quick question about the Rat

[kristopher612]

hi there all.  i have a friend that wants me to build him a rat.  He wants it to have more range in the cleaner side of the gain control.  looking at the schematic, i've noticed that it's set up almost exactly like the tubescreamer(minus the diodes).  would increasing the post value take care of this?  or do you think it would maybe be better to have 2 pots in series?  say a 50K pot and the 100K pot.  i would think this would give more control over the eary part of the gain than just using  250K pot, plus keep the maximum gain closer to the original.  I'm just wanting to see if i'm thinking this one out clearly, or if maybe there is a better way to do it.  Maybe making the 1K resistor a 5K pot or trimmer? any suggestions would be most welcome.

[Mark Hammer]

I would say the smartest thing for your budy to do is to install a switch that can throw a 10k resistor across the 100k gain pot.  Since that is bound to elicit some "which lug?" questions, let's just say you want to place theresistor in parallel with the 100pf feedback cap. (C3 in this schematic: http://www.generalguitargadgets.com/pdf/ggg_rodent_sc.pdf )

Why do this, and what will it accomplish?  The Rat is set up to very easily produce gobs of gain.  Unfortunately, it doesn't take very muich change in the rotation of the 100k gain pot to start generating a fair amount of that gain.  If the total resistance range of the Gain pot can be reduced with the parallel resistor, your friend should be able to dial in subter sounds.  A 10k resistor in parallel with 100k gets you a max resistance of 9.09k, which translates to a maximum gain of x17 for the low end, and x194 for content above roughly 1.5khz.  That should enable easy dialing of gentler grit.

[bipedal]

What version of the Rat are you looking at?  (Can you link to schematic you're using?)

IIRC, Rats generally use a 100K log pot set up as a variable resistor for the gain/distortion control, and I believe that in this circuit's arrangement, raising the pot's value would only increase gain potential -- analogous to changing the Tube Screamer's 500K drive pot for 1meg for more gain.  Sounds like more gain would be exactly opposite of what you're aiming for.

I don't see why using two smaller value pots in series wouldn't work...  For example, if you use two pots in series rather than a single pot (say, two 50Ks instead of a single 100K), you'd essentially have the same range of variable resistance (0 - 100K) in the loop, but controlled via roughly 600 degrees of available pot travel (2 pots x 300 degrees of rotational range), allowing more granular, precise control at various levels when compared to a single pot.

[kristopher612]

IIRC, Rats generally use a 100K log pot set up as a variable resistor for the gain/distortion control, and I believe that in this circuit's arrangement, raising the pot's value would only increase gain potential -- analogous to changing the Tube Screamer's 500K drive pot for 1meg for more gain.  Sounds like more gain would be exactly opposite of what you're aiming for.

Changing the tubescreamer's 500K to a 1 Meg also changes how clean it is at the early part of the rotation from what i've gathered.  having never built one i don't know the truth on this one, amd may have even misread that bit of info.  sorry if that bit seemed confusing.  i guess it made sense in my head, and i didn't get it typed clear enough.  I believe you're right though.  if i use  two 50k pots it would be the same as the 100k pot, thus keeping the max amount the same as normal.  Maybe i should just plan to use 2 50k pots, and the switch with a 10K resistor to give enough control to get what he wants.  i really dont want to ship this thing, have it sent back, and then ship it again.  he lives up in Maine, so its not a quick drive over to make sure it's what he wants before i box it up.  Thanks again for the ideas, guys. obviously i wan't completely thinking on this one. 

[Mark Hammer]

There are two things being confused here: gain and dialability.  The Rat covers a range of gains from very little to very much.  The "very little" part is absolutely fine for what you want, but the sweet spot where you find it sits in a zone covering maybe 2% of the pot's rotation.  Your friend is going to be like someone going back to the same mega shopping mall hoping to bump into "that cute girl" again.  maybe it'll happen, but likely it won't.  The suggestion I offered of switching between a low and high gain setting will allow for the lowish-gain settings to be easier to find and dial in.  If you use a toggle to connect that resistor in parallel or take it out of circuit, you'll have access to all the glorious high-gain settings and low-gain ones too.

[oldrocker]

You could try the LED's.  For me it makes the pedal louder but cleaner when it comes to fuzziness.  I think it's the Turbo Rat mod.  Anyway I have that switch for SI diodes or the the LED's.  I was just wondering if this would work for him.  Try breadboarding the original circuit and try some different things.

[kristopher612]

i will doing your switch mod, as well as using the two 50k pots to make dialability much easer.  i definitely want the settings he's after to be repeatable.  I will probably try it out before i say "done" with the normal 100k pot instead of  two 50k pots, but wont know what i'm doing until i try it.  so i may still use two 50K pots.  I also planned on doing a diode switch too, just hadn't mentioned that.  it'll probably be a week or two before i get i finished.  i have a lot of flower bed work to do right now, and saturday is a city wide garage sale!   BTW, i knew "that cute girl".  she worked at the mall when i was 14 and full of hormones, so i went back a lot

[grolschie]

I also really like the Rats sound when set to very low gain (almost the very bottom of the dial), but sometimes also up to around 2-3 o'clock area for heavy stuff. To me it seems that there are some problems with the stock gain knob setup (well, there is on mine):

 a).there is a reasonably sized, completely dead spot between off and low gain settings.
 b).the overall gain of the pot could be reduced to about 75%.

It would seem to me that reducing the overall gain at the knob, and removing the dead spot, would make the low end more easy to adjust, at the expense of the unusable fuzz (well, for me it's unusable) at the other end.

[brett]

Hi
here's a couple of ideas.
First, that 100k "log" pot in the Rat is probably not what you think it is.  "Log" pots, especially cheaper ones, often consist of 3 or 4 linear regions of increasing "steepness", that approach an exponential function.  (and yes, they should be called "exponential" pots, not "log" pots).  This means that the first part (10 to 20%) of the pot's rotation has much greater effect than a true "log" pot.  There's not much you can do about this except modify your "log" pot (or a linear one) to a more non-linear function.  See RG Keen's "Secret life of pots" article at geofex.com for details.  It's good stuff to know.
Second, because the change from 50k to 100k ohms, where the Rat is really wild and not useful to you, occurs over only the last 20 to 30% of the pot's rotation, you might consider changing to a single 50k log pot or even a 25k log pot.  You'll be surprised at how much gain you still get at maximum (ie it'll be about where 2 o'clock is now), and how much the control improves at the bottom end (that 5% sweet range will probably expand to 10% or 20%) 
cheers

[ACS]

...like someone going back to the same mega shopping mall hoping to bump into "that cute girl" again.

You come up with the best metphors mate!!