# Calamity Fuzz jfet bias.



## kizzer (Aug 29, 2020)

Hi there

Does anyone have the optimum drain voltages for Q1 and Q2?

I take it the bias resistors are 16k for Q1 and 33k for Q2.

Cheers for your help
Kieran


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## Chuck D. Bones (Aug 29, 2020)

I just fixed a Calamity for a friend.  Once I got the right J201s in there, Q1's Vd was about 4V, Q2's Vd was 0.5V.  Not every J201 will work unless you feel like mucking about with the drain resistors.  That circuit is very picky about JFETs.  You need ones with a very low Vp, like -0.2V or so.  Fairfield must have to hand-pick the ones they put in their production pedals.


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## Bobbyd67 (Aug 29, 2020)

I had better results with smd j201 than the regular genuine ones. Like Chuck said you should use lower spec j201, around .2v.


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## SYLV9ST9R (Aug 29, 2020)

Chuck D. Bones said:


> I just fixed a Calamity for a friend.  Once I got the right J201s in there, Q1's Vd was about 4V, Q2's Vd was 0.5V.  Not every J201 will work unless you feel like mucking about with the drain resistors.  That circuit is very picky about JFETs.  You need ones with a very low Vp, like -0.2V or so.  Fairfield must have to hand-pick the ones they put in their production pedals.


Yes, they do (from what I remember my friend who works there telling me about the Barbershop, that's why I replaced the 2 bias resistors with trimpot on my Chop Shop build).


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## kizzer (Aug 30, 2020)

Cheers for the info guys.


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## kizzer (Sep 21, 2020)

I ended up biasing Q1 drain to 4.6V by swapping the 16k resistor for a 13k. This pedal sounds great!


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## Dali (Sep 21, 2020)

I have a Calamity Fuzz waiting to be build. I also have about 20 PedalPCB MMBFJ201 arriving this week but I don't know how to test drain voltage on these at all.

I'm a total ignorant in electronics (even after more than 15 PedalPCB built, kudo to our leader for that!). Just check my signature. 

I have this multimeter but never use one: 




Any tip, trick, link? I tried YouTube without success (got cars battery checks mainly...)

Thanks i advance.


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## kizzer (Sep 21, 2020)

Hi there

You'll be testing a 9V circuit, so set the Multimeter to the nearest higher value, which looks to be 20V on your Multimeter. 

I used the SMD J201's on my build, had to solder them to the adaptors myself, bit of a pain!!! First of all I would suggest (if you don't already) using SIPP sockets, so you can change transistors if need be. And along the same lines socket the R4 resistor, this is the 16K resistor and this is on the drain of Q1.

With the circuit together measure the input voltage to the circuit with your multimeter. Touch a Ground Pad or Grounded point with the Black probe and touch the V+ pad of the circuit with the Red Probe. In my case I had 9.27V. I always start unless specified otherwise to bias the drain at half the input voltage, which for me was around 4.6V.

The J201 adaptor is marked DSG for Drain,Gate and Source. So with your Black probe touching Ground, touch D (Drain) with your Red probe to give you your reading. In my case it was at 3.62V with the 16k resistor, so I needed to lower the resistor value. 13K got me 4.65V, which sounds great.


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## Chuck D. Bones (Sep 21, 2020)

kizzer said:


> I ended up biasing Q1 drain to 4.6V by swapping the 16k resistor for a 13k. This pedal sounds great!


Did it not sound great with the 16K drain resistor?


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## Dali (Sep 21, 2020)

Thanks a million @kizzer ! I'll do it all and yes, after a few problems, I'm using pin sockets now on all my transistors.


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## kizzer (Sep 21, 2020)

Chuck D. Bones said:


> Did it not sound great with the 16K drain resistor?



Sounded markedly better biased to 4.65V than at 3.6V.


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