Cauldron of Gain Schematic and PCB

[Gilles C]

Gilles,
I think that would be VERY useful.  I've built several emu circuits from ROG and schematics around here and haven't come up with similar tone on ANY.  The only one that seems to work better for me than most is my Dr. Boogey (probably the one I lucked out on).  I'd love to be able to build multiple pedals that sound the same (for my main rig, my practice space rig, my bandmates, etc).  I have several emus on my workbench right now that just don't sound right (Matchbox - low output/bias trouble, OrangeMKII - low output/bias trouble, Umble - worked great initially - suddenly ridiculous amount of gain). 

What technique/measurements are you thinking?  Shall we all set up a rig as described in the ROG 'A Closer Look at the Fetzer Valve' article (for measuring Vi and Vp)?

I'm definitely on board with this one...The sound clips here and at ROG are so tempting...I'd love to be able to match my FETs to their values so the only suspect part is my building skill!

Anyone else on board? How would you guys at ROG feel about posting your JFET values?
Mike

Just as a start, I almost used the method from ROG to find IDss and Vp, and used the values I found with the calculator on their site to find the resistors values. But I added 2 trim-pots to experiment a bit from there with an oscilloscope and a meter to verify the bias point from there.

I also used the inspiration from these sites:

http://www.qrp.pops.net/fetbias.htm

http://www.brookdale.cc.nj.us/fac/engtech/aandersen/engi242/

http://ampdesigns.tripod.com/FET_Amp_Designing.html

http://ee.stlcc.info/132/fetbias.htm

and a couple of others which I can't find again...

That was for a start anyway, and I'll had more in a new thread if there is some interest later.

Thanks for the come back Mike, and nice comment Jay,

Gilles

[MartyMart]

I see a slight problem here, although the idea of "matching fets" would be fine for each
"single" build  ie: in a similar way to matching fets for the P90, perhaps you'll get 12 J201's
that are "a certain gain group" from 100. ( RG or Jacks Fet matching circuit, sorry don't remember who's ! )

You may then end up with 8 "gain groups" within the 100, so if you build from within these "groups" they will sound
very similar, but which "gain group" do you choose ?
And if it's only one or two "sets" do you throw away the other 72 J201's from the 100 ?

That gets expensive VERY quickly ,,,

Hope I'm making sense here !
Would that "fet matching" circuit be enough, just to select from ....

MM.

[GreenEye]

I recently bought a Danelectro FAB metal, then gave it to someone and bought another.  I swear the first one had sounded better than my second, but I'll never know...

So maybe it's not just FETs that end up sounding different per pedal.

[Gilles C]

For the FETs, I would group them first by Vp (or Vgss) to help select the ones I want to use with a bigger gain/lower input signal, or for a lower gain/bigger input signal.

I may be wrong, but to me right now, it seems to be what makes the biggest difference between FET. I'll see in the following weeks if that's the way to go. As I said, I just decided to experiment about FETs differences, so I hope to learn from these tests.

For the FETs that don't fit in one kind of circuit, they could be good for another configuration. No need to throw them away. For example, the upper FET in the Mini-Booster that doesn't need the same specs than the lower one.

Btw, here is another place where FETs are used, and with a slightly different info about how to calculate Vd.

http://www.albertkreuzer.com/preamp.htm#descr

Added: After reading a few other articles, it seems it would be more logical to group them by Idss. It's never too late to change, so let's go with Idss...

Gilles

[mydementia]

Hey guys.
Just wanted to let y'all know that I got my CoG working.  I bought a dremel tool today and a conical scroll bit and dug out all my re-solder goo.  I also had a ground problem on my output cap... ahh the wonders of a continuity check... but now it works.

On 18V - it was noisy as heck!  I put 9V to it, rebiased the J201's to ~4.5V and now it works great.  There's a weird gating thing when the note dies out...kinda fizzles/farts away...maybe an artifact of this circuit being designed to run on 18V (lacking headroom?)...  The gain is tremendous!  It's not as tight for palm muting (like my Dr. Boogey) but does the way-out gain thing great.  Not sure if it will get a box...but it's definitely interesting...

Here's my layout from Bane's layout tool - if anyone else is interested in building from this, I'd like to hear how it goes.

[theman]

hi mydementia,

try changing the 100k resistor to 10k. that's what is on the dr. boogey. also, you might want to try this for biasing to get rid of that sputtering decay:

* set the drive to a rather low setting
* crank the volume so you can clearly hear a note decay
* set the trims all so that you get a sound, maybe in between the range
* strum a chord, and listen
* set the q1 trim so that you get a good sound with no sputtering decay
* repeat for all other trims

you'll find that you can get rid of the sputtering while still keeping a good gain range. i've had one pedal (a bsiab derivative) where setting the trim to get rid of the sputtering caused the gain to be reduced. it actually opened up the pedal and made it less compressed, more crunchy.