Skip to Content

Pro Co Rat Input & Output Buffers

haggy's picture

Hi, I'm thinking about building a ProCo Rat, and I'm wondering about the input and output buffers. There are many schematics in the internet, and they differ a lot. In some versions you don't have the input buffer at all, and in some versions the buffer transistors have different biasing or different transistors. Some have even BIG errors. Errors with the biasing, errors with the transistor symbols.

http://www.kilback.net/homebrewtweaks/pedals/pcratc.gif
http://diystompboxes.com/pedals/PCRAT1.GIF
http://www.montagar.com/~patj/pcrat.gif
http://www.montagar.com/~patj/pcratb.gif

Since I have no access to 2N5458 or BF245A transistors I'm thinking of designing my own buffer circuit for the output. Here are the data sheets of these transistors:

http://www.nxp.com/acrobat_download/datasheets/BF245A-B-C_2.pdf
http://www.fairchildsemi.com/ds/2N/2N5459.pdf

I understand that the devices are symmetrical, and that means it really doesn't matter which way you connect the drain and the source. The second thing that catches my eye is that it seems the devices are supposed to be run with negative Vgs voltages, and that at 0 Vgs, the transistor is fully on. In other words the cut-off voltage Vgsoff is negative.

I read somewhere that some guy has made this circuit with different in/out buffers, even with no buffers at all. It's looks like the output buffer is more crucial, since in the latter Rat versions the input buffer was discarded anyway. What would the circuit be like without both buffers? I guess a guitar amplifier shouldn't be that big load... But I guess it influences the LM308 too much?

What do you think?

Any ideas?

Also all modding ideas and other comments about the Rat are welcome!

No votes yet
haggy's picture

And forgot to ask: does

And forgot to ask: does anyone have any suggestions for the buffer transistors?

I guess some JFET with about the same Id values... But if the source resistor is changed I guess many other JFETs would work too?

Guitar101's picture

I always go with the

I always go with the schematic for starters then mod using my ears. I would start without any buffers if it was me.

No day better then today!

haggy's picture

I'm just wondering, that in

I'm just wondering, that in case I don't have the output buffer, the amp I connect the effect to and even the cable might have really big effect on the outcome - the filter and the distortion of the main circuit (LM308 & co.) would probably work in a different way.

I guess the circuit before the output buffer depends on the fact that there is a high-impedance load (a JFET should have at least a higher input impedance than a BJT). If that load is instead composed of the impedance of the amp I'm using and the cable, anything might happen? It might even oscillate?

Has anyone here tried this with a rat?

haggy's picture

There is a LOT of big errors

There is a LOT of big errors in the schematics you can find in the net. I think the best one is at generalguitargadgets, the Rodent. I would scratch the LED driving circuit but otherwise it looks good.

The others have several problems: wrong biasing for the buffers (it's a JFET for god's sake!), positive feedback instead of negative (we don't want to build an oscillator, do we?), and so on. The B.Y.O.C. mighty mouse schematic even has the supply voltage protecting diode the wrong way around, effectively shorting the supply voltage to ground. Nice job!

It's a shame the free schematics are of such bad quality.

Limbe's picture

Hi Haggy! The second and

Hi Haggy!

The second and fourth schematic you posted are correct for the Rat and will both work fine!(I know because I´ve built both of them). The 2N5458 shouldn't be hard to find and must be connected the right way!

I've seen prices from 45 cent so I see no reason for not buying it since the switch and box will cost you a whole lot more!

I wish you good luck with your project.