Rat biasing

[scaesic]

i have a stock new rat 2 which sounds a lot more extreme than a couple of other rat 2's i've used before.

i changed the chip out for an lm308 but its still noticibly extreme distortion.

the nobs seem to be a lot less interactive too, compared with other ones.

is this just a case of biasing the fet that drives the op-amp? if so, could someone point me to the resistors relevent, and i can take some readings?

I'm not sure what "version" this rat is but it uses a 3pdt and has only one transistor, which i'm assuming is used to drive the op-amp instead of switching, as it has a 3pdt for tb?

[brett]

Hi
the JFET buffers the output, so it has virtually no effect on tone (unless its not working).
It might be worth checking the input and output caps (0.022uF).  Also check that the 1uF bias filter cap is connected.  If it isn't, things will sound awful.

Check your battery.  Also, the voltage at each end of the 1M bias resistor should be half the battery voltage. (ie about 4.5 to 5V)

[scaesic]

hok, i'll check those things outif i can trace what components are what on which schem, but i suspect its changes to the circuit from proco rather than a fault. Whats the best schem to use for the newer proco rat 2's?
i had seen one diagram that had a tranny as an input buffer before the op-amp.

this rat sounds more like a fuzz pedal than a versatile distortion box, anyother cures?

[WGTP]

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

What clipping diodes does it use?  Is it LED's, 4 SI's or 2Si's?  Just 2 Si's might make it sound Fuzzy.  I've included Tonda's mods scheme for reference. 

What are you driving it with?  A super distortion pickup, or a Strat single coil?

If you find everthing working, you might try disconnecting the 560 (less bass) OR 47 (less gain) ohm resistors for less gain and distortion.   

[scaesic]

it sounds harsh (i should probably say fuzzy, it has lots of gain - not lots of highs) no matter what's driving it, iv used a strat both single coil neck and hot rail bridge, and a humbucking sg style guitar.

there's just 2 back to back (i think)si diodes, i would of thought this'd give the softest distortion type?

it's stock, but the rats have gone through so many revs it makes it hard to figure out whats different here, it cant be something as simple as gain because even at low gain settings its still quite harshly fuzzey.

what about increasing the resistor going in to pin 2 to reduce the gain?

its so difficult to tell what schem to use for this pedal, there's about 3 or 4 all slightly different. It definetly only has one tranny though, 3 100k pots and a 3pdt.

[WGTP]

If there are just 2 SI's, that would be pretty distorted, but it should clean up at low settings.  My Rat II had 4 SI diodes, but it was dead and I rebuilt it.  Sounds like something isn't right, but not sure what it would be.

Those are the resistors I was referring to and increasing them will lower the distortion.  You could also just disconnect one or the other as I said. 

One thing you might try is to disconnect the clipping diodes and see how much the op amp is distorting at low settings.  If it doesn't clean up without the clipping diodes, again, something is wrong. 

[dosmun]

My Rat II had 4 SI diodes, but it was dead and I rebuilt it.  Sounds like something isn't right, but not sure what it would be.

2 of those are for the bypass circuitry

[scaesic]

im pretty sure this one has only 2 back to back clipping diodes, its also possible theres another one for polairty protection.

i might try increasing the resistor just before the op-amp input, ok bye!

[dosmun]

You are correct all Rats had a polarity protection diode.  If it is a recnt Rat then it won't have the switching circuit as they all use a 3PDT these days.  I think the current circuit is also pretty true to the original.  I recently compared Rats from all versions and made notes of the changes. 

[brett]

Hi
There's a mystery about why you get high gain on low gain settings.
That would most likely be caused by a problem with the gain pot.  Either the internal wiper not working, or a poor connection in the external soldering.
You could check that the restsances between lugs 1 and 2, 2 and 3, and 1 and 3 all change when the knob is turned.
cheers

[scaesic]

the pot works, and it adjusts the gain, the range of gain avialable is pretty low though. i'll probably increase the resistor into the chip and see what happens.

[PenPen]

Does your version have a compensation cap on pin 8 to pin 1? Since it used stock a different chip it is possible the compensation cap wasn't used. Should be I think a 30pf small ceramic cap. That is for the LM308 version. This MAY be your problem, I can't tell.

Aside from that, do you mean it doesn't have ENOUGH gain and you want more, or it has too much and you want to limit it?

[brett]

Hi

i'll probably increase the resistor into the chip and see what happens.

Which resistor is that?
Also, which op-amp did it have before the LM308?  Some single op-amps are non-standard pinouts.
cheers

[scaesic]

well it works with the lm308 in so it must be the same pin out, i cant remember what it was, something long and unmemorable. i havnt worked out what resistor to change yet, il just have a look at where the track goes. i decided that since the gain was really high and the gain pot was only acting as more of a volume pot then the gain set by the fixed circuit must be saturating the chip/diodes really early. If i reduce the gain by changing out the resistor before pin 2 then the ratio of the pot and that resistor will be reduced.

[PenPen]

Ok, let me start this by saying this circuit was one of the ones I did A LOT of studying on. I broke down the whole circuit to learn how each part affected the sound, so I knew what to change, then breadboarded and tested a ton of different values.

First thing you can do is search for my posts on the Rat. I had a few threads discussing this, detailing how each component worked.

Second, do you have a compensation cap in your version. This will be a small cap between pins 1 and 8.

There are a few ways to lower the gain. One, you can reduce the value of the pot, by putting another resistor in parallel with the lugs of the pot. Two, you can change the pair of resistors in the feedback loop, increasing their value. I recommend against this, since the gain formula is R1(pot)/R2(loop to ground). In the Rat the value of the R2 pair is also run into some caps for a frequency dependant resistance value. If you want less gain you can do the usual mod of removing one of the pair in that loop, which increases the resistance there and lowers the gain.

However, I think you should check that compensation cap first, since you need to know why your's isn't behaving properly.

If i reduce the gain by changing out the resistor before pin 2 then the ratio of the pot and that resistor will be reduced.

Sort of. There should be TWO resistors on that track that lead into caps and then to ground. The other side then goes into pin 2. You are looking in the right spot but you should probably change the resistance of the pot before messing with those.

[petemoore]

this rat sounds more like a fuzz pedal than a versatile distortion box, anyother cures?
 fuzz pedal than a versatile distortion box
 Adding value to the clipping diode cap or output is a nice place to trim harsh off the HF hash, this can necessarily take the Fizz out, transforming any fuzz instantly into 'distortion'.
 When possible, I like to do this with insulated grip testclip wires only, non-invasive...as long as they don't get to sparking...
 Power down, twist a thin wire under a component lead like the signal path side of the clipping diode, add cap connected to a convenient ground, say signal signal jack sleeve lug, test values of HF rolloff @ clipping...using tone knob and amp...see if a 'rounder' clipping doesn't help the extremeness...
 Actually 'extremeness'...leads me thinking to possibly move that range down. Perhaps you can trace the gain knob to OA circuit, also the feedback to ground from OA input...
   
 

[scaesic]

Ok, let me start this by saying this circuit was one of the ones I did A LOT of studying on. I broke down the whole circuit to learn how each part affected the sound, so I knew what to change, then breadboarded and tested a ton of different values.

First thing you can do is search for my posts on the Rat. I had a few threads discussing this, detailing how each component worked.

Second, do you have a compensation cap in your version. This will be a small cap between pins 1 and 8.

There are a few ways to lower the gain. One, you can reduce the value of the pot, by putting another resistor in parallel with the lugs of the pot. Two, you can change the pair of resistors in the feedback loop, increasing their value. I recommend against this, since the gain formula is R1(pot)/R2(loop to ground). In the Rat the value of the R2 pair is also run into some caps for a frequency dependant resistance value. If you want less gain you can do the usual mod of removing one of the pair in that loop, which increases the resistance there and lowers the gain.

However, I think you should check that compensation cap first, since you need to know why your's isn't behaving properly.

If i reduce the gain by changing out the resistor before pin 2 then the ratio of the pot and that resistor will be reduced.

Sort of. There should be TWO resistors on that track that lead into caps and then to ground. The other side then goes into pin 2. You are looking in the right spot but you should probably change the resistance of the pot before messing with those.

for the first post - the pedal has way too much gain, compared to another 2 rat2's it is really fuzzy, even with the pot on low gain settings, also with the gain pot right down it has no signal.

secondly, changing the op-amp seemed to make little difference to how the circuit sounds, which made me think that maybe the original op-amp was set for a really high gain.

i dont really fancy changing the pot because as i said, i dont think this will help as even with low pot settings it still sounds fuzzy (lots of gain). ill check that cap and post with some more info once iv done some measuring of resistor values.

thanks for posting btw.

[scaesic]

this rat sounds more like a fuzz pedal than a versatile distortion box, anyother cures?
 fuzz pedal than a versatile distortion box
 Adding value to the clipping diode cap or output is a nice place to trim harsh off the HF hash, this can necessarily take the Fizz out, transforming any fuzz instantly into 'distortion'.
 When possible, I like to do this with insulated grip testclip wires only, non-invasive...as long as they don't get to sparking...
 Power down, twist a thin wire under a component lead like the signal path side of the clipping diode, add cap connected to a convenient ground, say signal signal jack sleeve lug, test values of HF rolloff @ clipping...using tone knob and amp...see if a 'rounder' clipping doesn't help the extremeness...
 Actually 'extremeness'...leads me thinking to possibly move that range down. Perhaps you can trace the gain knob to OA circuit, also the feedback to ground from OA input...
   
 

thanks for some more ideas to look at, but, what does OA stand for? :/

*edit, hang on, it means op amp doesnt it?*

[scaesic]

just thoguht i'd point this out for clarification

in jacks corrected rat schem pins 2 and 3 arethe other way around from the multirat schem: http://www.diystompboxes.com/pedals/PCRAT1.GIF , which is how i assumed the schem was, and also makes more sense to me as pin 2 is the non-inverting input for the lm308.

[PenPen]

Ok, let me start this by saying this circuit was one of the ones I did A LOT of studying on. I broke down the whole circuit to learn how each part affected the sound, so I knew what to change, then breadboarded and tested a ton of different values.

First thing you can do is search for my posts on the Rat. I had a few threads discussing this, detailing how each component worked.

Second, do you have a compensation cap in your version. This will be a small cap between pins 1 and 8.

There are a few ways to lower the gain. One, you can reduce the value of the pot, by putting another resistor in parallel with the lugs of the pot. Two, you can change the pair of resistors in the feedback loop, increasing their value. I recommend against this, since the gain formula is R1(pot)/R2(loop to ground). In the Rat the value of the R2 pair is also run into some caps for a frequency dependant resistance value. If you want less gain you can do the usual mod of removing one of the pair in that loop, which increases the resistance there and lowers the gain.

However, I think you should check that compensation cap first, since you need to know why your's isn't behaving properly.

If i reduce the gain by changing out the resistor before pin 2 then the ratio of the pot and that resistor will be reduced.

Sort of. There should be TWO resistors on that track that lead into caps and then to ground. The other side then goes into pin 2. You are looking in the right spot but you should probably change the resistance of the pot before messing with those.

for the first post - the pedal has way too much gain, compared to another 2 rat2's it is really fuzzy, even with the pot on low gain settings, also with the gain pot right down it has no signal.

secondly, changing the op-amp seemed to make little difference to how the circuit sounds, which made me think that maybe the original op-amp was set for a really high gain.

i dont really fancy changing the pot because as i said, i dont think this will help as even with low pot settings it still sounds fuzzy (lots of gain). ill check that cap and post with some more info once iv done some measuring of resistor values.

thanks for posting btw.

I didn't mean putting a new pot in, I said put a resistor in parallel with the pot. Put a 100k (arbitrarily chosen value) resistor wired to one leg, then wire the other leg to the wiper. That pot should be set as a variable resistor, so it should have one outside lug connected to the wiper, the other not. Bridge between the non-connected-to-wiper outside lug with a resistor to the wiper lug. That will divide the value of that pot, and lower your gain.