This page attempts to pull together all the information available about all versions of the ProCo Rat.  If you have any more information, email me (rstrand@ihug.com.au) and I'll put it up.  By the way, I'm not a ProCo Rat guru.
 

Due to the ever changing world of the internet some of the images for the earlier Rats are no longer available.  You should still be able to get an idea from the available images and the Box comments.

The Proco RAT was designed Scott Richard Burnham who started at proCo in 1975.
 

Type	In Buffer	Out Buffer	Clipping Diodes	on/off LED	Switch	Box	Comments
The Rat (no image)	NONE	FET	Silicon	N	DPDT True Bypass	Wide Sheet Metal Labelled: "proco The Rat" small sound inc. down side of proco	The Rat First issue: Big Box-Tone-Rat Tone control is called TONE - counterclockwise cuts highs.  1977-1981. 100 prod. early 1978,  Mass production started in 1979. (about 12 hand painted versions were made in 1976) Original circuit  (Rev B) Bat: 600uA(-1.6mA)
The Rat (no image)	NONE	FET	Silicon	N	DPDT True Bypass	Wide Sheet Metal Labelled: "proco The Rat" small sound inc. down side of proco	The Rat Second issue: Big Box-(Filter-)Rat Uses original circuit Tone control is called Filter. - clockwise cuts highs. 1981-1983
Rat(w/face)	NONE	FET	Silicon	N	DPDT True Bypass	Narrow Heavy Gauge Steel Labelled: : "proco RAT" RAT is in black/inverse	Third Issue:  Small Box Rat (white face) Uses original circuit Tone control is called Filter. 1983?
Rat (no image)	NONE	FET	Silicon	N	DPDT True Bypass	Narrow Heavy Gauge Steel Labelled: : "proco RAT" RAT is in white	Third Issue: (Fourth Issue??) Small Box Rat Uses original circuit Tone control is called Filter. 1983?-1987
Rat 2	NONE	BJT?	Silicon	Y	DPDT True Bypass	Narrow Heavy Gauge Steel H 73mm x D 103.2mm x W 79.4mm luminous/glowing Markers Labelled: "RAT" and "pro co" on edge face RAT is in white	LED Rat Slight differences to original circuit 1987- Bat: 3mA
Vintage Rat	NONE	FET	Silicon	N	DPDT True Bypass	Wide Sheet Metal H 79.4mm x D 104.3mm x W 127mm Labelled: "The Rat"	Reissue 1990?- Uses original circuit Bat: 800uA
Turbo Rat	NONE	BJT?	RED LED	Y	DPDT True Bypass	Narrow  Heavy Gauge Steel H 79.4mm x D 103.2mm x W 88.9mm Red + luminous/glowing Markers Labeled: "Turbo RAT"	1987- Bat: 7.8mA
Brat	NONE	BJT + FET	Silicon	N?	FET sw	Small Red Box, Black Text H 66.7mm x D 130.2mm x W 95.3mm Labelled: "BRAT"	1993?- Low Cost Bat: 4.95 mA
Roadkill (no image)	NONE	BJT +  FET	Silicon	N?	FET sw	?	1993?- A  re-packaged Brat "It was available at Guitar Center, though I don't know if exclusively." - Daniel (sonicvi)
R2DU	?	?	?	?	?	Rack Mount	Don't know too much about this one either. Rack mount version containing two Proco rat's and an external two-botton footswitch. My guess is this unit uses it's own variant of the circuit, but is close to the Vintage and Rat2.

General Notes:

• Rat 2, Vintage Rat, Turbo Rat, Bratt are later/current models
• The LED is in the A of RAT
• All except brat: From left to right controls go for Distortion/Filter/Volume, or, Distortion/Tone/Volume for the very early units.
• Rat 2, Vintage Rat, Turbo Rat, Bratt  all quote 1k output impedance and 1M input impedance - possibly dubious figures.

• People state the Rat 2 and Rat sound different, Rat 2=slightly fuzzy, Rat=mellow, others say no difference  The circuit has minor differences to the original.
• The Turbo Rat  is a harder/edgy sound due to the LED clipping diodes.  The circuit is different to the original.
• The Brat is a low cost unit, many say it doesn't sound as good as the Vintage.  The circuit is different to the original.
• For the word of the masses go to Harmony-Central and search for Proco or Proco Rat.

It is not clear what the correct circuit are for the different versions of the Rat. The original seems to be OK but the others seem to have contradicting information between the available schematics and the actual on/off LED circuit is a unkown. Other unknown pieces are are the actual FET/Transistor part numbers used in production.  With the information so far, only the Turbo Rat shows the use of Bipolar transistor buffers, are there any older circuits using transistors?

• Original/Rat 2/Turbo rat  Original looks OK, Rat 2 (LED Rat) and Turbo Rat disagree with other circuits.

NOTE: The pcratrev.gif on Aron's site is a modified version of this and is missing the component values.
This Turbo Rat indicates 1N34A germanium diodes which diagrees with the belief that RED LEDs are used for clipping.  Both the Rat 2 and Turbo Rat circuits show 220K input impedances, this disagrees with the Proco's 1M input impedance spec.
• Unknown Rat Schematic the 1nF cap never looked right to me.   I don't know where this circuit fit in to the chronology

A bit of research has revealed this schematic, with an additional 1k resistor between the FET source and the 1nF cap, was posted on alt.guitar in April 1994 by M.Verhoef, he states "This version of the RAT has been modified since it was intended to be used in a rack mounting.".  So it's either R2DU or a  variant of Mr Verhoef's.

• Rat 2  Same as Turbo Rat circuit below except 1N4148 diodes used instead of Red LEDs.
• Turbo Rat  According to ProCo, the switching circuit transistor is some for of FET.  Output circuit correct, small uncertaintly if it's a FET or BJT.

VERMELHO is portuguese for RED.
• Brat - rough overview of the Brat schematic - not a trace

• Hand Painted Big Box - Tone-Rat 1976
• Big Box - Tone-Rat 1978
• Rev B: 1981
• Rev L: 1987

A special thanks goes to these people:

• Daniel  (sonicvi),  who was particularly helpful with the date information
• GFR who confirmed the Turbo Rat and Rat 2 schematics (I think he actually traced them)
• Ook who opened up his Proco Brat to reveal a bit of the schematic, he also confirmed
  the Brat and Roadkill are the same circuit, and PCB, in a different box.
  
• Kate Ebneter who gave some insite into the innards of the Roadkill.  She
  also states the PCB is the same as the Brat.