Simple but effective Rat mod

[anchovie]

A friend asked if I could thicken up the tone of his Rat, but didn't want the Reutz mod as it would change the character of the distortion too much and lose the Rattiness.

After a study of the schematic and 20 minutes of experimentation, I've discovered that a 220nF in parallel with the clipping diodes sounds bloody lovely! Haven't seen this mentioned on any of the Rat modding pages I've read so thought I'd share - apologies if I've reinvented someone's wheel.

[Kipper4]

Does it matter where in parallel, before?after?between?

[anchovie]

Electrons don't care where you put it on the board, they'll just see the two diodes and a cap as one entity - they're all between signal path and ground at the same node of the schematic.

[Groovenut]

Not being a jerk, but how is this mod any different than turning down the filter knob? Your .22u shunt cap is interacting with the 1k series resistor to give you a corner freq of 723Hz. You can attain that same roll off with the filter pot at roughly 65% resistance.

Maybe I'm missing something here?

[Mark Hammer]

You're half right.  In conjunction with the tone control, the added cap makes for a 2-pole filter.

[Groovenut]

Sure Mark, but I was meaning either or. Plus I would think you would need to run the filter control almost wide open to have a useable tone with both installed unless you were running the gain really high.  It's all subjective I suppose.

[anchovie]

Test it with your ears, not a calculator. For example, the tone I was getting with cap added and filter wide open wasn't achievable with the pedal stock at any filter setting.

I don't know enough of the science behind diodes and caps to know if there's any "order of preference" as far as the signal is concerned with having them joined at the same node - electronically speaking, is the rolloff occurring before the diodes clipping or afterwards? I'll throw that question out to the engineers!

By the way, I'm still finding the filter control very usable - the mod hasn't simply darkened up the whole pedal.

[Kipper4]

I love my Rat. I'm gonna try this Anchovy thanks.
This might be a usefull addition to the zen drive too.

[knutolai]

Test it with your ears, not a calculator.

IMO the most frustrating part of filter design. It's seldom something ever sounds as I expected from the calculations

[Groovenut]

Test it with your ears, not a calculator. For example, the tone I was getting with cap added and filter wide open wasn't achievable with the pedal stock at any filter setting.

I don't know enough of the science behind diodes and caps to know if there's any "order of preference" as far as the signal is concerned with having them joined at the same node - electronically speaking, is the rolloff occurring before the diodes clipping or afterwards? I'll throw that question out to the engineers!

By the way, I'm still finding the filter control very usable - the mod hasn't simply darkened up the whole pedal.

That's the only way to test anything audio. Calculations mean nil except as an estimate of how it will sound.
The placement of the cap at the node means the clipping and the filtering are happening simultaneously.
I personally find the stock filter adequate for any Marshall style amp but a bit too bright for Fender style amps at anything less than noon.

[hymenoptera]

The Madbean Slow Loris straps a 10nF cap across the LED clipping diodes, but not the standard 1N914's..

I might have to mess with a 220nF instead this since I have a RAT breadboarded out in the shop.

[Groovenut]

The Madbean Slow Loris straps a 10nF cap across the LED clipping diodes, but not the standard 1N914's..

LEDs tend to be very noisey when they conduct ("crunchier" by some descriptions). The 10nF would provide filtering above ~16KHz, allowing the conduction noise to get to ground (mostly) before getting into the downstream circuit.

[plexi12000]

hey, just chiming in for the Rat fans.  i built AMZ's "Fat Gnat".....REALLY nice circuit.

[Kipper4]

Did this to my latest and to my ears its very but effective.
Nice one Anchovie

[Guitar Poppa]

Not being a jerk, but how is this mod any different than turning down the filter knob? Your .22u shunt cap is interacting with the 1k series resistor to give you a corner freq of 723Hz. You can attain that same roll off with the filter pot at roughly 65% resistance.

Maybe I'm missing something here?

Hello !

You're right on a static point of view, but your analysis should be extended to the dynamic level : when the diodes become more or less conductive depending on the level applied to them through the 1K resistor.

>>> When the diodes are inactive, they are like a very high impedance parallel to the cap... You may neglect their effect, and as you wrote, the 1K resistor and the cap works together like a low pass filter. Playing the guitar soft gives a warmer sound. The response curve should look like a bell, with its top between 400 and 500Hz.

>>> When the diodes becomes conductive, they look like a dynamic resistor, whose value will instantly change according to the level of the signal. This dynamic resistor parallel to the cap will affect the serie resistor's value of the low pass filter (Thevenin's theorem). The more they become conductive, the higher the low pass frequency shifts.
The sonic effect his like a dynamic filter, a kind of enhancer : soft notes, release of them, will sound more warm and dimed, heavy notes, attacks, will get all the sharpness the rat can give. This effect is not drastic, but gives hues to a pedal that his normally rather violent.

NB :People who tryed to put a 100pf to 470pf cap between collector and base of the second transistor in a Fuzz Face may have listen to an analog dynamic effect : this effect his characteristic of a capacitor parallel to a solid state clipping circuit.

>>> All that depends of the way the guitar his played, and it should interact with the built in filter. Every one can experience differents values of the capacity. It seems that 220n is well choosen, as the diode system in the Rat works on low impedance.

Happy to share this experience.
Guitar Poppa

PS : excuse me for the way I write english... Hope it won't be too stange !

[LightSoundGeometry]

I own a slow loris and love it to death. it makes almost every other distortion obsolete.