 1991 re-issue RAT and I'm wondering if the RAT opamp that's offered by Small Bear Electronics is something different.  
 
Has anybody here bought their RAT opamp?
 
9/20/2004 9:27 AM
JS
Hi,  
 
LM308N was produced by National Semiconducter.  
LM308AN is produced by Motorola.  
 
Specs of LM308AN are a bit different from LM308AN and in normal audio circuits it won't matter.  
 
In a RAT the chip will be heavy overdriven (remember the chip was not designed for this) and in this operation mode the sound of the chip can change dramatically.  
 
IMHO LM308AN can vary a lot due to production tolerance. I have a few and in a RAT curcuit they sound from sweet to harsh and anything inbetween.  
 
LM308N are more consistant and you can switch them without hearing an audible difference.  
 
JS
 
9/20/2004 2:33 PM
LP

Thanks JS you have enlighten me with your knowledge.  
Also I forgot to tell that my Rat is the vintage reissue (big box1991). May be the reason of much tube like distortion ? After the LM308AN installed, even in a solid state amp it sounds sweet and good.In cord playings the seperation becomes perfect. But palm muting or hard cord playing was gone.  
However I am not a metal player(I play blues,oldscool rock) I have got this pedal for its flexiblity which is a little bit gone. But I am not missing the old harsh sound.  
 
Is there any mod to improve the flexiblity?  
 
Soldering the chip is a torment,I am going to install a socket next time to understand the diferences between chips.  
 
Also,is there any difference with the metal can version LM308 that Keeley uses and the others?  
Sorry for too much questiones....
 
9/20/2004 3:47 PM
JS
For mods look up (my 2nd. reply).  
 
I've never ever seen any metalcanned chip, even in an old RAT. Maybe it was metalcanned for schielding purpose but today any good chip is metalshielded under the plastic housing.  
 
By the way only for info:  
 
Is your 91-RE-RAT input/output-buffered or only output-buffered?  
Which type off trannie is/are uses in the circuit?  
 
JS
 

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