GCS Overdrive

[DougH]

http://www.diystompboxes.com/pedals/gusOverdrive.gif

I tried this on the breadboard last night and I must say, for a good, simple, low-gain overdrive this can't be beat. With my PAF's this is like the perfect overdrive. It adds a nice "sweetness" to the guitar tone and it sounds like it will really cut through a mix, even with the amp clean. Crank up the amp a little and it will sing nicely. More distortion from the amp and this will push it over the cliff into "almost metal" territory. And it interacts with the guitar volume real well too.

And hey, Look Ma! No knobs... I can tell that Gus spent a lot of time tuning it, as the output level is perfect. The amount of distortion is just what you would expect from an overdrive. The timbre of it is perfect- no bass cut yet a nice midrangey sweetness that will really cut through. It sounded great with both my neck and bridge PAF's through my "baby vox" amp. It's the perfect overdrive for this rig. With single-coils I think it could use a hair more gain. The emitter bypass cap may be a good candidate there for a little tweaking, not sure, need to experiment. I used a 2n3904 with hfe of 137 and it sounds wonderful. I tried another bjt with hfe around 400 or so and it lost some of the charm. It sounded too trebly, for my tastes. So, IMO it seems lower hfe is better with this one.

This circuit is a real sleeper. If you like tube screamer overdrive stuff, check this out. It's definitely a simpler build than a TS, and in many ways IMO sounds better. I think this would make a good first stage for a multi-stage circuit as well. Time to experiment... And it's time for me to start spending more time in the "GCS" schematic section.

[Mark Hammer]

Given the number of gold ribbon circuits you've come up with yourself, this is quite the recommendation.

[raulgrell]

Daaaaamn... I just realised this topic was started by Doug...

I'm deinately trying this circuit out...

[ItZaLLgOOd]

I was wondering if you could use the "boost" control set-up from the beginner project to have adjustable gain.  I'm new to all of this so maybe I'm way off base.  It would be nice to have the control if your using both HB's and single coils.

[yertle]

I guess the 1k resistor and the 220n cap make a highpass filter (set at 723.8 hz), if you would replace the 1k with a pot you will mess with the filter. So that wouldn't be the best thing to do I guess, maybe it's a possibility to up a high pass filter in front of the circuit so you can mess with the gain control without messing up the freq. cutoff. Maybe a single switch to select between to r/c combo's would be a possibility?

Correct me if I'm wrong

[snoof]

I saw this mentioned in a thread recently, and was wanting to give it a whirl.  Thanks for the reminder guys!

[Gus]

Doug thanks for trying it.

  This and the NPN boost were made to see what I could do with one Si NPN transistor

  Hopefully it is a fun useful circuit

  It was voiced using a turned up fender deluxe type amp and single coils. Guitar, overdrive, amp

  Being a one transistor circuit that is doing a few things, parts interact.

  For people that  want to change stuff

  Gain is set by R6, R4 and R5
  Mid boost part of the eq is R5 C3
  Lowpass is set by R6,R7 C5 and some loading by the stage after this was meant for a 1 meg input
  Bias can be adjusted by R2 and/or R3 I would start with R2 there is a reason
  hfe vs input R
  input output caps
  R1 value
  etc.

[jpm83]

Some times the most simple things are the best for example sparkle boost. What is the output level in this thing, is it how much over unity?

Janne 

[DougH]

Gus has some of the most elegant designs of anyone on this board- minimal parts, maximal sound. Shows what you can do if you sit down with a pencil and paper and think about what you are doing.

Gus I played with doubling Rc and Re, halving C1 (to attempt to get the freq response the same, and I should have doubled R8 as well), but it didn't sound as good to me. My single-coils are stock in a cheap guitar, which may be the issue with them. I think the gain is tuned near perfect for my humbuckers. I was looking for a hair more response, not clipping per se but singing feedback stuff. It's definitely made for a loud amp. I was thinking maybe a low gain stg added to the output to just give it a little push might work, if I could do that without destroying the sound of it.

[Gus]

I kind of like the DC points of this circuit.

I got an Idea from the post add say a 100uf cap and a resistor from the emitter to ground (three legs off the emitter).  Maybe a 4.7K to start. 

The stock circuit gain is ~= Rc/Re
X 4.7 to around X 9

Or maybe another transistor stage with a gain of maybe X2 an a input R of 100K.  so a high hfe like  a MPSA18 and would need to work on getting the collector current and bias point right.   Or an opamp set to a gain of two. I am thinking about it.

[DougH]

Or maybe another transistor stage with a gain of maybe X2 an a input R of 100K.

This is what I was thinking. As long as it had high enough Zin not to load the output of the overdrive. Of course, you could do a jfet buffer driving a low gain stg or a low gain jfet stg, but that overcomplicates it IMO.

I subbed a 470 for the 1k in series with the .22u in the emitter circuit, then just shorted it out. It didn't really change the gain that much. I'm real happy with this stage as is, just wondering if something simple could be done to add some response.

[DougH]

I tried something really simple with this today. I just built up two of them on the breadboard and used the exact duplicate as the second stage. I subbed the two 100k resistors with 100k pots. The first one is the "gain" control and the second one is "volume". I duplicated the circuit completely, with the output LPF and everything. It was kind of a brute force method, but it really sounds good! Gives it just a little more "oomph" but still sounds pretty sweet. The volume control is good for controlling drive to the amp. It sounds different than the single stage, and it would be fun to add a second stomp switch to bypass one of the stages for "one or two stage mode". This is a fun little circuit to play with.

[raulgrell]

Any sound samples available?
I'm very curious about this circuit, but I lost my solder reel (how the hell that even happens, I have no idea) and I don't have a breadboard, so I can't try it out myself...

[lenwood]

+1 on the GCS overdrive.
Simple , Great sound to further overdrive a tube amp.

[slacker]

I gave this a try today and like everyone else has said it's a really nice sounding circuit.

Into the clean channel of my Twin it makes a great boost, like Doug said it's got just the right amount of volume and fattens up a strat nicely, it's very smooth even using the bridge pickup.

I tried Doug's idea of running 2 of them in series and that adds a nice bit of extra colourisation. I thought it might be interesting with a bit more grit though so I rejigged the first one into Gus's NPN boost, but kept the output lowpass filter and the mid boost in place. This gave a nice bit of extra crunch and with the first stage on max it has a nice amount of distortion on it's own.

It's definitely worth playing with.

[DougH]

I'm glad you tried and like it, Ian. Yes, the NPN Boost should make a good first stage if you want more gain. I will give your idea of using that with the mid boost and LPF in place a try as well. It's hard to decide what to do with it because I like everything I've tried with it so far. 

[vortex]

Thanks for the heads up on this one!
I did a quick vero build last night and had a little time to play around with a few NPN transistors. I tried a 2N3904, 2N4400, 2N5089, PN2222 and a germanium OC140.

A couple of quick subjective observations:
I read that Gus suggested that one could use most NPN trannys with a HFE over 200HFE. Most of my stash are in the 200 HFE and under range with the exception of the 2N5089. I prefered the lower HFE trannys in the 170HFE range.

In general the sound of the circuit is a nice dry "tuff" kind of overdrive, I didn't notice much in the way of extra sustain but as Gus mentioned it was designed to  kick an overdriven tube amp in a nice way, and it does. Nice balanced full range tone. As I substituted transistors some seemed to represent the high end a bit better than others. There is a hint of "transistor-y" sound but for a one tranny circuit this is darn sweet. I can tell that the slightly aggressive high end will work well with a hot tube amp.

Just for fun I popped in a OC140 germanium and things got even better! I know that the circuit isn't designed with germanium in mind but sonically the OC140 added extra dimension, richer lows and sweet chiming highs. I was only getting 3 volts on the collector but it still sounded sweeter.

This afternoon I breadboarded the circuit and made a fruitless attempt  to bias the OC140 a bit better with socketed pots in place of the resistors.  My lesson today is, being a hack only gets you so far....What can i say, I'm no EE! Yet. Sorry Gus...

Just thought I would say thanks and add to the thread.

[Gus]

Try adjusting R2 for the bias.  Different operating points are fun to try.  In one circuit I like Collectors at about 2.5VDC with a 9VDC supply

It is a hopefully good starting point with the values selected.  I was wondering when someone would try a Ge.

[snoof]

i tried a russian GE in mine on the breadboard, that particlular tranny didn't really sound all that different, but it did work!  i didn't take any reading with my meter though.

[joegagan]

i will try this with a ge = ac187