How to set the Bias on the Quarantine Fuzz: Viral Mutation

Chuck D. Bones

Circuit Wizard
Cooder asked for specific instructions on setting the bias on this pedal, so I'm posting them here for all to see.

Here's the schematic for reference. Bear in mind that this is built on the Quarantine board, but with some jumpers, parts mounted "sideways" and different part numbers and values. This is not a lesson on how to build the mod. Read the schematic, look at the pix in my build report. I've built four of these so far: two on Quarantine boards, one on Vero and one on a Flugelhorn board. They all required slightly different bias resistor values due to transistor variations. The schematic shows the values I used in my 2nd build; they are a good starting point. The target bias voltages I provide work well for me. You're free to try other voltages if you like. No need to set the bias on Q1, stock resistor values work fine.

Quarantine Fuzz - cb mod schematic v5.0.png

These are the select-in-test (SIT) parts:
R7, D3, R11, R15.

The easy way to do this is to mount the board on a non-conductive fixture, like a board holder, helping hands, etc. The only external wires you need on the board are the power leads. You do not need the pots installed. Do this before installing the TONE pot, otherwise you'll end up bending it out of the way. I temporarily install parts by leaving their leads long and using the spring tension of the leads to hold the part in the thru-plated holes and make electrical contact.
We'll start with the 2nd stage. First, you need to measure Q2's Vp. Fortunately, it can be measured in-circuit. Don't install D3. Temporarily install a jumper where R7 would go. Connect a DMM from Q2-S to ground, set it to measure voltage. Power up the board. The voltage reading on the DMM is Vp. If Vp is 1.5V or less, use a red LED for D3, If it's greater than 1.5V, use green. If Vp is over 3V, you're using the wrong JFET. Power off. Solder in D3. Remove the R7 jumper. Temporarily install 220K for R7. Power up and measure the voltage on Q2-D. The target voltage is 5.0V to 5.5V. If the voltage is too high, increase R7. If the voltage is too low, decrease R7. Once you've found the correct value for R7, power off and solder it in.
Now for the 3rd stage. Q3 is germanium and they are temperature sensitive. If you just soldered Q3, then wait 10 or 15 minutes for it to cool down to room temp. Temporarily install 33K for R11. Power up and measure the voltage on Q3-C. The target voltage is 4.5V to 5.0V. If the voltage is too high, increase R11. If the voltage is too low, decrease R11. If you have to go below 22K, your transistor is way too leaky. Power off and install R11.
Almost done! The 3rd stage bias influences the bias of the 4th stage, so make sure the 3rd stage bias is set before proceeding. Temporarily install 470K for R15. Measure Q4-D. The target voltage is 4.5V to 5.0V. If the voltage is too high, decrease R15. If the voltage is too low, increase R15.
After all of the SIT resistors are installed, go back and check the bias voltages once more just to be sure.
You can use other part numbers for Q2-Q4, the process is the same. I used 2N1308 for Q3, but any NPN Ge transistor with hFE > 100 and not too leaky will work. Lower hFE might work, but I haven't tried it.
 
Oooooh for awesome!
Thanks Chuck, celestial explainations and thoughts again to wrap my head around.
Would it be a good idea o any downsides apart from more space required to replaced R11 with a 100k multi turn trimmer to adjust Q3 instead of a fixed resistor? Seeing that GE can be depending on room temp and what day it is in the menstrual cycle? Just to have some easy go back to later?
 
Wow.
You could put in a trimmer for R11. 50K trimmer in series with 22K. I don't think it needs to be multi-turn. I should temp test one of mine to see how much Q3 drifts with temp.
 
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