Author Topic: Old Rat  (Read 1670 times)


Old Rat
« on: May 24, 2019, 11:06:39 PM »
(Edit:Now that I know it is not so uncommon, I changed the title of the thread so I don't mislead anyone.)

I've got this Rat pedal that I think is an oddball transitional era Rat pedal. I was looking for a schematic to make a repair and came across a multi-rat schematic that originated from here, so I thought I'd share.

First I wanted to quickly talk about my repair. I had a broken tab on the distortion pot. I searched all over for an identical replacement. I thought I found an exact match, but the shaft was too short, so I tore down both pots, and just replaced the part that had broke. You can hardly tell.

Now the reason I think this Rat is uncommon is because it is dated 12/86 down below the foot switch. Then it says, "Design SRB 1974-1986".

So is this the original circuit in the GID box? I don't know, but that's what that would lead me to believe, though I haven't been able to find any information on it. Even ProCo ignored my question and answered my other question instead.

I bought it used in '91 off my neighbor.

« Last Edit: May 25, 2019, 05:49:41 AM by Crazydave »


Re: Uncommon Rat
« Reply #1 on: May 25, 2019, 02:06:20 AM »
Yours pot-dates to the 40th week of 1989.  Should be a "Rat 2" I think.  It has two transistors (JFETs), the earlier models only had one.  I'm pretty sure the added JFET is an input buffer.  SRB would be Scott Burnham, the engineer at ProCo who developed the circuit in the 70s.  12/86 was probably when that particular PCB layout was designed.


Re: Uncommon Rat
« Reply #2 on: May 25, 2019, 05:45:23 AM »
Awesome! That's the most info anyone has been able to give me before!

I mean it is disappointing it's not what I thought,  but that's kind of irrelevant because  I always loved this pedal, that's why I keep fixing it instead of buying a new one. Any rate, just nice to know.



Re: Old Rat
« Reply #3 on: May 25, 2019, 12:16:34 PM »
Definitely worth fixing, I like how you repaired the pot.


Re: Old Rat
« Reply #4 on: May 26, 2019, 01:40:15 PM »
I'm pretty sure the added JFET is an input buffer.

It's not an input buffer - but that's a common misunderstanding. The only RATs that had an input buffer were (if certain RAT schematics found on the web are to be believed) the ~12 hand built prototypes known as "bud box" RATs.

This extra FET was added to the RAT2 in order to have a status LED while in bypass using a DPDT footswitch. It's part of what's become known as the "RAT bypass" circuit which inspired the "Millennium Bypass" popular among DIYers:

This pedal is an early RAT2. These are sort of valuable/collectable now assuming you can find a buyer (prices on the bay can be ridiculous but we never know whether they actually sell or not). It is probably not a first issue or first run because the earliest RAT2 PCBs (based on photos on the web) are green on the component side. The 8940 date code fits with this PCB run (my Turbo RAT has the same PCB and has 8950 pots).

It uses the "RAT2 layout" which was later used by the Turbo RAT as well. Generally speaking, it is the "original circuit" - as all RAT2s are - except for the Rat Bypass circuit. Details are in my Mutli RAT Schematic below - the RAT Bypass section is shown in green.

Pro Co RAT Schematic - "Multi-RAT" Comparison


Re: Old Rat
« Reply #5 on: May 26, 2019, 02:52:56 PM »
Thanks for the clarification, Gord.  I saw the LED and DPDT, but it didn't register in my foggy brain.