Corduroy question (trim pots)

[Pierre67]

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?

 

[Pierre67]

...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.

 

[Chuck D. Bones]

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

 

[Pierre67]

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the first version of the Corduroy PCB, when I build it I couldn't get Q3 drain voltage right, then I saw that there's a newer version of the Corduroy with the trim pots, and reading the new schematic I realized that the first version didn't have the 1M resistor from Q2 gate to ground, so I added a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)...

 

[moonlightpedalbuilds]

Legit J201’s?

 

[Chuck D. Bones]

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.

 

[Pierre67]

Legit J201’s?

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

 

[Pierre67]

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...

 

[Pierre67]

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

 

[Pierre67]

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.

 

[Chuck D. Bones]

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.

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.

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.

 

[Pierre67]

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.

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 is some wire, the stomp switch and the output jack.


My guess is the grounding arrangement is the root cause of the squealing. The only ground wire I can see in you pix is the power ground. If you are grounding the input, output and power to that one wire, then any resistance and inductance in the ground wire provides a coupling path between the power supply, input and output. Try grounding the IN and OUT jacks separately to their dedicated ground pads on the board, that should fix it.

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....

 

[xefned]

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.

 

[xefned]

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.

 

[Pierre67]

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.

 

[xefned]

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	4.5v	4.5v
4.5v	0.7v	4.5v
4.5v	5v	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.

 

[Chuck D. Bones]

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.

 

[xefned]

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?

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.

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

 

[Chuck D. Bones]

I used thru-hole J201s from SB.

Thanks for the kind words.

 

[Chuck D. Bones]

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.