Proco Rat bottoming out?

grolschie · December 16, 2008, 06:14:05 PM

Hi there,

I have a Proco Rat with LM308 in it. I am finding that with GAIN at 11-12 o'clock and higher it sounds good, but bottoms out when playing power chords. My bridge pickup is reasonably high output which I think may be the problem. My Bad Monkey don't seem to have this issue when gain up full though.

Any tips on where to start tweaking please?

Thanks in advance,
grol

Kerly · December 16, 2008, 08:49:14 PM

What do you mean by "bottoming out"
The rat does has quite alot of compression when the distortion is turned up

grolschie · December 16, 2008, 10:04:47 PM

Farting out. Rather than distorting nicely, it farts. Like it cannot handle the signal.

petemoore · December 16, 2008, 10:15:00 PM

Yup. I believe so, only from experience with Rats.

hanumanlangur · December 17, 2008, 01:41:42 AM

Yeah, it definitely does get farty with mine, too. Does anyone know how this could be changed? I'd like the pedal to sound full and clear.....

grolschie · December 17, 2008, 03:45:56 AM

I thought maybe decrease bass at input cap and increase at output cap? Or perhaps mess with the 4.7uf +560ohm and 2.2uf+47ohm combos?

gnognofasciani · December 17, 2008, 03:50:01 AM

Quote from: grolschie on December 17, 2008, 03:45:56 AM
Or perhaps mess with the 4.7uf +560ohm and 2.2uf+47ohm combos?

This is definetely the way to go

grolschie · December 17, 2008, 04:16:10 AM

Ok, trying to figure out how this works. So, does the 560ohm + 4.7uf combo (a low pass filter, right?) let frequencies below 60.5hz pass to ground? And the 47ohm + 2.2uf combo let frequencies below 1540hz pass to ground? I was using Jack Orman's Simple R-C Filter Cutoff to get those values.

As an aside, what is the difference between these four filters please:

0.005uf + 200k = 159.2hz
0.1uf + 10k = 159.2hz
5uf + 200ohms = 159.2hz
200uf + 5ohms = 159.2hz

They calculate to the same value. Thanks in advance.

gnognofasciani · December 17, 2008, 04:24:18 AM

Basically the difference is the gain (100k pot/560 ohm)...change the 4,7u with a 470n or 1u cap and the farty low-end should disappear

grolschie · December 17, 2008, 04:32:42 AM

Thanks for that. So that would let freqencies below 605hz pass to ground if I keep the 560ohm resistor?

grolschie · December 17, 2008, 02:21:35 PM

I will try a 470nf cap in the next few days and post back. Thank you very much. :-)

grolschie · December 18, 2008, 01:19:20 AM

I have swapped the 4.7uf for a 1uf and that has done the trick! I guess a 470nf might tighten it up more, but I accidentally bought a 470uf cap by mistake. I have a 470nf MKT cap, but the schematic shows electrolytic caps there so I didn't try it. A 1.5uf might be less dramatic.

Thank you very much gnognofasciani for the 1uf/470nf tip! And thanks to everyone else who replied.

grolschie · April 23, 2009, 01:51:44 AM

Hmmm... still not quite right. I notice according to the schematic traced by Soul Sonic that some Rats don't have an input buffer, whereas this schematic seems to.

I put a buffered pedal in front of the Rat and it definitely sounds different. Maybe I should reverse my mod and try a buffer in front?

lynessmy · April 23, 2009, 02:05:28 AM

your chip is from which manufaturer? Try to get the NATIONAL SEMICONDUCTOR one.....
I have this problem when using some unknown manufacturer version.

grolschie · April 23, 2009, 02:09:18 AM

I bought an LM308 for it a few years ago. My humbuckers put out a fair amount of volume.

aron · April 27, 2009, 05:38:34 PM

grolschie,

Did you get this fixed? I have the same problem with the Rat I just fixed. Mine sounds like grid overload on a tube amp or extreme sag. Every power chord will lower in volume and then fade back. Crappy sound. I don't remember this on my original big box Rat.

aron · April 27, 2009, 05:48:03 PM

Never mind. I had a booster in front of the Rat! As soon as I plugged straight into it, no problem. Actually it doesn't sound half bad now.

grolschie · April 27, 2009, 05:50:54 PM

Hi Aron,

I haven't pulled it apart yet, but putting a buffered pedal in front reduces the bottoming out. Maybe try that as an experiment.

Funnily enough I get the sound that you describe with my ROG Eighteen pedal when I use humbuckers. Exactly as you describe with "Every power chord will lower in volume and then fade back. Crappy sound.". I never solved that, so I don't use the pedal anymore. I wonder if changing the first stage of the pedal to be more like the first stage of the Professor Tweed might help me?

What does the diode in the input of Joe Davisson's Vulcan overdrive do? It says "The diodes in the bias network prevent hard-saturation of the transistors."
Anyways, I wonder if something needs to be added to the front of both circuits? A buffer, or maybe something to limit the input?

grolschie · April 27, 2009, 05:52:21 PM

Quote from: aron on April 27, 2009, 05:48:03 PM
Never mind. I had a booster in front of the Rat! As soon as I plugged straight into it, no problem. Actually it doesn't sound half bad now.

Ahh... that makes sense. So an exaggeration of what happens when same pedal is hit with high output pickups perhaps?

aron · April 27, 2009, 07:07:39 PM

Maybe but I do have high output pickups - well fairly high. Super Distortion in the bridge.

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Proco Rat bottoming out?