Gar Gillies' infamous STINGER adapted for stompbox use!

[whoisalhedges]

monitor the drain voltage when you remove your "Rf" (which, incidently, is called something else on your circuit). does the voltage drift up to 9V?

you need R5, whatever value, to provide DC bias to the mosfet gate. otherwise, your input signal will need to be VERY BIG to turn the mossfet on. see also SHO, orman boost.

Yup, that makes perfect sense. I suppose if I wanted to be really devilish, I could directly couple Q2 to Q1's drain and get a DC bias that way? Probably not the best idea I've ever had....

[Gus]

Why are you using a MOSFET?
You can use a source biased JFET with gate to ground resistor

[whoisalhedges]

Why are you using a MOSFET?
You can use a source biased JFET with gate to ground resistor

I wanted to maximize gain and minimize introduced noise (this is unlikely to be a dead-quiet pedal, but no reason not to try). A pedal-building pal said MOSFETS will perform better, give me more gain for the same noise floor/power needs - he also said "they work more like BJTs, so it'll be easier for you to work with" or something like that, which I now understand to mean that it's an enhancement mode device.

If I could get a JFET to do the same thing, I'd be fine with that - or would I need a pair of JFETs?

You mentioned contact bias way back on page one. When I mentioned removing C3 and coupling the gate of Q2 directly to the drain of Q1... is that what you mean? That the V+ coming across R4 would then provide the bias voltage for Q2?

If I were to do that, I assume I'd have to change up the drain & source resistors for Q2 - and popping JFETs in there would require reconifguration as well, I got no problem with that, just want to know what's on the horizon....

[whoisalhedges]

FWIW Gus, I still intend to breadboard your Sziklai circuit - it's way more complicated a build than I've done before, but I need the practice!

End results are what matters here. I'm not married to a MOSFET for Q2, I'd gladly try a JFET or a BJT (or Sziklai/Darlington pair thereof). Bootstrapping, mu-amps - even op amps, which I've never gotten to work right in a drive circuit, I still see no reason a skilled builder/circuit-interpreter couldn't find a way to get the input, output, and fuzz stages out of a quad op amp. Part of what has me wanting this to work is that I like the sound so freakin' much... there are just some bugs that I want to fix - and that might not be fixable in the current topology.

[Gus]

I mentioned contact bias because one of the schematics grounded the pentode cathode and had 10meg  at the grid to ground.  This has its own type of sound and as Eb7+9 posted noted there is a large value plate resistor.

You might not need crazy amounts of gain because of running at 9VDC

I mentioned the JFET because I don't like small MOSFETs I think they are easy to break.  Also a JFET does not need a Drain to Gate resistor

Contact bias has its own sound I do not know if it can be replicated in solid state.

Maybe if you build the circuit with the mosfet on vero the issue might diminish.  All the preamp schematics have a switch to remove the fuzz.

I have a breadboard but I do not use it.  I build on vero or stripboard and modify the circuit as needed.  However if you build enough and use sims you can often get close on the first build.

I use JFETS only when I have to, like when building a condenser microphone.  I think BJTs can sound good in effects.

[whoisalhedges]

I usually use BJTs, this is only my second build with FETs - and my first where they're really "doing" anything (I build an OD with JFETs acting as input buffer and output boost, but a BJT handling the clipping).

[whoisalhedges]

Well now, there's an idea....

After auditioning JFET and BJT options, because I don't want to close any doors, if I choose to stick with the MOSFET?

Well, I can just put in a stinkin' buffer before it, can't I? Maintains the integrity of the signal going to the fuzz circuit, keep anything hanging out on the gate from getting into the clean circuit. Easy peasy.

[whoisalhedges]

Splitting the signal at the input cap, having two first gain stages (as Q1 in the schemo) providing a more-or-less identical but isolated signal to the two signal paths. That works. Now I've still got to work on a few things. The Stinger side is way louder than the clean - it's got another gain stage and no complicated tone stack, after all - so I've gotta put in a little attenuation. Not too much.

[Gus]

You could try a high beta(so you can have a higher value input resistance and stable bias) emitter follower(EF) after Q1, or you could try a source follower. 

Make the emitter resistor a volume control or a fixed voltage divider(kind of like what the RM does with the collector resistor)

The EF buffers and reduces the signal going into the fuzz section.

[whoisalhedges]

Ooh, I like that....

Something else that comes up often in my builds: this one definitely has a volume boost no matter where I plug it in; but the boost is significantly more in the little SS practice amp at my test station than it is in my tube amp. Is this a problem with output impedance?

Some builds I've done (again, not this one) actually have a volume drop when plugged into the Laney, vs. a big boost with the practice amp.

[whoisalhedges]

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 ...

Hm....

I popped in a J201 with 27k (worked my way down from 100, all the way down to 33k there was too much gain/all gated and splatty - 27k was the biggest resistor that worked!) at the drain and  100r at the source. It sounded pretty "good!" I mean to say, it sounded like it's supposed to. Not to mention the whole lack of bleed off the (now nonexistent) feedback. The frequency response was such that it didn't like being mixed with the clean signal, though, there was phase cancellation all of a sudden... so, I popped another gain stage into the clean side. Kept the distortion down; just a nice volume boost before summing, and a phase inversion - two birds. The signals are more even in volume (they're about at unity, give or take, with clean dimed and Stinger at 6-7) so I can have preamp with a little fuzz in the background, a 50/50 blend, or a nice volume-boost on the fuzz; plus there's no cancellation cutting off the decay, sustain being kind of important to me here.

So, it's all JFETs for now, all common source: one at the input, one in each signal path, and then a summer/output stage. The cleanish preamp side I like as is: the Bax stack is flexible enough and I think I've chosen my RC values well. The Stinger side still needs work - I love the way the MOSFET sounds, both its fuzz and the weird and complex comb filtering that goes on up and down the summed waveform due to the phase cancellation; but I'll probably stick with the JFET for simplicity's sake, keeping parts count lower - and honestly embracing some of the limitations of the original. There's still work to do on it, though. The fuzz is great, fuzz-wise, but the frequency response needs addressed. The two variable HPFs formed by the "color" (bleeding highs to ground) and "volume" (more highs are passed through at higher fuzz volumes) circuits certainly work, but "color" is less effective with the JFET than it was with the MOSFET. Even all the way up (bassiest), the treble is shrill and somewhat annoying. A simple treble bleed cap may fix that. I think just about any good fuzz should be annoying at some settings, but I'd like the Stinger side tone control to be something more than "extremely irritating - irritating - slightly less irritating." The volume control works well enough, better than before.

I just did this tonight, haven't even drawn it up. So I haven't had the chance to experiment with some basic filtering. It will be basic: I am trying to emulate a simple circuit, there's no reason to do so with a complicated one. But all in all, I think I'm almost there.

[whoisalhedges]

So, guys... only a little bit relevant to this project....

Is this common for new obsessives? That we bang our heads against something for weeks, and then....

Okay, I'm also working with JFETS, but that's irrelevant to my "whoisalhedges is an idiot" moment. I may at the end of the day wind up with a MOSFET providing the fuzz, and I spent a good week trying to deal with the fuzz feeding back onto the gate that I need to provide a positive bias to the gate.

OR I COULD JUST USE A DAMN VOLTAGE DIVIDER. I mean, I'm going to try out BJTs as well, do I bias them solely with a feedback resistor? No, they've got a voltage divider, or they're directly coupled, or something like that. I mean, I know what the bias was from the MOSFET  circuit I liked... just solve for x....

So yeah, is this the usual path for learning about this stuff? A solution I've been searching for a week now - that for more experienced builders wouldn't have even come up - as they'd have biased it right the first time! - pops into my head while going to bed....

[Eb7+9]

It will be basic: I am trying to emulate a simple circuit, there's no reason to do so with a complicated one. But all in all, I think I'm almost there.

emulating a simple circuit using simple(r) means is possibly a good idea ...
when I met Gar I asked him about it ... he admitted that he designed the Stinger for bass primarily

in the 60's there were very few bass distortion circuits built into ampplifiers
the only known bass fuzz was found in the Acoustic 360 heads
and the fuzz there was of the slamming type, "on-off-on" with signal ...
and maybe some other waveshapping going on ... still, *very* crude ... but also effective

the Stinger was very similar in operation to the 360 fuzz excpt it only had one stage to do it with ...
so, a one-sided slammer

difference between triode and pentode, soft and hard clipping

you'd get hard clipping with a jFET or MOSFET, simply because they have a pinch-off region in their transfer curves
like a Pentode does ... and unlike a Triode which soft-clips (on the saturation side)

so, emulating a single pentode slamming circuit with a jFET or MOSFET is likely the closest class-A emulation possible
short of going a more complicated math-analysis method, which I don't think is needed here

imo Gar's Stinger didn't work well on all his amplifiers // and esp. on Gtr ...
in my 60's Sessionman I found it completely un-usable on Gtr, otoh the amp itself had an amazing plexi-like tone ...
for some reason, in the BTO it fits in well when the amp is cranked - the only Garnet amp I know that works good that way
and, possibly the most insane amp tone I've ever

I hate to say it but I don't think you can package the Stinger sound in a pedal
you can try, but you might end up disappointed ... at least it's a very peculiar challenge
cuz, as you noted above, it wasn't something meant to work on its own ..
at east as far as Gtr playing goes

still, it's interesting to watch someone try

[whoisalhedges]

I think being a newbie puts me in a good position to try.

I don't know what I shouldn't be able to do.