Proco Rat Redesign

[azrael]

I am currently fiddling with the Proco Rat design, and one of my questions was about the 560/4.7 and 47/2.2 pair coming from the clipping section of the LM308.

Is it really necessary to use both of those pairs? They have different frequency rolloff values, approximately 60 and 1500 hZ respectively.

But do they both contribute, or can they be replaced by a single pair?

I often see the popular "lube" mod, and wonder if it could be more effective if there was only one pair.

[petemoore]

  They're included for a reason.
   They are indeed involved with frequency/gain, the impedance rises or falls @ certain frequencies.
   . and could be replaced with 1/2 as many components, it'd be a variation on the theme.

[jorri]

they are of course frequency response so i'll say its taste... and gives the particular rat sound in my opinion.

I have a rat clone with tons of mod switches, and one of them almost does what you are suggesting. it just jumpers the two together(increasing capacitance though) for a bass fuzz, but the complexity is gone and its quite fuzzy in a boring kind of way.

another mod just cuts out the 47R side, and thats quite nice a soft overdrive, but all the highs are lost and it becomes like a dark low-gain drive. you can boost the 560 side and you get a more standard drive with the 47 still cut. and cutting out the other path seems to make a sound that's all ice-pick and no mids. just the
so the47 side adds that icepick type sound (which is why i use the rat, and why others want to change it beyond recognition), the 560 adds most the body of the sound- but its quite useful having the resistors as trim pots. i usually increase the 47r resistance to avoid feedback, and decrease the other for some added fatness. its really worth experiementing with it!

[PRR]

> Is it really necessary to use both of those pairs?

"Redesign" is relative to try-and-see. If you were tuning the SkyLab, it might be cheaper to figure it out first. In THIS case, it is cheaper to add/cut the 30-cent parts than to do ANY thinking.

> different frequency rolloff values, approximately 60 and 1500 hZ respectively

Gain is different in the 20Hz to 200Hz band. The 560r and 4.7u add bottom... the pure 1500 bass-cut could sound gutless.

This would be tedious to compute by hand. Cheating with a computer took me about a half-hour. At my brain-rates that would buy all the parts in a Rat.

BTW, 10 seconds in Google found a Wikipedia article which addresses the 560 and 47 ohm resistors as a sonic mod......

[azrael]

Well, I'm well aware of the Ruetz mod.

I was just wondering if the pair actually do something separately, or if they just act as a single resistor/cap combo.
The answer seems to be that each one does contribute something, and simulating the same frequency response with a single pair will not yield identical results.

[dhan523]

Can anybody explain to me the jfet near pin 2 of LM308 from the Keen/Orman Rat? I am more interested in the effect and tone changes of this Jfet in my build since I have been opting this build than the tonepad diagram. Does this create more of a tube tone?

Thanks for the help

[GGBB]

I have far less electronics understanding than may others (probably most) here, so I am at a loss as to explaining the intricate technical details of the two filters.  However, what I understand is that, as mentioned already, they are more critical than ordinary filters because they are controlling gain via the feedback loop.  For my simple mind, it helps to look at them that way, not just as a passive filter.  So again as already mentioned, there two of them for a reason - changing the setup will affect the tone and move you away from the classic "RAT" sound.

IMO, the Reutz mod gained popularity because maybe the most common change people wanted from the RAT was more bass, and it was a really simple mod to clip out the 47R/2u2 to achieve this.  The variations (trimpots, clip out the 560R/4u7 side, ...) stemmed from the fact that that mod changed the tone way too much, making it sound far less RATish.  I never tried the Reutz mods myself, but have read a lot of comments about them, and I did some playing around with the circuit to try to get a better sounding bass boost.  As I see it, the problem with playing with the resistance values only, as in the trimpot version of the Reutz mod, is that as you change resistance you also change the filter's knee.  When you do that with the bass gain part of the pair (the 560R/4u7), you raise the knee so you end up getting higher gain but higher cutoff of the HP filter - the mod kind of works against itself.  Lowering the resistance while raising the capacitance will increase the gain but keep the filter's knee at the same point.  I found these changes worked really well as a bass boost mod:

http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/RATBassBoost.gif.html?g2_imageViewsIndex=1

[R O Tiree]

Aren't the opamp inputs on that diagram the wrong way around?  Pin 2 is the -ve input and should be connected to 100pF, the LHS of the Drive pot and the R-C pairs to ground, and Pin 3, the +ve input, should be connected to the 100k (4.5V bias) and output from the input buffer JFET.

[Pollinator95]

Can anybody explain to me the jfet near pin 2 of LM308 from the Keen/Orman Rat? I am more interested in the effect and tone changes of this Jfet in my build since I have been opting this build than the tonepad diagram. Does this create more of a tube tone?

Thanks for the help

I think it's just a buffer, like in a TS (same principle, anyway).

[GGBB]

Aren't the opamp inputs on that diagram the wrong way around?  Pin 2 is the -ve input and should be connected to 100pF, the LHS of the Drive pot and the R-C pairs to ground, and Pin 3, the +ve input, should be connected to the 100k (4.5V bias) and output from the input buffer JFET.

Good catch.  I didn't really look at it, but I've seen that schematic before and it has other problems as well.  Original RATs don't have the BF245 input stage.  The voltage divider is different, and there's no power pin 7 on the IC.  150K distortion and 1K5 resistor are accurate for very early RATs, but 30p and .0033u are not.  I was never sure if this was some super rare early version or the bud-box rat, or if it is a forgery not from the indicated authors, but the pin 2&3 reversal problem would seem to indicate the latter.