Swapping out JRC 4558DDs

To start I am very new to this, as in no experience whatsoever. The gist is: I want to swap out the jrc 4558dd on the over drive channel of my Marshall mg100dfx for the lm308. My thinking is it will make it sound like the pro co rat. Thanks. Please tell me if this is stupid, or if there’s a way to execute on the idea.


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So many distortion myths get passed along on the net.
Maybe good intentions. Many can be misleading.

Main key with distortion is gain stages and clipping thresholds, and filters.
Marshall design is way above pro co design far as filtering
and clipping stages.
Most the magic is filtering, then being aware of what gain levels work
and the actual real world diference between diodes in feedback path
or diodes after the opamps.

Pro Co sound is mainly the smart way they did the highpass filter
which is not as aggresive as some to clean up the distortion. and allow
more bass.

Otherwise most the sound is my more favorite way to do things
is just get distortion from the opamp. then use additional output diodes
to bring down the output levels. Diode sound not really there to much.
Its raw opamp distortion. I would say the Pro Co , Marshall Governor
and the Big Muff opamp are all good examples of opamp distortion.
Not to mention filters being more important. So digitech did well with
the Grunge pedal. since the filters and Eq are better.

Usually you dont need more than gain of 100 to get rocking distortion.
Pro co blast the opamp up to 300. So most keep the drive level low
because even at low settings the gain is at 100 at all saturation you need.

Otherwise if you crank it up. People like the super saturation crackle and noise
and splatter you get when you go to far with gain. Which is artifacts of the opamp.

Far as blasting opamps for distortion. I find the Fet TL072 / 74 series the best and smooth.
Far as the " slow" slew rate myth that gets stretched to all ends with the pro co.

It does have a certain splatter and rawness like old fuzz pedals. But filter pretty important.
High end is horrible. but the feels in the mids is like old transistors circuits. Spongy
Grungy, doomy blah blah.

Youll get it with any slow opamp. I got good slew rate splatter with just ua741
worked better really for getting " fuzz" pedal splatter with opamps.
Years later I did find the bigmuff opamp. And low and behold the specifically use
a different opamp to drive hard for the fuzz. With real world listening test
741 always worked the best. Low n Behold that what was in the big muff opamp.

But after awhile is just a opamp getting blasted, most the magic is the pre filters, and
post filters. And if you want heavy sustain metal tone with multiple stages.
Or just splatter the krap out of it for fuzz tone. Gain either 100 like normal.
for splatter go to 300. just one stage with to much gain. Otherwise metal pedals get saturation by using
2 or 3 stages with filtering to get smooth but heavy crunch.

Pro co works fine, final tone filter is limited. As mentioned Grunge pedal and numerous
other digitechs mainly took the opamp sound or " Rat" technology.
And just used better filters and added better
EQ. like 2 band or 3 band with mid cut. More important for getting good final sound.
Or you can highpass the opamp distortion more. then with active 2 band add the bass after.
instead of allowing to much bass into the clipping circuit
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