Cauldron of Gain Schematic and PCB

[ragtime8922]

coud you repost the PnP again?

Hopefully Nikolay still has it and can chime in and post it. He designed the pcb and lo. As I said before, I had it saved in bitmap format and it got corrupted. I must have started converting everything to PDF after this design. I lost years of research and design (dammit). I still have a bunch!

I have an AWESOME sub-mini tube/Ge transistor circuit that I'll post soon. It does the warmest classic rock (Floyd, etc). The breakup is just gorgeous!

[spudulike]

So Gus, lets just name a few circuits that you'd consider a bad design:

Dr. Boogie
Boogeyman
The English Channel
Fetzer Valve
Umble
Thunderchief
Uno

Consider the above circuits (great FX pedals) and the fact that they are all original designs of the greatest guitar amps of all time. Now factor in the fact that the Cauldron of Gain is basically the exact circuit used in the Soldano SLO-100 preamp section. This should give everyone an idea of Gus' credibility. Hey Gus, you need to Google: Runoffgrove, Marshall, Vox, Dumble and Electronics.

No, you need to read up on cct design and FET biassing techniques. Just because a technique is accepted doesnt mean its the best way of doing things. Check up on the Thunderchief biassing problems.  

[ragtime8922]

So Gus, lets just name a few circuits that you'd consider a bad design:

Dr. Boogie
Boogeyman
The English Channel
Fetzer Valve
Umble
Thunderchief
Uno

Consider the above circuits (great FX pedals) and the fact that they are all original designs of the greatest guitar amps of all time. Now factor in the fact that the Cauldron of Gain is basically the exact circuit used in the Soldano SLO-100 preamp section. This should give everyone an idea of Gus' credibility. Hey Gus, you need to Google: Runoffgrove, Marshall, Vox, Dumble and Electronics.

No, you need to read up on cct design and FET biassing techniques. Just because a technique is accepted doesnt mean its the best way of doing things. Check up on the Thunderchief biassing problems.  

I have to admit that this is one of the few FET "designs" that I've done. I was just screwing around with the idea of cloning amp circuits with fets. I then left out the power amp section due to massive ammounts of gain that I already had. I used that biasing method based on the above listed effects and got good results, right or wrong. Almost every distortion circuit I've built since uses bi-polars due to the absolutely gorgeous desings of the great Joe Davidson. Now for overdrive/boost circuits I've been using Ge and tubes (sub-mini even).

Gus, sorry to get so defensive. It's this Italian blood that just sets me off in short bursts sometimes. Peace! But just for the hell of it, set the rules aside and breadboard this circuit. A guy with FET bias experience like you will probably get even better results. In fact post the correct way to bias the FETs in this circuit....so I don't have to Google!

[Connoisseur of Distortion]

i heard somewhere that biasing from the source pin would be a more realistic approach to tube operation... on this forum, i think, but can't say for sure.

i don't know what advantages it would have over the current system, but am curious about how it could change current circuits. is it easier to bias? better gain reproduction? more consistent voltages?

[Gus]

People I meant my post in a nice way.  I still check here sometimes.  When I saw that circuit I saw possable problems things like input headroom to each stage.  I did not say it did not sound good what I meant it might be hard to reproduce, one person could build it and it could sound great another might not find the same thing.  Now by playing with a drain R and source R values that can be good but hard to reproduce.

FETS are a pain to work with if you want reproduceable circuits.  BJTs are easy.

Go to the vishay site and find the fet section then find the FET AN (app notes) all 6 are very good (can't direct link)

Things to take note of IDSS(why IDSS? it is easy to measure and things seem to track in the same company and batch).  Say you find a fet you like in one part of the circuit measure its IDSS if you use the same fet number brand and batch there is a good chance a fet with the same IDSS will drop in place.

Now the fets some like here theJ201 do have a tighter spec.  Ones like the 2n3819 are 2 to 20 ma IDSS, all over the place

Google the # and fairchild,  Fairchild has some good data sheets

I sometimes buy fets 1000 at a time

FWIW if you work on neumann microphones you will see the source r is what is adjusted in the u87,km84 etc.  The 1176 has its fets selected etc....

If you build with fets and you can afford it buying fets in batches of 100 or more and sorting them might help with your builds.

I think R.G. has a nice IDSS tester posted.  A real simple one is you DMM set to Ma a 9V supply and an IC socket for the fet short the gate to source and to ground. connect the meter to the drain and the other side of the meter to 9V that should be your IDSS at the circuit voltage.  Write that down and sort your n channel fets.  Then find were they sound the best in the circuit.

[Jay Doyle]

Gus is right. If you built this three times my bet would be that you would get three different sounding pedals. JFETs are extremely finicky.

http://www.vishay.com/fets-small-signal/ssfanp/

Above is the link that Gus was talking about. Great app notes on FETs.

[theman]

A few responders seem to forget that this is a DIY forum. If the pedal is being made for the enjoyment of music, then who cares whether the FETs are biased such that repeatability is suspect. The 100K trim pots should take care of most issues, though there are some occassions where it might be difficult to bias up (I've not experienced this yet). BTW, the Cauldron of Gain looks like a Bogner Ecstasy preamp design, which is slightly different but somewhat similar to the Dr. Boogey Dual Rectifier. Both can be made no problem and sound fantastic!

Unless you're trying to make (and sell!) many units and have them all work the same, then repeatability does not make any difference. Granted, if the circuit design is such that it may be unstable, have very low life (parts burning out), is not safe, or is simply is too sensitive to environmental conditions -- then yes, these would be valid concerns since most people would like to enjoy long-term the things they make.  But for repeatability reasons-- this is chiefly a manufacturing concern, and thus apply to people in it as a business.

It's not any different than a DIY'er making a deck for his house, or furniture for his family room. I'm sure there are things people do that aren't "correct" in terms of mass production, but as long as it works, is safe, and gets the job done -- I say go for it. That is the whole point of DIY. It's also a very custom thing. Each piece created is different and thus unique.

[theguy316]

does anyone have a pcb transfer layout for this pedal?? anyone have any feedback on their builds... was it any good?? what amp/boutique amp does this amp emulate or even closely sound like?? thanx

[WGTP]

I think the new redesigned Fetzer Valve at ROG addresses some of these issues.  Check it out if you haven't already.   

[theman]

the fetzer valve at runoffgroove is: "A closer look at the FET booster based on a vintage Fender 12AX7 input stage." it does talk about how you do a 12ax7 -> JFET gain stage conversion.  the cauldron of gain architecture is similar to the bogner ecstasy red channel. with some different filtering and attenuation in between gain stages, it can also be made into the dr. boogey dual rectifier.  this is all about very high gain distortion.

[Tubebass]

Okay, I have a question: has anyone built a circuit using a variable resistance in the source path, and then compared how it sounded with different FETs in the sockets? The distortion pedal I'm using on my pedalboard is just two 2n5457 stages with  trimpots on the sources. I've never done the experiment of swapping FETs and re-biasing to see if the sound changes. This might be worthwhile.

[mydementia]

PCB/Perf layout anyone?

If nobody has a good PCB/perf layout for this guy - I might give it a try this weekend using bancika's software.  Could someone verify that this: http://i2.tinypic.com/t82kp4.gif is the latest and greatest (only?) schem?  Bit me in the a$$ on the Orange emu...

I might have to try my Umble on 18V...maybe even Dr Boogey....mmmmm.....charge pump....

Thanks.
Mike

...just what I need...another hi-gain jfet emu...

[theman]

after fine tuning the trim pots to maximum gain on the dr. boogey for 9V operation (some squeals at high gain and treble settings, but manageable), i tried running it at 18V with the godlyke power pump. there was all sorts of nasty noise right from the start with the guitar input at minimum. i stopped right there and didn't bother to re-bias.

the cauldron of gain should work okay at 9V, as the architecture is similar to the dr. boogey except there's some slightly different attenuation going on between stages, and the cauldron has the 470k/470p filter in between many of the stages. i think the 100k source resistor for the voltage follower at the last stage could be changed to what's in the dr. boogey (10k). the dr. boogey also has a presence filter at the end, while the cauldron has the 10k/2n2 filters.

spice sims show that the dr. boogey has more gain than the cauldron ...

[MartyMart]

Got this on the breadboard, have a "working" circuit but some problems running at 18v and 12v
Both gave good boost and some drive, but not a lot of drive .. also had big noise problems .
Biasing in Q3/Q4  was impossible, Q4 would not go under 15 volts ! ( or 10.5 @ 12v supply )

Subs : all 500k/511k's are 470k
Gain pot : 1M log with 1m r across lugs 1&3 = 500k log !

SO :
Made the 100k from Q5 10k ( ala Dr Boogie ) and added bypass caps to Q2/Q4 of 1uf and re-biased for
9v supply .... BINGO !  all sitting at 4.5 - 4.7 v and sounding great, but STILL a medium gain cauldron ??

Idea's ?

MM.

[mydementia]

Hi Marty,
I perfed this one up this weekend too... I used bancika's layout tool and got the J201's right this time!
I'm having trouble biasing Q3 (maxed trimmer only gets to ~12V instead of ~9V) and the output volume control doesn't do anything (wtf?).  Aside from having no volume control... it sounds like there are some mean high gain sounds in this emu...  I didn't have a problem biasing Q4... but the Q5 gate is getting the high Q4 drain voltage...probably not good...
I'm using the charge pump from GGG (MAX1044) to get ~17V out of my 9V battery.  Only sub from the schem was a 20k pot for the MID.
Help/advice appreciated...

Here's a link to the thread I started on this...I guess I'll work here too (didn't want to hijack this one).
http://www.diystompboxes.com/smfforum/index.php?topic=46891.0
Here's my layout:

[MartyMart]

Try reducing R9 , I've made mine 3k3 and I also removed & jumpered R17
and changed R5 to a 1k2.
Remember that I was having HUGE noise problems at 18/12v, so mine is now a
9v circuit.
These changes have at least doubled the gain and I'm quite pleased with it so far.
The only problem I'm having is that it gets a bit "thin" sounding at lower gain, so perhaps
I'll play with a few cap values.
I have working eq and volume too, but mine has become something else, getting quite
close to a Dr boogey right now !!

MM.

[Gilles C]

While reading the thread, this schematic reminded me of the Mini-Tubes Preamp from AMZ.

I built it from the schematic when it was still available on AMZ site, and it uses the same kind of bias scheme. I agree that the bias has to be adjusted for every FET depending on the FETs used, and I even had to use bigger values for the trim pots because of the FET I was using at that time. I even had to use a specific FET number for that circuit because the others I had were not working very well with that schematic.

But it sounded so good... had so much gain, that I didn't care the trouble.

Btw, these days, I still have problems biasing a FET with the Drain resistance. And the Fetzer Valve article helped me understand a lot about FET specs and FET biasing.So I am still in the experimenting stage again. But one thing I am trying to achieve is being able to have a circuit that works for everyone, not just on my bench. And I like to be able to build a prototype, and being able to build a second circuit based on the prototype, and have a working one again.

So what would help I think when it comes to FETs would be to give the specs of the FETs used in a successfull circuit. That way, it would be easier to build another successfull one, one that would sound the same.

That was my 2 cents.

Gilles

[mydementia]

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

[Jay Doyle]

A few responders seem to forget that this is a DIY forum. If the pedal is being made for the enjoyment of music, then who cares whether the FETs are biased such that repeatability is suspect...

Unless you're trying to make (and sell!) many units and have them all work the same, then repeatability does not make any difference...

But for repeatability reasons-- this is chiefly a manufacturing concern, and thus apply to people in it as a business...

Repeatability DOES most definitely make a difference ESPECIALLY when posting to this forum. What would be the point of posting a circuit otherwise? Why would someone post a circuit that another builder would have a hard time replicating? Isn't that the whole point to sharing schematics: for someone else to be able to reproduce the abilities of the circuit you just spent so long creating?

Read the app notes I referenced above. Using a trim to V+ and ground through the 1M on the gate allows you to bias via Vgs instead of Vd. Varying Rd to achieve the desired bias will change the voicing of the gain, current consumption, headroom etc. Not to mention max headroom isn't at 1/2V+ in a trasistor amp stage anyway...

Look, I'm not trying to harp or sound old-fogeyish but there are people that post here who may not be familiar to most of the people who currently follow this BBS but they are folks who have helped out and designed many circuits that have helped to create the trove of designs available.

Gus is one of those people. I defy you to use as few parts and create a better sounding circuit than his NPN Boost.

Respect: Not too long ago, it didn't have to be earned around here.

And just because you don't recognize the name, doesn't mean that they don't have something valuable to impart.

Regards,

Jay Doyle

[Gilles C]

Sorry...