Turbo RAT Trace

[GGBB]

I recently picked up a used Turbo RAT and traced the circuit today.  It is very close to Soul Sonic's RAT-2 schematic.

What really surprised me though was that there was a 2.2M resistor on the back (trace side) of the board connecting the drain and gate of the JFET.  I know next to nothing about circuit design so I have no idea what this would be for, and I can't know for sure if it's original since I just got the pedal.  It's probably from 1990 (pot dates are 8950) and has the 12/86 Rev. L PCB.

Can anyone shed any light on what this might do?  Does anyone have a Turbo RAT with the same thing?

Here's a revised version of the Soul Sonic schematic with the Turbo RAT differences noted:
http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/ProCo_TurboRAT_amp_RAT2_Schematic.gif

And a couple of hi-res images:
http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/ProCo_TurboRAT_PCB-1.jpg
http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/ProCo_TurboRAT_PCB-2.jpg

[Chugs]

The addition of a 2.2M resistor from drain to gate, along with raising the 1M from gate to ground to 2.2M provides extra headroom for the Fet buffer. Using two 2.2M resistors in this fashion give the buffer a roughly 1M input impedance. This is a better form of biasing than the usual 1M from gate to ground. I have done the same thing to a couple of Rats I have modded.

[GGBB]

Thanks Chugs.

So this and the 2.2M from gate to ground are part of the same mod - is that correct?  How does this mod change the sound?  Less compression?  More responsive to touch?

[Chugs]

Yes, the 2.2M to ground is part of the same mod. The two 2.2M resistors bias the buffer and set the input imdeance of the buffer stage at roughly 1M. This mod gives the buffer a little more headroom. The buffer is getting hit pretty by the preceding circuit and distorts very slightly which, in turn, imparts an additional tonal change to the sound. Giving the buffer a little more headroom gives a touch more clarity and tiny bit less gain to the circuit. It is pretty subtle change, but I like it.

[GGBB]

Thanks again Chugs - very informative.

[GGBB]

Just an interesting follow up note.  I took a close look at my Vintage RAT reissue which has the 1995 Mutli-RAT PCB.  This mod is actually designed into the Multi-RAT PCB - it has a spot to connect a resistor between drain and gate.  But in the Vintage RAT reissue it is not implemented - it just has the single 1M resistor from gate to ground.  I would infer that this means some RATs were actually manufactured with this circuit variation.

[GGBB]

Update:  I've redrawn the schematic and included every difference between all the various Rats that I know about:

http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/Multi-RAT.png.html

[Chugs]

Nice, Very handy. C11 should be 3.3N.

[GGBB]

Good eye - thanks!  Correction has been made.

http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/Multi-RAT.png

I've added a couple of gut shots of my 1998 Vintage Rat to the gallery as well.

http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/ProCo_VintageRat_PCB-2.jpg
http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/ProCo_VintageRat_PCB-1.jpg

[aelling]

Bit of a bump here but, if it's any useful here you go.

I have a newer Turbo Rat with the RAT 3 Rev. B board, it has the OP07DP chip, but it also has a 1µF to Volume 3 instead of a 10µF.

picture for reference

[GGBB]

Yes - all newer RATs with 3PDT switching use the original RAT circuit. On the schematic see the box titled "The Original RAT Circuit is Restored!"

[aelling]

Yes - all newer RATs with 3PDT switching use the original RAT circuit. On the schematic see the box titled "The Original RAT Circuit is Restored!"

Ahh, I see now. I got confused cause of the component differences in the columns, and I am surprised I didn't notice that when I opened mine up that the other components where different too  . Thanks