haggy
First Lieutenant   Posts: 4 Registered: 10/1/2008 Status: Offline
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posted on 10/1/2008 at 03:38  |
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 datasheets 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
quess a guitar amplifier shouldn't be that big load... But I quess it
influences the LM308 too much?
What do you think?
Any ideas? 
Also all modding ideas and other commenst about the Rat are welcome!
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haggy
First Lieutenant Posts: 4 Registered: 10/1/2008 Status: Offline |
posted on 10/1/2008 at 03:40, Reply 1 |
And forgot to ask: does anyone have any suggestions for the buffer
transistors?
I quess some JFET with about the same Id values... But if the source
resistor is changed I quess many other JFETs would work too...?
[Edited on 10/1/2008 by haggy]
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Guitar101
Moderator   Posts: 453 Registered: 12/22/2003 Status: Offline |
posted on 10/1/2008 at 19:57, Reply 2 |
I always go with the schematic for starters then mod using my ears. I would
start without any buffers if it was me.
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haggy
First Lieutenant Posts: 4 Registered: 10/1/2008 Status: Offline |
posted on 10/2/2008 at 06:04, Reply 3 |
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 quess 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...?I
It might even oscillate?
Has anyone here tried this with a rat?
[Edited on 10/2/2008 by haggy]
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haggy
First Lieutenant Posts: 4 Registered: 10/1/2008 Status: Offline |
posted on 10/9/2008 at 07:18, Reply 4 |
There is a LOT of big errors in the schematics you can find in the net. I
think the best one is at www.generalguitargadgets.com , 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 a bad quality...
[Edited on 10/9/2008 by haggy]
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Limbe
First Lieutenant Posts: 4 Registered: 10/23/2008 Status: Offline |
posted on 10/24/2008 at 04:43, Reply 5 |
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 .
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