Rat differences

scaesic · January 25, 2007, 08:18:26 AM

hi, iv posted this before but i thought i might try again.
i have two pro co rat 2 pedals, one of them has a sloped top, and one of them has a flat top. Basically i got a rat because i liked the sound of the proco rat 2 (flat top), but the new version i got had a sloped top and sounded very different.

iv changed the op-amp out for a lm308n but it still sounds very different.

here are the differences between the two when i a/b'd them with all settings flat.

The new (sloped top) rat is a lot higher in volume and gain. It also has a lot less highs, and a lot more mid ranges. The pots are a lot less responsive, ie the gain seems to go from slight fuzz to full on fuzz, whereas the old one went from slight drive/slight fuzz/full fuzz.

This makes me think that the op-amp circuit must be set at a higher gain, and there must be more output overall.

i want to mod the rat to sound more like the original (flat topped) rat. Which parts should i think about looking at?
I'm assuming in order to increase the amount of high end/reduce mids i'll need to change a few caps. And which parts of the op-amp circuit should i change to reduce the gain?

i've seen a few rat schems, some show the fet on the input and some on the output, the rat im modding uses a 3pdt, so i'm assuming the one fet on the circuit is an input buffer, which resistor do i switch out to decrease the gain of the circuit? the pot values are the same so i thought it must be the one resistor between the signal line and vref just before the opamp??

i thought about it a while and realised that if i reduce the gain of the op-amp config. then it will return more bandwidth, and therefore increase the high end response. Does anyone noe else have some good ideas to look at while i have the thing open?

i'd hate to have to keep constructing/de-constructing the pedal. Also, i don't know why the new sloped rat has more output, any suggestions?

http://www.diystompboxes.com/pedals/PCRAT1.GIF heres the schem iv been looking at.

WGTP · January 25, 2007, 09:53:46 AM

Can you tell what the clipping diodes are? LED's or 2 sets of SI's would be louder and have a higher clipping threshold than 2 GE's ofr SI's. It would also change the character of the distortion.

scaesic · January 25, 2007, 09:54:04 AM

had a look inside both, turns out the older one uses two fets, and a 3pdt. i guess this is probably where the major difference is but what can be done to close the gap in the sound?

scaesic · January 25, 2007, 09:54:53 AM

i think they both use a pair of si diodes to ground for clipping.
there is actually 4 si clipping type (the orangey ones?) diodes on the older one and three diodes on the new one, but one of those is definetly a protection diode. i dont know why there is 4 diodes on the old one, i cant find any schem with 4 diodes, although im pretty sure its not 4 diodes back to back because as they are far apart on the pcb.

it seems to me that this is a classic case of cutting costs by reducing the number of components, and in the process totally killing what made the rat 2 so good.

the turbo uses leds as clipping diodes i believe, but these are both rat 2's.

gaussmarkov · January 25, 2007, 10:04:36 AM

slightly OT: is bob strand's site the definitive source on rat comparisons?

scaesic · January 25, 2007, 10:07:40 AM

Quote from: gaussmarkov on January 25, 2007, 10:04:36 AM
slightly OT: is bob strand's site the definitive source on rat comparisons?

thats cool but the thing is, these are two "rat 2's" and the pcb's are pretty different. i just dont understand these companies. they seem to think that once the brand name is out they can afford to make an inferior product in the same package. the thing is, they're probably right. Morally though? i'm not so sure.

scaesic · January 25, 2007, 10:10:00 AM

one more thing i've noticed.

the older rat, even though it has a 3pdt, isnt using one of the poles, so i guess that's what the extra fet is for.

the new circuit board also says "pro co rat 3, rev b", so i guess im trying to turn a rat 3 (even though its sold as rat 2) into a rat 2?

Meanderthal · January 25, 2007, 06:08:48 PM

Hmmm... here's a schematic that lists different parts values for different versions... maybe you already saw this:

http://www.happybob.com/rstrand/rat/pcratrev.gif

Edit: This is for the single fet version, the schematic you posted is for 2 fet version. This must be what you have.

Meanderthal · January 25, 2007, 06:12:57 PM

Ok, look at the values of the "origional" Rat. That might cause the lower output, and even sound a bit... 'clearer'.

scaesic · January 28, 2007, 04:27:09 PM

those schematics dont acount for the different fet/diode arrangements i have observed in the two "rat 2's". Doesnt "original" rat refer to the rat before the rat 2?

i think the new rat 2, labelled rat 3 on the pcb, uses the op-amp in an inerting configuration insetad of non-inverting, but im not sure.

amz-fx · January 29, 2007, 07:23:23 AM

I've worked on a lot of Rat pedals and they were all the same basic design with different clipping diodes and very minor changes of other components.
I have never seen one with the input jfet buffer and have heard that was only on a couple of dozen of the first produced examples.

Rats often have two transistors in them but one is the output buffer and the other is the LED switch

There are a lot of different pcb layouts, however, which makes it difficult to tell people how to modify their pedals.

regards, Jack

scaesic · January 29, 2007, 03:08:53 PM

There is definetly a dramatic difference in the flat topped rat 2's and the sloped topped rat 2's. The circuits look pretty similar, but the pcb is totally different with lots of extra components. I understand that some of it can be acounted for by the 3pdt instead of a 2pdt, but the sound is totally different too. it's sad. if i had time id trace out the schematic from the pcb.

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Rat differences