2-Bit Rat Rod

[Chistopher]

I modified my Rat 2, using a mere 2 components. Heres a basic analysis for anyone who cares to know.

The way a Rat works is that it a signal gets boosted to the high heavens through an opamp, then further distorted through a hard clipping arrangement of clipping diodes, then it runs through a tone control no more advanced then the one on a guitar.

The way the opamp works is that it has two high pass filters in the feedback loop. I won't bore you with the specifics, but the first filter makes it so any frequency above 60 Hz gets boosted a decent amount. The second filter makes it so any frequencies above 1.5kHz get boosted quite a bit more. The signal then travels through the hard clipping circuit and causes the signal to clip and compress at higher gain settings. This is why Rats get thin at low gain, the bass treble frequencies hit the maximum headroom and boosting the gain further allows the quieter bass frequencies to get there too.

My mod aimed to change two things: a more usable gain control and more bass at low gain settings. I didn't want to majorly overhaul the sound as the famous Ruetz mod does either.

So I looked at GGBB of diystompboxes "bass boost" mod and did some tweaking to the component values. I took the 47R and 2.2uF leg (R8 and C7) of the feedback loop high pass filter and doubled the resistor value to 100R and halved the cap to 1uF. This lowered the overall gain of the circuit and gave a near 6 dB boost to frequencies below 1.5kHz. At high gain settings this isn't too big a difference, at low gain settings it's a major difference. The reduction in gain has made it so that diming the gain is the equivalent to about 2 o clock on the stock circuit. This makes it so that the famous sweet spot at 9 o clock is much more spread at at the expense of losing the upper range of travel on the gain knob that doesn't do much anyway. Seriously, on a stock unit SSL-1s can reach saturation at about 1 o clock.

This mod doesn't dramatically alter the Rat. All of the high gain sounds you can get on a stock pedal are still there. It does however make it a lot easier to dial in distortion and (now thicker) overdrive settings, where the stock unit wants to jump right from thin fizzy OD to full on fuzz

I'm more than happy with the mod. I like the Rat as is, but anything that makes me spend less time finding the perfect setting on the knobs is a good thing.

 

[jeremy]

sounds like a good mod and pretty simple to do

 

[Chistopher]

Yeah, it's definitely in a different category though. A lot of people expect modding their distortion to take an okay sounding pedal and turn it a fire breathing monster. What this mod does is take an already great pedal and make it easier to use. Just watching someone A/B this against a stock RAT wouldn't tell you much because the real night or day difference you get is in how the controls respond. If this were the first pedal mod I'd want to try something with a little more wow factor.

I've noticed since the original post that the added bass gives it a spongier fuzzlike feel to the higher gain levels. Playing the lower strings has more of a lo-fi feel to it. If you wanted to take this a step further and make it even more friendly for lower range instruments, replacing C6 with a 10 uF capacitor will let 30 Hz signals slide on through, which would let the fundamental of the low B on a 5 string come through untouched.

 

[Driver Blues]

What guitar were you putting into it? I may try this mod on my new Rat. Is there anything I should look out for modding a Whiteface Rat instead of a Rat 2?

 

[Chistopher]

I tried this with a vintage output Strat and a medium high output Wolfgang. There are a few minor differences probably between the reissue and the Rat2, but none that would affect the mod.

 

[Chistopher]

Also worth noting, R9 at its stock value makes it so the leftmost 1/3rd of the filter control does nothing in the audible human spectrum, so you can further simply the controls of your Rat by swapping it out for a 3k resistor, which makes the filter easier to use without limiting its range at all

 

[BlackhawkRise]

I made the R8 and C7 mod to my you dirty rat this morning and I must say I am impressed. This may just be the germanium diodes, but it's closer to the fuzz face ballpark. I never knew about the tone control thing, I'll have to give it a look when I get off work

 

[BlackhawkRise]

I tried the R9 mod, although on my board it was R10, and replaced it with a 3k3 resistor, which set the max cutoff frequency at around 15k Hz. This made the usable range of the tone control a little more usable, but their is still a small deadzone on the lowest two brackets of the tone control. After looking at a few charts such as the one attached, I think I'm going to bump it up to 4k7, which puts the corner frequency at 12k Hz, which is still high enough that fully counter clockwise, the filter shouldn't take off any highs. I probably won't be able to do the mod until Monday, but I will report back my new findings. I may also try a 5k6 which cuts off at a high of 8.5k Hz

 

[Chistopher]

I would suggest you put a 10k trim pot in there if you can. I put a 4.7k resistor in there, and even that's a little more subtle than I'd like

 

[BlackhawkRise]

10k creating a minimum cutoff value of 4.8 kHz at brightest filter settings going all the way down to 438 Hz at the darkest setting. What is the brightest guitar frequency that can be heard in a mix? My chart says round about 5 kHz. Would it neuter my signal to have a cutoff at 5.8 kHz with an 8.2k resistor? Obviously a trim pot can answer this for the individual, but speaking in terms of other pedal makers, where does the cutoff frequency of a passive tone control usually set as the maximum?

 

[Chistopher]

Anything higher than that and you'll start to lose high frequencies, whether that's a good thing or not is up to you.

After fiddling around with it, I don't think the resistor makes too major a change to the filters range, but above 5k-ish the pedal with the gain at 0 and the filter fully left started to sound darker. With a 4.7k resistor I was able to achieve a setting on the pedal such that the pedal functioned as a transparent buffer. Is that necessarily a useful thing to shoot for? Not really, but it is a fairly unique trait of Rats. On a stock Rat at 0 gain with minimum filter, the opamp sees a gain of near as makes no difference 0 dB, the filter cuts out no audible frequencies, the diodes don't clip, and you get a moderate output buffer. Can't say that for too many other pedals.