Corduroy PCB - retry to build - J201 Idss and VP

[harmaes]

I got a new Corduroy PCB and received new MMBFJ201s.
I also build a small JFET tester this time so I can measure Idss and Vp Of the J201.

So the MMBFJ201 I used before in the failed build have the following Idss and Vp values:
- Idss 0,67mA, 0,76mA and 0,78mA
- Vp -0,81V, -0,86V and -0,88V

The new MMBFJ201 are much lower:
- Idss 0,34mA, and 2x 0,43mA
- Vp -0,57V and 2x -0,64V

Older J201 I have are spanning Idss: 0,46mA to 0,5mA and Vp: -0,69V to -0,73V

So the first ones are possibly the best ones to use in the Corduroy (Wampler Velvet Fuzz) circuit?

In the Fetzer valve calculator it mentions these values:


- My questions are which J201 would suit the Corduroy circuit best and are the closest to “Wampler Ideal” values?
- Are the Rd (and Rs) values tbe ideal values to set for the bias on the trimpot?

Thanks for your help!

 

[harmaes]

I read here that the J201 in the Plexi circuit are biased with a 15k resistor.

Plexi-Drive JFET misery

Plexi-Drive JFET misery

www.diystompboxes.com

If I calculate the MMBFJ201 I received today which have lower values for Idss and Vp then these get close to 16k resistance:


So is my assumption correct that these lower measuring ones would suit this circuit better?

 

[harmaes]

@PedalPCB Can you inform me which fixed resistor values you had in v1 of the Corduroy when it didn’t have the trim pots yet? And did you measure the J201 in Brian Wampler’s Velvet fuzz pedal? Would be great to have a starting point to select the best J201s ! Cheers!

 

[harmaes]

So the previous build had issues like the brightness pot not working and I think something else must've been broken because I had issues with biasing the J201s.
This new build worked out fine and everything works. I tried the lower valued J201 and the ones I had from the first build.
Both work when biased around 4,5v but the lower value ones have less dirty fuzz, it's more like a mild fuzz.

I've used these ones now and they're set at 4,5v at the drain now.

- Idss 0,67mA, 0,76mA and 0,78mA
- Vp -0,81V, -0,86V and -0,88V

So I will put the circuit in an enclosure and then will tune the biasing a bit by ear.
Now it's oscallating a little but that's probably related to grounding. I tested it on the breadboard now so it's picking up a lot of signals then.

I'm going to build a Duality Fuzz (Darkglass) soon, will be cool to compare.

@PedalPCB I'd still be interested in an answer to my questions, thanks!

 

[harmaes]

I need your help with something:

I’ve played the Corduroy some more and found it too dark so I removed the 25kB pot and replaced it with a 50kB. It’s brighter now but the brightness pot doesn’t do anything anymore (which it did with the first build as well!)

Removing the pot was sensitive because it’s a tight space and it overheats quickly.
So after removing and replacing the pot I measured the connections and it connects to C19, and mass so it should work?
If I read the schematic correctly this 10n connected to the brightness pot to mass looks like a normal guitar tone pot?

- Chuck mentioned that the bias of Q4 influences the brightness pot as well. Could the bias be set to a level that the brightness pot doesn’t work anymore? I can’t imagine it but just asking.
- Not sure what else to measure from the brightness pot? Anything make a false contact maybe?

Thanks for your help!

 

[MichaelW]

I need your help with something:

I’ve played the Corduroy some more and found it too dark so I removed the 25kB pot and replaced it with a 50kB. It’s brighter now but the brightness pot doesn’t do anything anymore (which it did with the first build as well!)

Removing the pot was sensitive because it’s a tight space and it overheats quickly.
So after removing and replacing the pot I measured the connections and it connects to C19, and mass so it should work?
If I read the schematic correctly this 10n connected to the brightness pot to mass looks like a normal guitar tone pot?

- Chuck mentioned that the bias of Q4 influences the brightness pot as well. Could the bias be set to a level that the brightness pot doesn’t work anymore? I can’t imagine it but just asking.
- Not sure what else to measure from the brightness pot? Anything make a false contact maybe?

Thanks for your help!

Might get more traction if you started a new thread over in the Troubleshooting section.

 

[Feral Feline]

Perhaps ... if the pedal is working, open a thread in the Modifications sub-forum.