Anyone here modifying (Marshall) pedals?

I recently bought a Marshall ED-1 (Edward the Compressor) mainly for the name :)

It does it's job well enough, but I did a simple mod that makes it much more usable. Don't know why Marshall didn't think of this, but I linked the bypass/compress LED to the gain voltage signal. So now as you play you can see how much compression is being added. Loud signals makes the little red LED dim.

I was so impressed with the mechanical construction of the ED-1 box I then bought a Marshall Jackhammer OD pedal.

As always with these things it has far too much gain for my taste and the overdrive sound is (in my opinion) awful. Part of this is the front end TL072 op amp clips easily and adds a nasty edge to the sound. So I reduced the gain by about 12dB, added some other tone shaping, and reworked the basic diode clipper radically, so now it offers asymmetric 2 stage soft clip (think overdriven FET or triode) or symmetric 2 stage soft clip for an overdriven amp sound using the OD/Distortion switch. This is no longer a shred pedal, more a smooth blues noodling pedal. Perfect for my uses.

Then I bought a Marshall Guv'nor GV-2, mainly because it has gold knobs... I did the same gain reduction mods and put in a 3 stage soft asymmetric clipper for that smooth class A overdrive sound. Also excellent.

Both of these pedals work really well with the compressor up front, so it can hold the signal in the sweet spot.

So: Do they sound like my home brew class A valve amp being overdriven?Not quite, but they are getting really rather close

Pete
 

Gnobuddy

Member
2016-03-01 4:10 pm
...2 stage soft clip...

...3 stage soft asymmetric clipper...
I like your recipe!


Whenever I've found an overdrive or distortion pedal I like the sound of (which is rare), I usually find it has multiple soft-clipping stages, AC coupled, with resulting DC bias shifts as the signal level changes, which in turn cause changes in the duty-cycle of the output waveform with changes in input signal level.

A single op-amp with symmetrical clipping diodes sounds awful to me, and yet this is exactly what you find in the vast majority of distortion pedals out there. At least the original (Big Muff) used multiple stages like this, AC coupled to each other.

This pushed me towards the conclusion that multiple stages, AC coupled, and drawing some kind of polarity-varying input current, is the only way to avoid that rather monotonous and lifeless, almost synth-like, sound that so many distortion pedals generate.

Then I Googled to see if this idea was already out there. Yup, an engineer named John Murphy figured this out decades ago, and built amps to prove the concept: https://www.trueaudio.com/downloads/EET_jlm interview-lg.pdf

It turns out Murphy also designed a line of guitar amps (in the 1990s, I think) using this concept for Carvin Audio (recently defunct). His circuits used op-amps, but also diodes and resistors and caps arranged to cause level-dependent changes in output waveform duty cycle.


-Gnobuddy
 
Thanks for that article. It confirmed a load of theories I have floating around in my old head. In fact it seems my two home brew amps follow his ideas. I have a front end JFET with adjustable bias and load. That certainly inverts and soft clips asymmetrically. You can see the result is more 2nd than 3rd harmonic. The signal then goes to the usual 12AX7 which also soft clips asymmetrically, and the result sounds quite harmonious to me.

Contrast that with the usual clipped op-amp or hard diode clip in countless overdrives pedals...

My fight with the Marshall GV2 and JH1 has been to reduce the ridiculous amount of front end gain so the op-amps don't clip all the time. Then I've adjusted the diode section to be less aggressive and the result is starting to be more to my taste.

Now I need to rename the pedals**

"Jackhammer" seems a bit aggressive for the new soft overdrive. I'm thinking something like "Slightly cross Sloth" - maybe?

**I think most of the creative energy that goes into stomp box design is expended in thinking up a suitably nasty name for the thing.

:)
Pete
 

Gnobuddy

Member
2016-03-01 4:10 pm
Contrast that with the usual clipped op-amp or hard diode clip in countless overdrives pedals...
Here is one of only two or three pedals that overdrive in what I consider to be a good-sounding, amp-like way. It's a rather unusual circuit: 3 Legged Dog

In my starving student days I've used 4049 digital gates biased into linear operation as cheap, crude op-amps. But I've never built this particular circuit. On-line sound clips are impressive though, when turned up to classic-rock levels of distortion. (I don't know if this circuit does subtle bluesy distortion well.)

However: whenever I tried it, I found that CMOS inverters biased for linear operation were very noisy. I wouldn't be surprised if the rich distortion from the Three Legged Dog was accompanied by lots of hiss.

Another good-sounding pedal that might be to your taste is the old Wampler Plexi-Drive. A Youtube demo: YouTube

Google will turn up an unofficial schematic for the Plexi Drive. The JFETs will not bias properly if built as drawn; I think there was an error made by the person who traced the circuit. But with your engineering background you'll have no trouble fixing that little problem - just tweak the JFET source resistance values as needed to set all three drains a little above Vcc/2, i.e. about 5 volts with the usual 9V power supply.
I'm thinking something like "Slightly cross Sloth" - maybe?
What do you think of a slight tweak to your idea: "Cross-eyed Sloth"? :)
**I think most of the creative energy that goes into stomp box design is expended in thinking up a suitably nasty name for the thing.
That and the mandatory childish comic-book graphics painted on the outside of the pedal...

Now that a large fraction of the world's adult population appears to be suffering from arrested psychological development that has left them with childish minds in adult bodies, perhaps "Buzzy McBuzzFace" would be a more popular name? :eek: ( 'Boaty McBoatface' wins naming poll for $300 million ship - CNN )


-Gnobuddy
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Actually, Fuzzy McFuzz Face would probably sell well.
Using British understatement, I should name it "The Not That Bad Pedal"

Up the road here in Maine USA, there was a shop "Pete's Pretty Good Ice Cream". Maine's own form of understatement.

Did it sell well? Pete closed last year.
 

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very nice!

I got a Guv'nor II as a project. Here's what I figured out about it, and did:

Marshall Gv2 ' Guv'nor Plus' Overdrive/distortion Pedal - How It Works & Mods | MarshallForum.com

Yes, I swapped the infamous C22 for something much smaller, together with all the asymmetric diode mods and considerable gain reduction. It is now a much better behaved pedal, and has a second use as a clean pre-amp with useful EQ if you keep the gain low.

Turn up the gain and it starts to crunch nicely. End stop the gain and it goes fizzy and nasty. Not my cup of tea, but it is available in a rock related emergency.

Pete