Dunder Chief

H

HamishR

Well-known member
I'm really enjoying my Dunder Chief/Dirty Deed overdrive. It's surprisingly good at a classic marshall kind of OD - perhaps a little too dirty at the uppermost ranges of gain but overall it has a cool midrange and reasonable thump. It has that most important aspect of any pedal - it makes you want to keep playing.

But as with a real Marshall it can get a little furry on the low strings and i like a bit more twang. If I compare it with a Rockett Majestic it has a slightly chirpier midrange which is great but it hasn't got the definition in the low strings i like. So I'm wondering - if I use a lower gain pair of transistors rather than the MPSA18s would that make things a little less fuzzy? I only use the gain at around 10.00 so there's plenty of dirt to spare.

Turning the bass knob down doesn't really achieve any more twang. Thoughts?
 
Changing Q3 & Q4 won't get you there, they do not make gain or distortion in this circuit. If you want less bass and you don't use the high end of the DRIVE pot, then I'd suggest making R8 smaller so you have to raise the DRIVE higher. Turning the DRIVE up cuts the bass. Try 33K for R8. If that doesn't cut out enough of the bass, reduce C4 & C7. Try cutting them in down to 1uF. If you still want more bass cut, take them down even more. Those two caps control the bass content before the clipping, so they should have the most effect on the fuzziness.
 
It's not the bass I want to cut, it's the fuzziness. So it sounds like if I reduce the value of C4+7 that might help. I like the amount of bass when the bass knob is at 12.00. If I reduce the low frequencies before the dirt can I get them back after?

Oh, and thanks, BTW!
 
Sure, just turn the BASS up to compensate for bass lost by the mod. When you said "it can get a little furry on the low strings" I took that to mean that the clipping stage was getting too much low freq content. Do you have any clean boost with EQ you can put in front of the Dunder to try out my theory?
 
Hmmm... I don't think so. But I agree with your theory that too much low end before clipping is probably the culprit. I'll just have to do some experimenting. I am worried that changing things too much will lose what it is that I like about the pedal, but I think changing C4 + 11 for smaller values sounds like a good idea.
 
Well I have built another Dunder Chief with a few mods to try to achieve better low-string definition. Following Chuck's impeccable advice I swapped C4 and C7 to 1µF and used MLCCs.

Also, flouting Chuck's impeccable advice I swapped C1 for a 47µF simply because I think 10µF is too small for the main filter cap. From building amps I have experimented with filter cap values a lot in my quest for tight, big low end. And I do think using around 47µF helps in this regard. 100µF is necessary in some pedals but I think 47 seems to work just fine here - It helps get a little thump where there was none before. I know a lot of people who know much more about these things disagree and maybe they are right but I am sure I can hear/feel a difference. And it can't hurt anything!

While I was at it I made C18 10µF - and maybe I shouldn't have!

But the proof is in the testing, and so far I like the difference. I am using the pedal at quite low-gain levels - Maybe I should have used a smaller value gain pot. I have gain at around 9.00-ish and the bass strings are definitely tighter. Keeping bass below noon I have enough low end and even a little thump which is cool. Using a Gibson 335 I am getting some really cool sounds - grindy but with enough hi-mids to make it sing. It's definitely more "my sound" and I suspect a lot of players would like it.

Comparing it with my standard (at the moment) Majestic the modded Dunder Chief is a little fatter sounding and has a slightly more pronounced hi-mid peak but is not a raw or quite as natural sounding. But it's still a great OD and I suspect I will get quite a bit of use from it. Haven't tried it with a Strat yet - I think it will sound killer.

So thanks again Chuck! Your advice was spot-on.
 
Hey, whatever works, Man. I like large main filter caps too. If you do decide to change the gain range, you'll want to make the DRIVE pot and R5 larger. Or make R8 smaller.
 
Phew! I was worried you might think I was nuts!

So to reduce the amount of gain make the driver pot bigger? That goes against everything I have learnt so far. But then I rely so much on others to understand this stuff. If I want to have more room on the gain pot between clean and dirt should I decrease the value of R8?
 
Here's the formula (assuming that you are at mid frequencies where C4 and C5 are not limiting the gain):

gain = 1+ R8 / (R5 + DRIVE)

At 9:00, the DRIVE pot is around 6.5KΩ. That would make the gain 15.9 or 24dB.
At 12:00, the DRIVE pot is around 1.5KΩ. That would make the gain 59.8 or 36dB.
Notice that the DRIVE pot is wired so that as you turn it CW, the resistance goes down.

I'd use this process to arrive at resistor values:
1) Find the min and max DRIVE settings that you use now.
2) Figure out the resistance of the DRIVE pot at those two settings. You can use the graph below. The standard C-taper we buy is the 15C curve.
3) Don't change R8.
4) Calculate a value for R5 that gives you the desired max gain assuming the DRIVE pot resistance is 0Ω (full CW rotation).
5) Calculated a value for the DRIVE pot that gives you the min desired gain when the DRIVE pot resistance is maxed out (full CCW rotation). Pick a pot resistance with the next higher standard value.
6) If the pot resistance is < 10x R5, use a linear B-taper pot instead of C-taper.

Questions?

Alpha C-Taper curves.PNG
 
Oh no, no questions... Just going for a lie down now...

Actually some of that almost made sense to me. And suddenly it dawned on me - I kinda like how it works now - why change it?

o_O
 
So all of that typing was for naught? :eek:

I've hear that for some people, algebra has nasty side-effects like headaches, nausea and flashbacks to high school exams.
 
Haha! No, It wasn't for nothing. I always appreciate your responses Chuck; it just takes me a little while to absorb them. I've never heard of a gain control which works in reverse before.

I'm pleased to have been able to adjust the response of this particular pedal because I like the basic tones it has. I have removed most of the fuzziness on the bass strings thanks to you and will experiment further.

It's weird how this stuff works - my brother would understand everything you've written in a heartbeat - he's a computer genius kinda person who works in such complex, basis of all knowledge kind of computing that I don't even know what it is he does. But he can't play guitar and I can.:cool:
 
HamishR said:
Phew! I was worried you might think I was nuts!

So to reduce the amount of gain make the driver pot bigger? That goes against everything I have learnt so far. But then I rely so much on others to understand this stuff. If I want to have more room on the gain pot between clean and dirt should I decrease the value of R8?
Click to expand...

Everything chuck said is right on as usual... just thought I’d chime in with a real simple comment as to why larger pot in this instance means lower gain.

It’s a matter of WHERE the pot is. Most op amp dirt circuits (TS, rat, Timmy, etc) put the gain pot where R8 is, and in this spot larger R means more gain. Where the gain pot is on this circuit, a larger R means less gain. That’s why it’s counterintuitive from what you’re used to.
 
For the time being I will leave the gain pot alone because (a) I can get the low gain sounds I like with it as-is, and (b) it seems that the Strat likes it at around noon. With the mods I've made this is now a really impressive OD for a traditional Strat bridge pickup.

In hindsight I don't know why they used 4µ7 for C4 and C7. 1µ sounds so much better.
 
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