Modding a Rocktek ODR-01 Overdrive Pedal

[Kiff]

Hi, I'm new to this forum and decided to ask for some help. I recently purchased a Rocktek ODR pedal for my Marshall Valvestate VS65R Amp because the overdrive channel on the amp is appauling at best...its very muddy and there's more bass than treble even with the bass turned all the way down. Pulling off pinch harmonics etc is a frustrating chore.

The pedal adds about a third more drive to the amp but I want more (cant afford Mesa, Peavey, Boss etc very very poor uni student lol) basically I wanted to know if its possible to change anything on the pedal PCB that will increase my drive or increase/add distortion. Ideally I'd like to tune an octave lower than standard without a whammy pedal and this means I need more gain/treble/distortion to compensate for being in the tonal world of a bass

Hope that made sense, any help appreciated.

[MartyMart]

Hi,
go to  http://www.jpl-audio.com/pages/8/index.htm   here you will find lots of info and links to great sites, offering tips on these kind of mods.
It should be possible to get the result you want for about £2 worth of parts from Maplins or RS components, and 1 hr of your time !.
Hope this helps,
Regards,
Martin.

[Kiff]

Hi, thanks for your help I found some interesting pages and articles but nothing that couldn help me.

Unfortunately I'm not so "down" with electronics theory that I couldnt just open it up and figure out which part is doing what. That the only thing I havent been able to find out on the net. An in depth article or web page telling you how a distortion pedal works, what part does what and how to change that. I reckon if I knew this it would benefit me greatly and lead me on the path to archetyping my own pedals.

Any ideas please?

[Lonestarjohnny]

If you go over to the Geo site there is an abundance of info about what does what inside a effects pedal, very good reading and easy to understand on most count's,
JD

[Mark Hammer]

There is a VERY good chance that the ODR-1 is a variation on the Tube Screamer. or alternatively the MXR Distortion+.  You will find an excellent article that explains a great deal of what is going on with the Tube Screamer here: http://www.geofex.com/ .  If you click on the "Technology of..." link in the upper left corner, you'll see a link to the Technology of the Tube Screamer.

Give the article a read, even if your pedal is different than that.  When you're done, you should be able to ask some questions that will get you a little closer to what you need to ask.  The idea is notsomuch to make you able to figure things out entirely on your own, but to be able to ask the sorts of questions that will squeeze the best advice possible out of the rest of us.

I took a quick look at the schematic of the preamp in your amplifier (http://www.drtube.com/schematics/marshall/vs65-60-02-2.gif)..  If you look at the lower left hand corner of the diagram, you will see what may be the source of your "problem".

You should see a triangle, labelled IC4.  IC4 and IC2 beside it make up your amp's internal "overdrive pedal" and in fact are not that much different from a number of commercial overdrive/distortion pedals.  IC4 is an "op-amp".  It provides a variable amount of boost/gain (determined by the 1megohm control (that's what the A1M means) you see below labelled VR5.  That pot sets the amount of feedback from the output of the chip (pin 5) back to the input (pin 6).  The amount of gain created is set by the ratio of VR5 to R43 (just to the left).  As the value of VR5 gets bigger, the amount of gain goes up.

Because there is so much potential gain available in that stage, it is common practice to also stick a capacitor in there to keep high frequencies from going berzerk and oscillating.  That capacitor is shown here as C44.

One of the things about capacitors used in this way is that they roll off the high end by an amount depending on not only the cap value, but the value of the resistor it operates alongside of.  In this case, the value of 220pf means that when your boost control is maxed, the treble rolloff starts around 723hz, which is pretty low (this is also roughly where the Tube Screamer rolls off as well, but it also rolls off the low end as well so that the highs don't seem to be as muted - it's an illusion).  The long and the short of it is that your overdrive section is designed to tame the highs as gain is increased, but it may do so a little more aggressively than you would like.  

Fortunately, that capacitor can be changed at very low cost and effort.  Assuming you can identify it on the board (and I am hoping things are legended onthe component side), replacing it with a 100pf cap should tame the highs a little less aggressively so that you don't have to sacrifice bite for drive.

Hope this is helpful.  Welcome to the jungle.

[Kiff]

Wow thanks alot dudes, I'm going to read the articles on Geo right now and the explanation of the gain problem on my amp was fantastic. I think that I'll get someone more experienced with amps to change the capacitor, I'd do it myself but I've heard that its quite risky modding amps cuz of the high voltage, am I correct? or is this just applicable with tube amps?

Also a question that just popped into my mind. Is it possible to add distortion to an already overdriven signal without the sounds ending up..well...crap?

Thanks again everyone!

[petemoore]

Also, changing the input capacitor of the ODR to a smaller value will roll off more bass which makes the treble frequencies occupy more % of signal content...also you could mess with the coupling caps or output cap.
 An easy way to get more drive, and alter treble to bass content, is use a [simple] booster stage before the OD, and mess around with the voicing of the booster, by trying different value capacitors for the input [1rst], output, and coupling caps [if any].
 Since you said the highs are weak, disregard: adding high end rolloff cap from signal path to ground...trying different values/different places in the circuit.

[Mark Hammer]

Kiff,

Probably smart to have someone with more experience get in there and do the work neatly and safely.  Personally I'm more concerned about the amp's safety than yours.  Your safety is generally assured by pulling the plug while the power switch/LED is on and waiting for all lights to dim.  Would that  the amp's safety is so easily assured!

The smarter thing to do is to arrange to watch whomever does the work so you can learn a little practical technique.  I realize not everyone wants someone "hovering" while they work, but you may get lucky.

The amp mod I suggested is easily reversed if there is a desire for a stock unit, and even if you can't go back and reverse it, the tone controls can easily be used to remove whatever treble has been added.

[Kiff]

Hey Mark, I just read the Tube Screamer Article and some of it was hard to understand and some of it was easy to understand. The last I heard of electronics was GCSE Pysics lol

Any recommended reading I should do that would help me better understand whats happening.

I understand that creating distortion involves clipping the signal waveform, what I eventually want to understand is how to choose/put together different parts to achieve this.

The article was very informative and i have learnt quite a lot more in the last 2hours than I already knew.

What I'm going to do is take a picture of the ODR circuit and post a link here for you guys so you can take a look, from what I can see it doesnt look like its based on the tube screamer...then again I could be (and most likely are) wrong

[Mark Hammer]

Do not underestimate how much you can learn but printing out a number of the schematics for distortion/overdrive/fuzz devices you can find via the schematics link above and at other places such as www.generalguitargadets.com and www.tonepad.com.  These latter two sites also have a number of mods suggested by builders.  You'd be pleasantly surprised how much you can learn from switching back and forth between a couple of similar devices while on the bus or toilet.  A lot of "productive staring" can often be every bit as helpful as formal training!