Nick Greer - Green Giant query

[Kalimna]

Greetings,
I finished up another build last night (I have put my Ross Phaser on hold for debugging whilst I populate a new board with cuts/links - I intend to transfer all the resistors/capacitors to the new board, checking values along the way) and whilst it is working, the 'clean mix' control seems to do nothing - or at best something very subtle indeed.
The volume control works, and the overdrive that comes off is rather nice (through a Blackstar ID:TVP 15) indeed.
The layout is from this site :
http://tagboardeffects.blogspot.co.uk/2014/09/nick-greer-green-giant.html

And a schematic I've found is on this page:
http://revolutiondeux.blogspot.co.uk/2009/01/nick-greer-green-giant.html

Now, I have gone over checking for solder bridges and connections, and all seems OK - it is such a small circuit that even I could follow the schematic! I haven't measured transistor voltages yet, but I wanted to check a couple of things first.

I didn't have a 25k linear for the clean mix, and subbed in a 20k instead - might this be the cause of it's lack of effect? Would soldering a 5k in series help here? Or might a log pot be better? As it happens, I don't have any 25k logs, but I do have some 50k - could these be used with a 50k resistor wired in parallel across the used lugs?

Regarding the transistor used, the circuit calls for a 2n2904, but having tried a couple of types (3904, MPSA18, 2n5088), I ended up using a 2n2484 as the overdrive was 'creamier' for want of a better term.

If the effect is still too subtle (the clipping diodes being added in with the clean mix control, as I understand it), would it be reasonable to add in a second anti-parallel pair? Or change the 1n4148 for LED's or other type?

Sorry for all the questions, but I am trying to understand what the circuit is doing as well as have a fully working effect.

Many thanks again to all on here - this forum/blog is a fabulous resource.

Cheers,
Adam

[Ciaran Haslett]

Hi Adam.  Hope you have better luck on rebuilding the Ross.  As a quick experiment for me could you leave off the 10K I mentioned in the previous thread and see what happens?  

As for the Green Giant....it's an electra distortion.  One of the simplest, most moddable circuits going (look at any Lovepedal pedal and you'll see what I mean)

Basically we have a transistor amplifier pushing our guitar signal past some diodes in hard clipping configuration.  Hard clipping meaning going to ground..  The "clean mix" pot acts as a resistive element between whatever's coming out of the amplifier and those clipping diodes.  The more resistance, the less signal gets clipped etc etc.

The problem here is simply the name Nick Greer gave this control.  It's not a "clean, undistorted guitar to distorted guitar mix ratio" control.....its a "determine how much of the amp output signal is gonna get clipped by those diodes" control.  The problem now becomes obvious....if the signal is already clipping at the amp output, then what sonic difference can I expect with adding clipping diodes?

If you look at the data sheets of the transistors you listed (or better again, measure their gain with a special meter) you will see that their average hfe gains are all different.  The 2N3904 specified in the layout is the lowest of the ones you listed.  General rule of thumb is that the higher the hfe...the more saturated your signal (applies only to amplifiers...sort of....for another time lol).  

So if thats the case...you most likely have a situation where the amplifier is already saturated and clipping your guitar signal, so when you mix in the diode clippers....the difference is minimal.  Make sense?

So what can you do?  Quite a bit actually...as I said, the electra distortion is highly tweakable.

You could....

Swap out the transistor for a lower gain one.  (Zach here developed his germanium version of the same circuit).  Germs are considerably lower in gain than most silicons.  Have a good read here

You could lower the overall gain of the amplifier stage itself.  Try lowering the 2M2 resistor to 1M or lower.  This will limit how saturated the amplifier becomes so will allow a greater difference in mixing in the diode clippers.

You could up the Pot value to add even more resistance between the amp out and the diodes.

You could change the diodes to something with a higher Vf like LED's (as you mentioned).  This will not only allow more volume, it will change the tonal characteristic of these clippers.

The list goes on....but I highly recommend you give ALL this a go.  Even pick a tone control and try adding it.  You will learn tons about many of the different things will will find in most guitar pedals.

[Kalimna]

Ciaran - once again, thank you for your very detailed reply and willingness to help out :)

Briefly (because I *really* need to get some sleep before the next 12hr shift!), even though I like the sound of the 2484's, I have chosen a transistor with too much gain to allow the diodes to make much of a difference? I don't have any Ge transistors at the moment (eBay here we come), but there are a couple more NPN's in the box of goodies i could try.
You mention changing the gain of the amp by reducing the 2M2 resistor. Would it be reasonable to put a pot there instead, to allow a variable gain control? If so, would it need a resistor in series so that the gain never goes too low?
I will also try using a larger value pot for the 'clean mix' control. I have plenty to choose from...

Adding a tone control is something I plan on doing with my next Red Llama build (first one worked really well - gobsmacked how much volume it puts out), so the Green Giant would be an excellent oppurtunity to practice. (And once that is done, a Green Rhino and thence the Camel Toe...)

One thing I forgot to ask in the first post - there is a 1M resistor in the layout that doesnt appear in the schematic. Is that a pull-down resistor?

Either way, I feel the breadboard needs to be christened with this circuit.

About the Phaser - I havent started unwiring the first attempt yet, so will de-solder that 10k and let you know the result. Are you looking for sound differences, voltages, or both?

At some point it would be great to repay the knowledge, but I might be limited to baking, guitar building, and a bit of home brew!

Cheers,
Adam

[Ciaran Haslett]

RE the Ross....yeah I'm more interested in how the vref voltage changes.  I'm assuming it will now read around 4.5v.

Some "light" reading regarding common emitter amplifiers.  Don't worry too much about the math at the minute....just more on the general pieces of the puzzle.  And while you're at it...compare this to the Big Muff schematic.  Look familiar?  Only this time the diodes are in a soft clipping arrangement (in an amplifier feedback path)

http://www.electronics-tutorials.ws/amplifier/amp_2.html

RE the 2M2...gain might have been a poor word choice.  Usually amplifiers are biased at half supply voltage to allow the greatest "swing" of an AC signal, both positive and negative, before clipping.  That biasing is done using some kind of voltage divider (as I'm sure you've heard lol).  In this case there is no divider at the base.  It's biased by all 3 resistors BECAUSE of the beta (hFe) of the device.  I.E. a 2N3904 has a typical hfe of X so using resistors A, B, and C will give us a rough bias point of half supply.  The above article explains it in better detail than I can.  So yes changing the 2M2 will have a difference in gain, but in reality it's shifting the bias point of the transistor.  Definitely worth hitting the breadboard with this circuit!

RE the 1M.  Yip just a pulldown.  There to help prevent any "switch pop" when wired for True Bypass.

Tone control....if it is passive (i.e. there is no amplifier circuit) then expect "insertion loss."  A typical example...the Big Muff tonestack will cut around 6dB off your volume, which is why in the BM the tone section is followed by another transistor amplifier to make that loss back up.  Maybe take a look at these tone controls to get you started.

When you get to the Green Rhino...save yourself a few transistors and just use 1 for the input buffer.  I traced it and all those transistors in parallel confused the hell out of me....especially since the Tube Screamer (which the GR is a modified version) only used 1 transistor per buffer.  Frank here sim'd it and confirmed that 4 transistors in parallel make absolutely no difference to just 1 transistor.

We here all love cake, guitars AND booze.  If you can find a way to mix all 3 and post it as a vero layout, we'll all be truly thankful haha

[Kalimna]

Ciaran - a lot of info to digest there, between the links and the text. I will get this build working with two effective pots, and then build up another using the breadboard.
I'll post up the result when I'm happy with it :)

Regarding the cake/guitar/booze/layout combo. Well, we'll see - it might have to wait a week or two first....

Cheers,
Adam