Gar Gillies' infamous STINGER adapted for stompbox use!

[amz-fx]

Remove C2 and see how that sounds.

Replace R9 with a jumper.

regards, Jack

[whoisalhedges]

Remove C2 and see how that sounds.

Replace R9 with a jumper.

regards, Jack

Thanks (again) for the advice. What does it do? C2 bypasses R2, wouldn't removing it result in less overall gain? But again, I'm awfully weak on MOSFETs.

R9 is a 10k resistor between Q2's drain and V+... of course I get that less resistance there would bring Q2's drain voltage at V+, but how would you hypothesize that would effect the amplitude and clipping of the signal?

Apologies if I'm being a pain; but although I really want this to sound "perfect" I also want to know why the bits do what they do. The more I understand now, the fewer questions I'll have to ask later - or at least the questions I do have will be better.

[Eb7+9]

I've long been interested in obsessed with onboard tube fuzz circuits, like the Laney Klipp or the subject of my latest (freakin' sweet) pedal, the Garnet Stinger.

Keeping in mind the imperfections of emulating tubes with FETs (and even that's typically been triodes), the prospect of nailing down the Stinger's pentode-derived fuzz was pretty daunting.

So... thoughts?

it will be especially daughting if you make a wrong assumption right off the bat ...

(a) carefully compare the Plate transfer curves for a Pentode against the Drain transfer curves for n-chan jFET
(b) next, figure out what makes the Stinger circuit clip the way it does ...
(c) answer to (b) will suggest what to try and how to try it

Gar's circuit doesn't have diodes and the like ...

[whoisalhedges]

I've long been interested in obsessed with onboard tube fuzz circuits, like the Laney Klipp or the subject of my latest (freakin' sweet) pedal, the Garnet Stinger.

Keeping in mind the imperfections of emulating tubes with FETs (and even that's typically been triodes), the prospect of nailing down the Stinger's pentode-derived fuzz was pretty daunting.

So... thoughts?

it will be especially daughting if you make a wrong assumption right off the bat ...

(a) carefully compare the Plate transfer curves for a Pentode against the Drain transfer curves for n-chan jFET
(b) next, figure out what makes the Stinger circuit clip the way it does ...
(c) answer to (b) will suggest what to try and how to try it

Gar's circuit doesn't have diodes and the like ...

Thanks... I do understand (as in, I can see plain as day on the schematics) the differences in topology between the original(s) and my attempt. Also, there are differences between the two versions Gar did make, the 6AN8 version having the Stinger circuit in parallel with the first preamp triode; the 6JW8 version coming after the first preamp in series (but still paralleling the tone stack) - it's the second one that primarily influenced what I'm trying to build, but it's still not meant to resemble a 1:1 conversion of the valve circuit's topology.

Now, why it clips the way it does... there's great value in understanding that, but I haven't been able to suss it out - what I've done has produced a sound that is very similar, but it's no doubt doing it in a different (and possibly ass-backward) way.

One assumption I've made that could be totally wrong: I didn't want to be constrained in needing this circuit to be first in line after the guitar. I have a MOSFET at the input in large part because I thought it could handle a higher voltage swing at the gate without going all screwy than a JFET (or BJT, for that matter), so maybe I could stack it with another drive pedal in front. Is this an incorrect assumption? Would a JFET be a better choice at the input? The IRF510 I had on hand (as well as the cheaper, smaller, more sensible 2N7000 and BS170) have a much better V_GS than does a J201 - but maybe an MPF102 would be a better solution at the input?

And I guess that also raises part of the problem with my limitations in understanding why the Stinger clips as it does: if I raise the gain too much, it no longer sounds like the Stinger. So, if I smack the front end with another drive pedal... that might turn my whole effort on its head.

In a couple of hours spent planning and breadboarding, I managed to make something that has a sound that's 90% of where I want it to be. But the devil's in the details, right?

[whoisalhedges]

Another thing crossing my mind....

The 2N5088 is there, more than anything else, because I understand how a BJT amplifying a signal which is then clipped by diodes works. But, not only is it the bit that's least analogous to the original, the collector reads around 1.8V, the emitter sits at 0, and the base is just a diode drop above that... is it even doing anything? Should I get out of my comfort zone here and concentrate on getting this sound with FETs?

[whoisalhedges]

Oh, and one last (for now) thing: I get why copying the Stinger's layout for my gain control doesn't do squat. The amount of fuzz in the original is controlled by how it's mixed back into the main signal. The second version, which was the source of most of my layout, mixes the output from the 6JW8 in after the tonestack (which is also why the Stinger has the "color" control - the main preamp goes through the Bax stack; the fuzz goes through its own tone control) - so it's really about how much of the distorted signal gets mixed back in with the original. And since they're now totally different waveforms, it doesn't really matter that they're 180^o out of pahse. Right?

So - if I'm not going to separate, invert, and re-blend the signals (which I still might end up doing, but...), I'll have to completely re-design the gain control.

[Gus]

Did you look at my post?
Look up contact bias.
Note there is no obvious DC path to ground for the grid.

[whoisalhedges]

Did you look at my post?
Look up contact bias.
Note there is no obvious DC path to ground for the grid.

Google has given me some conflicting information. Do you mean "contact bias" as often used synonymously with "grid leak bias?" Or are you referring to "contact potential bias?"

I've looked at the plate characteristics graphs on pentode datasheets, but haven't really squared that with FETs.

By the grid, I assume you mean the screen grid? The Stinger on the later version of the BTO isn't tied to ground at the grid, true... but on the earlier version it is (via a 10M resistor) and on another schem I found (different amp - I'll link) it is via a 1M... so it's hard for me to understand how that would comprise such an integral part of the sound; as all these versions sound pretty much the same. Seems to me Gar just smashed the hell out of a pentode with electrons, and made it work with what he had on hand? Though from what little I know of him, he was a little more deliberate than say, a Mike Matthews or something.

Anyway, here's the other one. Similar to BTO version 2, except without the HPF-to-ground "color" control and with a grid resistor:

http://schematicheaven.net/bargainbin/garnet_lb200f_pro200.pdf

It's a less well-known amp than the B.T.O. (and I like the treble bleed available via "color"), but it looks like the circuit is a damn sight easier to follow. I wish I'd seen it earlier.

Regardless, I'll try to find something on contact bias. Just seems that there's more going on here - and how does that effect my desire to vary the amount of fuzz via a potentiometer?

[whoisalhedges]

I'm still not understanding how to translate the biasing of the pentode into making a MOSFET fuzz... I did move away from the cookie-cutter boost biasing on the MOSFET: I now have 330K on the drain and 120 ohms on the source, which is more in the right ratio, but it didn't do much other than raise the gain a little. No idea how I should handle the feedback & gate resistors with this change, though....

[whoisalhedges]

Contact bias?



The MOSFET is getting its bias from the source of the JFET here... the split-signal topology is also more like the original. It needs the feedback resistor, otherwise... I don't get it - if I remove Rf while playing, there's no audible difference at first, then it gradually loses signal; like a capacitor somewhere is discharging very slowly... anyway, dunno that 1M is the ideal Rf for this circuit, it's just what I have in there now.

Note that the BJT + diodes stage is gone (replaced, appropriately enough, by  ). Might not be needed. Vs were very low on the BJT, might not have been doing all that much anyway. I can make this arrangement sound quite a lot like my first attempt, only now every pot does something. Do they do what I want? Well.... Some will be cleaned up by replacing linear trimpots with audio pots. Others, well, I just re-did the whole circuit, it stands to reason there might be some values need changed.

This is a much better circuit than before, even if it doesn't sound quite as good, if that makes sense. My first attempt sounded great, but it was only one sound: couldn't change the amount of fuzz, the tone controls didn't do much, all I could control was the output volume. I can almost dial in the tone I want, it'll just take some patience and some massaging of component values.

Right now, the junction of MOSFET and Bax stack goes straight to the mu-amp. Dunno if it'll stay that way, if I need another (probably unity-ish) amplifier stage, or just some filtering - it does need smoothed out a little.

But progress... and I hope it shows some basic understanding of what Gus was saying, otherwise I've completely missed the boat.

[whoisalhedges]

Hmmm... played through it a bit yesterday after work. Had family plans (today as well), so didn't have time to fiddle with it any, but just wanted to enjoy the fruits of my labor a bit - and it didn't sound as good. Hard to say exactly why, "less full/more harsh" probably the best I can do. The lows are there; oh, the lows are there - the highs are somewhat like glass shattering, though. I'd already planned to pop in a treble bleed cap here & there anyway; it's just odd to hear the same (untouched) circuit sounding different one day to the next. After all, there's no Ge anywhere in it.

The color pot and Bax stack are working great with what they have; just looks like I need to filter some of the frequencies they get. Not too difficult.

[Gus]

This is an idea for 9VDC and common transistors.  Gains might need to be adjusted. It should be a good starting point.  A lot of it is a cut and paste from the tube circuit.

The preamp gain stages have high input resistance and a gain of under 10(loading reduces the gain from X10).  I like this compound pair. Gain was selected for 9VDC

The bottom transistor fuzz stage is set at about X10 and 100k output resistance. The first stage at X10 and this is a little under X100 at the output of Q3.

Vbias might need a lower output resistance. I wanted to bias the 2nd preamp stage with the 1meg control for close to the same interaction as in the tube circuit

Not built, it might be fun.

Thoughts, questions?

EDIT R23 should be 100k to add to the 900k for 1meg
The connection at the 680pf to the volume should not be there

[whoisalhedges]

Interesting, I've never messed around with Sziklai pairs before.

[whoisalhedges]

Thoughts, questions?

I've got Fathers' Day stuff this afternoon, will be watching Game of Thrones tonight... then getting up at 3:30a.m. to go to work; so I won't get to play with this till tomorrow at the earliest.

I'm glad you cleaned up some of the (seemingly to me) labyrinthine switching. I've dispensed with the frequency boosts, figuring decent enough tone controls could handle that. I've been staring at the two versions of the B.T.O. schematic since before I even knew what to call those doohickeys in the tubes, or why the fuzz tube had two more squiggly lines in it than the 12AX7s, and I still don't get the switching arrangement.

Although I've made some attempt (and you've made a better one) at working with the topology presented almost 50 years ago by Gillies; what I ultimately want is a fuzz that sounds like the Stinger. There are a couple of things that separate it from other fuzz circuits: 1) it's tube-based, and while I can't get 100% of the way there with transistors, I think I can get pretty close - we're not talking about the sweet sound of a Deluxe at the edge of breakup here, we're talking about a pretty gnarly fuzz; and 2) it's not just about overdriving a single pentode - it's about the way that overdriven pentode interacts with the preamp, and how that combination effects the signal presented at the input.

So, while a dual-switch pedal could be interesting: one an overdrive/preamp with a certain Winnipeg flavor, and one a fuzz - that's not really my main goal. I'm perfectly OK with all or nothing.

I also would be remiss if I didn't say thanks - I really appreciate the time and attention you and Jack, two people who's work I've been familiar with since long before picking up a breadboard of my own, have spent on my white whale.

As far as questions: why the Sziklai pair? Actually, why BJTs in general? FETs seem to be everyone's favorite for "tube emulation" - to whatever level of success. Maybe it's simple: maybe you're seeing my desire for what it is; that I don't care what the schematic looks like, I care what the pedal sounds like - and that's true. I could give a fig about "tube emulation" except insofar as it effects the sound and feel of the pedal. If that's the case, you're an excellent internet psychologist as well. Still, what is it about BJTs that make them, in your opinion, a better choice for this attempt? If they are, great - I understand them better than FETs; even though I'm still unschooled on Sziklai (or even Darlington - though I did make a pretty sweet one-Si/one-Ge Bazz Fuss once) pairs.

In any case, in all my reading on effect design, I learned way more from actual answers to actual questions from folks like you, Jack, and R.G. here than I ever did from reading essays in a vacuum. Now that I'm getting advice on my own plans, it can only help a lot as I go forward with this.

[Gus]

Why BJTs? Reply #3 in the following link http://www.diystompboxes.com/smfforum/index.php?topic=114035.msg1057516#msg1057516

Sziklai pair good link http://sound.westhost.com/articles/cmpd-vs-darl.htm

I think BJTs sound fine with a good design.  A guitar +9VDC circuit can be designed so BJTs will work without trimming. 
However I posted something in the past about how I would use JFETs it requires a higher voltage or a + and - dual supply A +18VDC, -9VDC dual looks good for effects, JFETs will bias without trimming.  I first saw the circuit for guitar use in a older solid state Fender Harvard amp schematic.

[whoisalhedges]

If only I knew then what I'm learning now....

I've only read the first few paragraphs of that Sziklai paper, but I think I'm getting the hang of if. Also, I wired up my first attempt from some cookbook circuits and luck. Now I find http://www.electronics-tutorials.ws/ - you know how you can read the same thing over and over, and then finally it just clicks? Either through repetition or because someone finally explained it in a way you could understand? Well, I'm learning a lot more about FETs from the Electronics Tutorials folks.

Options! I like it.

So, hopefully by next week, I'll have 2 or 3 circuits breadboarded, will be able to more or less understand them all, and will be able to make a decision based on both viability and sound.

I'll report back.

[Eb7+9]

I've long been interested in obsessed with onboard tube fuzz circuits, like the Laney Klipp or the subject of my latest (freakin' sweet) pedal, the Garnet Stinger.

Keeping in mind the imperfections of emulating tubes with FETs (and even that's typically been triodes), the prospect of nailing down the Stinger's pentode-derived fuzz was pretty daunting.

So... thoughts?

it will be especially daughting if you make a wrong assumption right off the bat ...

(a) carefully compare the Plate transfer curves for a Pentode against the Drain transfer curves for n-chan jFET
(b) next, figure out what makes the Stinger circuit clip the way it does ...
(c) answer to (b) will suggest what to try and how to try it

Gar's circuit doesn't have diodes and the like ...

Thanks... I do understand (as in, I can see plain as day on the schematics) the differences in topology between the original(s) and my attempt. Also, there are differences between the two versions Gar did make, the 6AN8 version having the Stinger circuit in parallel with the first preamp triode; the 6JW8 version coming after the first preamp in series (but still paralleling the tone stack) - it's the second one that primarily influenced what I'm trying to build, but it's still not meant to resemble a 1:1 conversion of the valve circuit's topology.

Now, why it clips the way it does... there's great value in understanding that, but I haven't been able to suss it out - what I've done has produced a sound that is very similar, but it's no doubt doing it in a different (and possibly ass-backward) way.

One assumption I've made that could be totally wrong: I didn't want to be constrained in needing this circuit to be first in line after the guitar. I have a MOSFET at the input in large part because I thought it could handle a higher voltage swing at the gate without going all screwy than a JFET (or BJT, for that matter), so maybe I could stack it with another drive pedal in front. Is this an incorrect assumption? Would a JFET be a better choice at the input? The IRF510 I had on hand (as well as the cheaper, smaller, more sensible 2N7000 and BS170) have a much better V_GS than does a J201 - but maybe an MPF102 would be a better solution at the input?

And I guess that also raises part of the problem with my limitations in understanding why the Stinger clips as it does: if I raise the gain too much, it no longer sounds like the Stinger. So, if I smack the front end with another drive pedal... that might turn my whole effort on its head.

In a couple of hours spent planning and breadboarding, I managed to make something that has a sound that's 90% of where I want it to be. But the devil's in the details, right?

if you want to emulate something it should be the mechanism at play ...

which means you have to isolate and identify THAT mechanism

ok, I'll spill the beans for y'a

main mistake peeps make is assuming a jFET emulates a Triode ...

WRONG

jFET transfer Drain current curves most resemble (in profile) a Pentode ... not a Triode
of course, the voltage ranges are scaled, etc ...

but it's the clipping specs we're after

so, yeah ... go with a n-chan jFET


how does the Stinger circuit work ?!

simple,

huge plate load so the stage is idling near saturation ...
very little, or none, degenerative NFB in the Cathode/Source circuit

so, it is a very crude "open loop" clipping circuit ...
very 60's

and the Stinger does indeed sound very crude on its own

ie., needs to be mixed with other signal to make paltable
at least that's what it's like in my BTO

but first, we want to emulate the Stinger operation ...
then worry about blending later, etc ...

all you need is a driver gain stage preceding it, so you can set the drive signal right
(and not too much as you mention above)

and "some" jFET + plenty of Drain resistance ...

which jFET you choose will determine how much pre-gain you'll need
and how much of a load to stick on the Drain
(ie., yes, a balancing act using trimmers)

use a large trimmer in the Drain circuit and set to approximate clipping response of Stinger slammer

that should do it ...

remember, you're not looking for something that sounds especially "pretty"

[whoisalhedges]

if you want to emulate something it should be the mechanism at play ...

which means you have to isolate and identify THAT mechanism

ok, I'll spill the beans for y'a

main mistake peeps make is assuming a jFET emulates a Triode ...

WRONG

jFET transfer Drain current curves most resemble (in profile) a Pentode ... not a Triode
of course, the voltage ranges are scaled, etc ...

but it's the clipping specs we're after

so, yeah ... go with a n-chan jFET


how does the Stinger circuit work ?!

simple,

huge plate load so the stage is idling near saturation ...
very little, or none, degenerative NFB in the Cathode/Source circuit

so, it is a very crude "open loop" clipping circuit ...
very 60's

and the Stinger does indeed sound very crude on its own

ie., needs to be mixed with other signal to make paltable
at least that's what it's like in my BTO

but first, we want to emulate the Stinger operation ...
then worry about blending later, etc ...

all you need is a driver gain stage preceding it, so you can set the drive signal right
(and not too much as you mention above)

and "some" jFET + plenty of Drain resistance ...

which jFET you choose will determine how much pre-gain you'll need
and how much of a load to stick on the Drain
(ie., yes, a balancing act using trimmers)

use a large trimmer in the Drain circuit and set to approximate clipping response of Stinger slammer

that should do it ...

remember, you're not looking for something that sounds especially "pretty"

True, but something can be beautiful without being pretty.

I've picked up on a lot of this researching what I built (after I built it - backwards, I know, though I've completely rebuilt the thing by this point). I knew a FET didn't naturally emulate a triode - more misspoke than anything, it's that most of the circuits I see (here, ROG, some commercial pedals) are trying to bias FETs against their nature to try to make them sound like triodes. Of course, what I didn't know is just how much their transfer characteristics resembled pentodes; when I looked at datasheets for 6AN8 and 6JW8 my mind was sorta blown....

I plugged and played my first circuit, and it sounded quite a lot like the front end of the BTO that sat at a local music shop for a year before selling 2 weeks before I had the money but that's another story... I've figured out by now that was dumb luck. The ways my first attempt sounded right were luck, and the ways it was wrong were due to my limitations and mistakes. Learning a ton from this thread, though.

What's on my breadboard right now is a JFET boost splitting the signal into a Bax stack off the drain and a hot MOSFET fuzz off the source. This isn't identical to Gar's scheme, he had the two signals out of phase when they mixed together, mine are in phase at mix - I tried pulling the fuzz from the drain, it didn't sound good; BUT I've made some changes, will continue to make changes, and that could change. Then there's that mu-amp at the end making things very, very loud. Not hugely different from the most recent schematic I posted, but some of the component values have changed.

Here's how green I was when I plugged some FETs in from the cookbooks at the beginning of this project: I didn't understand that one of the key (probably THE key) difference between the JFETs and MOSFET in my circuit is the difference between depletion mode and enhancement mode devices. Um, yeah, that's gonna make a difference in how it works. Turning a JFET "off" isn't the same thing as turning a BJT "off." Once that penny dropped, I made some real progress.

Anyway, the MOSFET "Stinger" right now has its base at idle at 3.269V, while the drain is at 3.285V. That saturated enough?

[whoisalhedges]

OK, what's going on here?

I haven't tried any different circuits yet, because I just spent the past two days trying to figure out just what the hell is going on.... I removed the "master" output control and put it after the Bax stack - so now, like in the original, I have two "independent" volume controls, one for the clean signal and one for the fuzz.

Reply #29 shows the basic circuit, except R23 is now between R15 and R7. I've wired it a few different ways, I've pulled the fuzz output from both the source and drain of Q1 (that is, both in phase and out of phase with the clean signal) - and however I do it, clean is disengaged whenever fuzz is on. I have to disconnect the fuzz stage to get clean to go through (and it's a nice clean boost, with a nice EQ!). Looks like the clean signal is going to ground via R7...?

[whoisalhedges]

Well, OK then!

Crappy Radio Shack breadboard + a crappy jumper that might have an internal short + the world's worst 9v battery snaps (good thing I'm not ready to make stuff for sale just yet, I couldn't in good conscience...) = as it happens, R13 and R15 weren't grounded. So it didn't matter how I tried to adjust R7's connections, it was still the only path to ground. So, as soon as it was turned down to the point where its resistance was low enough - well, you can guess the rest. Turn down the fuzz, the whole signal goes to ground. Glad I didn't pay for it. 

Now I can experiment with Gus' BJTs and Eb7+9's suggestion of a JFET in lieu of the MOSFET.

I have decided to go the whole preamp route, though - when working correctly, with fuzz either turned all the way down or turned off, the JFETs make a really nice boost with a great tone stack: when I end with the mu-amp it's a decent overdrive with the "clean" side turned up; and with a single JFET wired like Q1 it becomes a clean-ish (but EQed) boost that can overdrive the front end of my Laney real nice.* And with that gnarly fuzz engaged, watch out.

*Do not under any circumstances cascade the one FET output into the mu-amp... which reminds me: I need to get my dog off the ceiling.

So, it'll be two footswitches and five pots. I won't be adding the frequency boost switches; the tone stack already has everything from window-shaking bass to icepick-treble. I am utterly clueless at designing PCBs, but even on vero the actual circuit won't be that big. The size of the enclosure will be determined by, well two footswitches and five pots. If I end up going a Sziklai route, the board'll need to be bigger, but still not as big as most circuits with two footswitches and five pots.