# Corduroy question (trim pots)



## Pierre67 (Apr 19, 2021)

Hello !
I have build the Corduroy fuzz, it sounds pretty good but I'm having a hard time dealing with the trim pots, I'm getting a high pitch squealing if I set the trimpots to 4.5V drain voltage...Squeals goes away at aproximately 6V- 6.5V.  I have tons of J201s and I already tried replacing them...Am I missing something?


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## Pierre67 (Apr 26, 2021)

...by the way, the squealing only happens in the "big" mode with the fuzz all the way up and volume a little more than half way up...
Any suggestions  will be appreciated.


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## Chuck D. Bones (Apr 26, 2021)

Suggestion #1: Post pictures so we can see how you built it.


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## Pierre67 (Apr 27, 2021)

It's the first version of the Corduroy PCB, when I build it I couldn't get Q3 drain voltage right, then I saw that there's a newer version of the Corduroy with the trim pots, and reading the new schematic I realized that the first version didn't have the 1M resistor from Q2 gate to ground, so I added a "flying" resistor in the back of the board, that solved the issue with Q3. I replace the 15k resistors with trimpots and left C18 out to get a little mor brighter sound. In the tight mode I can have all of the controls maxed without any oscillation (squealing)...


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## moonlightpedalbuilds (Apr 27, 2021)

Legit J201’s?


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## Chuck D. Bones (Apr 27, 2021)

When the drain voltage is dialed in, what are the trimpot resistances?

How are your in and out jacks grounded?

The other way to fix the missing gate resistor problem on Q2 would be to replace C12 with a jumper.


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## Pierre67 (Apr 27, 2021)

moonlightpedalbuilds said:


> Legit J201’s?


Yes, they come from smallbear and other reliable sources (I have 150+ of them).


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## Pierre67 (Apr 27, 2021)

Chuck D. Bones said:


> When the drain voltage is dialed in, what are the trimpot resistances?
> 
> How are your in and out jacks grounded?
> 
> The other way to fix the missing gate resistor problem on Q2 would be to replace C12 with a jumper.


I use common ground to test before boxing...


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## Pierre67 (Apr 27, 2021)

maybe I should ask this : does anybody get squealing/oscillation with fuzz maxed, and volume past half in the big mode?


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## Pierre67 (Apr 27, 2021)

Chuck D. Bones said:


> When the drain voltage is dialed in, what are the trimpot resistances?


To get 4.5V drain voltage, trimpots are between 12.5k and 13K.


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## Chuck D. Bones (Apr 27, 2021)

Pierre67 said:


> To get 4.5V drain voltage, trimpots are between 12.5k and 13K.


OK, that's good.  If you had to get up to 50K I'd be concerned.



Pierre67 said:


> maybe I should ask this : does anybody get squealing/oscillation with fuzz maxed, and volume past half in the big mode?


[Edit]
Mine (v2) squeals in BIG mode with all controls dimed.  It's loud as &^* when VOLUME is dimed; unity on my rig is just below 9:00.

The fact that it only squeals when VOLUME is cranked tells me that there is a feedback path from the input to something after the VOLUME control.  The only things inside the Corduroy after the VOLUME control are some traces, some wire, the stomp switch and the output jack.



Pierre67 said:


> I use common ground to test before boxing...


[Edit]
My guess is there is stray coupling somewhere on the board or in the outboard wiring.


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## Pierre67 (Apr 28, 2021)

Chuck D. Bones said:


> OK, that's good.  If you had to get up to 50K I'd be concerned.
> 
> 
> Mine (v2) does not squeal at any setting.  It's loud as &^* when VOLUME is dimed; unity on my rig is just below 9:00.
> ...


I use the same system for testing all my circuits, in this case, Corduroy has all grounds connecterd via a ground plane, so it should work like many other fuzz/distortions/etc I have build...The fact that this squealing is only in the big mode tells me that the problem must be in that part of the circuit or, maybe, I'm pushing the controls beyond the limits of the desingn. I have unity volume (in both modes) at 11:00. Squealing only in big mode starts past 12:00. 
To be honest, this is not my first corduroy, the first one had the same problem and got trashed trying to do some mods. One thing I have noticed is that I can lower the drain voltage changind the 2N5089 for something with less Hfe, but the fuzz doesn't sound that good....


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## xefned (Oct 10, 2022)

Pierre67 said:


> maybe I should ask this : does anybody get squealing/oscillation with fuzz maxed, and volume past half in the big mode?



Yep. Squeal here. Still freakishly good sounding for a fuzz (which is not my favorite effect in general.) I played it for most of an hour before I even encountered the squeal, because it took that long to find that combination of FAT + fully dimed fuzz. And when I did, it didn't even sound that "wrong." Just like an amp going off the rails. Mine only whines over 4pm on the fuzz dial. Turn it down, and it's back to good.

So if it makes you feel any better, you're not the only one. I'm not even sure it's indicative of a problem because you can dial it out with the trimpots. I realize your post is 18 months old and you've probably solved it, but in case future travelers wander here, I figured I should mention it.

On the plus side, there is no issue with lack of treble here. With so many people reporting a treble problem, this feels like a successful build. I dropped my 50k pot (recommended for more treble) back to the stock 25k and there's still plenty of treble.


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## xefned (Oct 10, 2022)

The only thing I'm doing out of the ordinary is raising the SMD jfets a little high off the board with some right-angle headers. I doubt that would be the cause. I suspect it's all down to variation in J201s. Probably should've socketed them. I'm using the pre-soldered ones from AionFX. They appear to have "62p" written on the top.


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## Pierre67 (Oct 11, 2022)

Hi xefned, tanks for your reply !
…I have never find out the cause of the squealing, I have a large stock of NOS J201 (from reliable sources), tested a few sets and finally gave up.


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## xefned (Oct 11, 2022)

Pierre67 said:


> Hi xefned, tanks for your reply !
> …I have never find out the cause of the squealing, I have a large stock of NOS J201 (from reliable sources), tested a few sets and finally gave up.



I've discovered that you can dial out the squeal and the pedal still sounds fantastic. (Did yours sound great with the fuzz knob down?)

The secret seems to be this: ONE of the 3 FETs has to be WILDLY out of calibration. It doesn't even matter which one. Very strange—some kind of weird interplay between the 3 stages. Chuck might understand what's happening among a small society of driven FETs. They appear to be self-governing individuals dividing up the burden.

For example: with all 3 FETs adjusted for 4.5v drain, squeal city (mind you, only in big mode with full-fat fuzz.) All 3 of the settings below fixed it for me. I settled on #3 only because it's closer to stock. (Stock? I guess standard for this forum. I'm not sure what Brian does.)


Trim 1 (Q2)Trim2 (Q3)Trim 3 (Q4)*0.7v*4.5v4.5v4.5v*0.7v*4.5v4.5v5v*7.5 v*

I'll build another one with socketed FETs and figure out the ideal values to avoid squeal, if that's even the cause. Good luck, I hope you can come back to it and love it.


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## Chuck D. Bones (Oct 15, 2022)

In the first two cases, you have one of the JFETs saturated.  In that last case, you have Q4 near cutoff.  In every case, you have reduced the gain of one stage to a very low number.  A real Wampler Velvet Fuzz would not be biased this way.  While it is not normal to dime all of the controls on a dirt pedal, a good design will not oscillate under those conditions.

I just tried running my Corduroy Fuzz with all controls dimed.  In Tight mode, it is stable.  In Big mode, it oscillates at a few KHz.  Backing off on the FUZZ, VOLUME or guitar Volume kills the oscillation.  This weekend, I'll open it up and see if I can track down the root cause.  Believe it or not, this is one of the few pedals I have built in accordance with the Build Docs.


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## xefned (Oct 15, 2022)

Chuck D. Bones said:


> While it is not normal to dime all of the controls on a dirt pedal, a good design will not oscillate under those conditions.



Yeah, that makes sense. I wondered if Brian just biases his FETs crazy, but that seems unlikely. 
Did you use SMDs too?



Chuck D. Bones said:


> I just tried running my Corduroy Fuzz with all controls dimed.  In Tight mode, it is stable.  In Big mode, it oscillates at a few KHz.  Backing off on the FUZZ, VOLUME or guitar Volume kills the oscillation.



Same with me and Pierre: it's stable in tight mode. It _only oscillates in big mode_. Too much gain on the 2n5089? On mine, FUZZ has to be fully dimed to get oscillation.  Backing off just a tad turns off the oscillation.



Chuck D. Bones said:


> This weekend, I'll open it up and see if I can track down the root cause.  Believe it or not, this is one of the few pedals I have built in accordance with the Build Docs.



You deserve a 🎖️ for improving basically every circuit in need of help. #godAmongMen


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## Chuck D. Bones (Oct 15, 2022)

I used thru-hole J201s from SB.

Thanks for the kind words.


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## Chuck D. Bones (Oct 16, 2022)

OK, here's what I found.  There is a trace running between pins 2 & 3 of the VOLUME control that connects to R2, R7, C6 & C7. Those 4 parts are right at the input to the Big preamp stage.  Even a tiny amount of capacitance between the VOLUME control pads and that trace is enough to sustain oscillations under worst-case conditions.  C6 is an RF filter and is too small kill the oscillations.




Here's my quick & dirty fix.  I lifted the ends of R2, R7 & C7 that are closest to the center of the board.  I removed C6.  That trace that runs next to the VOLUME pads is now out of the circuit.  I connected the loose ends of R2, R7 and C7 above the board.  I connected C6 directly from Q1 E to B on the back side of the board.  This is electrically equivalent to where Mr. Wampler put it in the original design.  I did it this way because it was easy.  That little blue cap peeking from behind the board just above Q1 is C6.  I ended up replacing C7 so I could have a cap with longer leads.


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## Pierre67 (Apr 19, 2021)

Hello !
I have build the Corduroy fuzz, it sounds pretty good but I'm having a hard time dealing with the trim pots, I'm getting a high pitch squealing if I set the trimpots to 4.5V drain voltage...Squeals goes away at aproximately 6V- 6.5V.  I have tons of J201s and I already tried replacing them...Am I missing something?


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## xefned (Oct 17, 2022)

Chuck, you're a flippin' genius. 
(I'm traveling atm, but I'll try this as soon as I'm back in town.)

Thank you so much for all the mods you've contributed.


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## xefned (Oct 18, 2022)

Chuck D. Bones said:


> Even a tiny amount of capacitance between the VOLUME control pads and that trace
> is enough to sustain oscillations *under worst-case conditions*.


Um yeah, about that. My wiring could be contributing to the situation. It's a mess of wire with ins and outs traveling haphazardly to a relay bypass PCB. I could stand to follow paniagua's scheme of shielded ins and outs. That's what I aspire to.


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## xefned (Oct 18, 2022)

Chuck D. Bones said:


> Here's my quick & dirty fix.  I lifted the ends of R2, R7 & C7 that are closest to the center of the board.  I removed C6.  …



*Great news!!!* This mod works! Thanks so much, Chuck.

I just cut the trace and added the bypass cap and now I can max the fuzz knob in big mode with no oscillation. All is well.
I'm shipping this off to my friend Aaron who is struggling with his band trying to make an OD sound like a fuzz.

I'm assuming I don't need to lift the components and cutting the trace is adequate (?) since you already lifted the parts to prove it works.

All trimpots have been reset to give 4.5v on the drain of each JFET. 
If I try really really really hard, I can still achieve a small amount of squeal, but that is with all 3 knobs maxed out and only with hot humbuckers. If I split the coil on either humbucker, the squeal goes away. (It's also only at the very top end of the volume pot.) Only mentioning this for due diligence for future travelers. There is no musical scenario in which you'd max *any* of the 3 knobs, let alone all 3. (Well, maybe the fuzz you'd want to max for mad sustain, but that's been fixed.) Unity volume is around 9 a.m. Max treble is a tad harsh. Max volume is socially irresponsible.

Played for another hour this evening. My daughter appeared at the door and asked me to turn it down.   Another successful build.

Thanks Chuck. Thanks all for being here in this land of sonic possibility.


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## Chuck D. Bones (Oct 18, 2022)

I wasn't sure that cutting the trace would be good enough because even with that one leg removed, the trace still runs too close to the VOLUME pot's pads IMO.  

Wire routing is important with high-gain circuits.  Your routing looks good.  The IN & OUT wires are far apart and routed agains the enclosure walls.  I would not recommend shielding. Some people use it as a band-aid.


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## xefned (Oct 19, 2022)

Chuck D. Bones said:


> I wasn't sure that cutting the trace would be good enough because even with that one leg removed, the trace still runs too close to the VOLUME pot's pads IMO.



It definitely did the trick. But I'll build my next one with the legs lifted away from the volume pads for comparison with this one.
Somehow I convinced all my friends that they need a Corduroy. Or maybe they convinced me. I can't remember.
I will
report back...



Chuck D. Bones said:


> Wire routing is important with high-gain circuits.  Your routing looks good.  The IN & OUT wires are far apart and routed agains the enclosure walls.  I would not recommend shielding. Some people use it as a band-aid.



Thanks dude. I'm loving these relay bypass boards. I wanna retrofit all my favorite builds with them.
Appreciate all your advice.


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## Chuck D. Bones (Oct 19, 2022)

I have it on good authority that the Corduroy board is in line for an update.  Don't know when.


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## xefned (Oct 19, 2022)

Chuck D. Bones said:


> I have it on good authority that the Corduroy board is in line for an update.  Don't know when.



Cool. I will compare lifted to non-lifted, experimenting with the distance to the volume pot to provide data to Robert for the redesign. (I already have 3 rev. 1 PCBs.)

Corduroy #2 now has an _elevated_ mod (green wire below,) farther away from the volume pot than just cutting the trace near the pot's pads. I'll compare them and report back.


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## xefned (Oct 20, 2022)

Here's a TL;DR graphic for anyone else experiencing oscillation with a rev. 1.3 PCB. It's Chuck's mod from above for people for whom words are hard. Connect the 3 red dots. Move C6 (100pF) up to the 2n5089.


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## xefned (Oct 21, 2022)

Side-by-side Corduroys. All 6 FETs at 4.5v at their drains.

Left one oscillates with 3 dimes. Right one (with elevated mod) does not.
This tells me *you should do the full elevated mod ABOVE-BOARD* and not just cut some traces.
That is all.





Course, there are other differences: Intelligent Relay bypass. Not likely to be the cause.
Left one uses LM-386-4 rather than LM-386-1. Also unlikely to be the cause, since it's in the "tight" part of the circuit.

Still, I'll go ahead and mod the left one with an elevated wire. I was just being lazy because I didn't feel like unboxing it.


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## Robert (Oct 21, 2022)

Chuck D. Bones said:


> I have it on good authority that the Corduroy board is in line for an update.  Don't know when.



It's done, waiting for PCBs to arrive to verify.

Anyone who has this issue, or has the PCB and hasn't assembled it yet can shoot me a message when the new revision is available and I'll make sure you get one, no charge.


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## xefned (Oct 22, 2022)

Once the PCB is out of the enclosure, the mod only takes about 10 minutes.
I'm not sure why I feel the need to share this pic. But the takeaway is that I'm now certain that cutting traces is not enough. The offending trace is still near enough to the volume pot to pick up the oscillation.





Having said that, cutting traces is still good enough to keep the fuzz knob from oscillating which is the main thing we care about because nobody will ever max all 3 knobs. Dial back *any* of the 3 knobs, including the treble, or use a single coil, or back off your guitar's volume knob, and you won't hear any squeal. I'm also using high-output humbuckers, so I may be the only person with this “issue” if you can even call it that.

One last thing: starving one of the 3 FETs with a 7v drain as a quick fix is just as well. This pedal still sounds like a sex machine with an anemic 3rd FET. I've now played many hours with this pedal and it's addictive in any mod state including just cutting the trace that runs between the volume pot lugs, or starving a fet.


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## Robert (Nov 2, 2022)

Updated PCB prototypes are here, no more oscillation at any setting, yay!    

What's that BB King song?  The Squeal is Gone!


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## Joben Magooch (Nov 3, 2022)

I just have to give some shout outs here. I know very little about circuit design, it's mostly paint-by-numbers for me, but am always looking to learn more...I don't have this particular pedal, but just wanna say it was cool to see this discussion go down, Chuck walking thru the circuit and finding a workaround/solution, and then Robert making a revision and offering it out for free to anyone with the old PCB. Cool gesture to see and I feel like I learned a little bit, to boot.


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