What makes a Rat circuit good or bad?

Cirrus

I've built a Rat clone, and it's definitely on the grungy side of things as the gain goes up.

I've had a number of different Rats and Rat-a-likes over the years, and they've all had their own thing going on.

The first Rat 2 I had was quite grungy and boxy sounding with the gain over 1/4. This clone I've made is is about the same. Once the gain's up towards half, you can run the filter at 0 - wide open - and it's not particularly harsh.

I had a '90s US rat with the lm308 which was a lot brighter - you had to run the filter up around 3/4. But consequently, if you wanted to turn the gain up you could roll the filter back and get some clarity/ articulation at higher gain. Wish I hadn't sold that one! And I had a big box rat in 2010 for a while, it had bigger lows and didn't feel as compressed.

People say there's nothing in the OP07 vs LM308, but it's such a simple circuit, so where are the differences, if not the chip?

My clone has the "Ruez" mod on a pot, I set it by ear then measured the pot, and I picked... 50 ohms (stock is 47).   

Danny1969

The original op amp had a really shit slew rate I think, that's basically how fast the output can change in response to a change in input. a lot of those early opamps needed an external compensation cap too and that value also affected the slew rate. Plus you got variation in capacitor and pot values which are quite wide on older components .. 20 to as much as 40% depending on the component. 
It all adds up 

stickyfiddle

The amp you run it into is critical imo. All Rats into Voxy-EL84s sound better than all Rats into blackface style amps.

I have a 2002-ish early MIC Rat2 with slightly shitty pots and no LM308N and a blue switch, and a Fuzzdog with decent pots and the "right" chip, and a cloned JHS All American. Of those I prefer the Rat2. The Fuzzdog is really really great in a gritty snarly sort of way, but I find the Rat2 is a bit more chuggy and sits in a mix better. I'm willing to bet it's as much down to component tolerances as it is the chip.

The All American is interesting, but I haven't spent enough time with it lately to remember a detailed opinion.

Heartfeltdawn

Cirrus said:

I had a '90s US rat with the lm308 which was a lot brighter - you had to run the filter up around 3/4. But consequently, if you wanted to turn the gain up you could roll the filter back and get some clarity/ articulation at higher gain. Wish I hadn't sold that one! And I had a big box rat in 2010 for a while, it had bigger lows and didn't feel as compressed.

Exactly what I've found and I've had a disgusting number of the things going through me when I was hunting down serial numbers and figuring stuff out. The big box jobs have a bottom end to them that gets you into fuzz territory when maxed out and definitely less compressed. 

Cirrus

I've got a couple of LM308s in the post. I wonder if they're real.

Do they still make them? How does a company like Mooer get enough to make their Black Secret (which I also thought sounded on the grungy side of things)? If they are legit, do they make them like they used to?

By which I mean, the heart of the circuit involves running an op amp into overdrive - sure, most of the distortion is coming from the diode clipping that follows, but it obviously matters what the op amp feeding that it putting out. Are all LM308s made equal, in regards to what they do when they're distorting?

ICBM

My experience is that the IC doesn’t matter - all it’s doing is producing a clean signal at much higher level than the input. The distortion is created purely by the diodes, and these do make a big difference - in fact, they’re one of the main differences between the several varieties of Rat, which use different types and all sound different.

The rest is down to the surrounding circuitry, especially the tone filtering (both at the filter control and the preset bass cuts), but the circuit is so sensitive that even the type and quality of the components, especially the pots, seems to matter - this is the big difference between the US and Chinese ones, not the IC.