Another Project: Sixteen Overdrive

[vanhansen]

Ok, so there's a million overdrive circuits out there but I couldn't help myself.  Since doing the Marsha Valve, I decided to add another FET stage to it to see what would happen.  I got a pleasant surprise after some tweaking.  This circuit is actually quite similar to the first two JFET stages of the JCM800 Emulator by Electric, I won't deny that, but with a few part changes/additions.  It's named Sixteen Overdrive in honor of the number of years the JCM800 amp was in production.  Being that it started with a Marshall preamp schematic, I wanted the name to somewhat relate back to that.

Schematic is here: http://aronnelson.com/gallery/vanhansen/sixteenoverdrive

To get more low end out of the circuit, the 0.022uf caps off the drain of each FET were replaced with 0.1uf caps. This really filled it out. The 1M resistor from gate to ground on Q2 is necessary to keep the bias of Q2 stable. The Volume pot is 1M because it simply sounds better and more open than a 100k or 500k with the addition of more headroom. During testing, a fixed 1M resistor was put in place of the Gain pot with good results making a nice 1 knob version. Finally, a low-pass filter using a 10k/0.001uf combo was added at the end to take out just a little bit more of the highs.

Enjoy!

[sonic66]

Hi. I've  made a couple of ROG Fetzers and liked them alot.

I had some extra parts around and thought i'd try the Marsh variant that you've presented. I. only had a 100n cap for the resistor cap pair to ground. What difference will this make?

I used a 10M instead of a 1M and a D100k pot for volume.
I aslo left out the 100 farad bipolar cap near as i didnt have one around.  Biased to 4.5 it works and sounds ok but there is a lot of hum. ?

[vanhansen]

Hi.  Thanks for building the Marsha Valve.  I'm not sure exactly what difference the 100n cap would make because I haven't tried it.  I know a 470n sounds ok but I haven't gone that low to see what it does.  If you can, try at least a 470n with the 2.7k resistor.  What's your FET source voltage showing?

I think a 10M resistor to ground on the input is too much resistance.  You could be losing a lot of the potential from it.  Go smaller.  If you don't have 1M, try a 1.5M.

What kind of hum is it?  My build is very quiet, even on the breadboard.

You may want to add the 100uF cap from +9V to ground.  That'll at least help with filtering the power.

[ragtime8922]

I would have bet my house that that cool booster of yours would evolve into an overdrive.

Speaking of evolving our boosters, I just used a sub-miniature tube to design a booster that is very close to my Bogzer boost and the Fetzer Valve. I posted it like 5 minutes ago. Check it out.

Keep on designing and tweakin bro

[vanhansen]

I messed around some more with this circuit last night and the Marsha Valve too.  I decided to try changing the cap off the Q1 source to 1uF since I had some laying around.  I had a .47uF there but since I put a 2.2k resistor in place of the 2.7k I figured it made sense to make the cap a little larger to try and maintain the frequence response, if it helps anyway.  It still sounds the same, maybe a little cleaner and more articulate.

Messed with the output pot value of the Sixteen OD too.  I still like 1M the best for headroom but 500K sounds good as well.  The 1M just allows it to sound fatter with the additional headroom and gives one hell of a boost in front of the clean channel of my amp.  You can really hear everything.

I think I'm about ready to perf it now and get it in a permanent home, if I can stop tweaking it that is.    Good thing I don't have any more JFETS, I'd probably be adding another stage.  Hmmmm, I wonder what adding 2n5089 transistor after Q2 would do.

[MartyMart]

Erik,
This is quite a sweet circuit, perhaps "Sweet Sixteen" would be better name ?
I made a vero layout, and had gate/source twisted around !! once solved
it fired right up with about 4.7volts at the drains ( damm those 100k trimmers can be fiddly !! )
I was thinking "transistor" rather than fet when I laid it out !!!

Would a 2meg gain pot drive a little more ? as it's quite a subtle crunch
at present, or do we need another stage !!

Thanks for the schemo

Marty.

[Gringo]

IIRC there's already a "Sweet Sixteen" named fx (fet driven dist by doug Hammond, i think)

[petemoore]

Way to Go !!
 I like seeing that you're working with 'recombinant' circuits.
 I see no reason to stick to the strictures of any given schematic, I think just grabbing parts and seeing what they do like 'this' when connected to 'that' gives serious insight as to 'circuit intuition'.
 Of course it ends up that your personal favorites just happen to be your personal 'offshoots', modifications or circuits derived from experiments.
 Tweeked  seriesed gain stages...great fun and great tones...

[vanhansen]

Erik,
This is quite a sweet circuit, perhaps "Sweet Sixteen" would be better name ?
I made a vero layout, and had gate/source twisted around !! once solved
it fired right up with about 4.7volts at the drains ( damm those 100k trimmers can be fiddly !! )
I was thinking "transistor" rather than fet when I laid it out !!!

Would a 2meg gain pot drive a little more ? as it's quite a subtle crunch
at present, or do we need another stage !!

Thanks for the schemo

Marty.

Hey Marty.  I'm glad you like.  My first thought was "Sweet Sixteen" but I found out that our own resident, Mr. Hammond, has an overdrive named that already.  Oh well.

I haven't tried a 2M gain pot, simple because I don't have any, but it may open that up a little more.  It's worth a try.  I've been fiddling with the idea of adding a transistor stage either at the beginning or the end of the circuit with a couple diodes.  I'm thinking that putting it at the end would have a nice effect.  I just haven't had the time to sit down and fiddle with that idea yet.