Pedal building problems... volatile FET bias? :s

[Pierre]

I bought two pedals from OLC circuits and I finished building them a while ago. Yesterday I bought new batteries and therefore redid the bias. I got the Thunderchief to work 100% but the Slow Century (SLO preamp emulation with FETs) is causing me problems.

Q1, 2 and 3 bias perfectly to 4.5Vs. but Q4 wouldn't go lower than 7.9! With the old batteries, everything went fine :s
When I move some components (switch and input jack mostly), the Q4 bias changes, and I can get it right. Q1, 2 and 3 DON'T change. It's proper weird.
I changed the FET (A J201) for another to see if I had a deficient one, but no luck. I replaced it with a MFD102 as well, nothing either. New battery did nothing either
I'm really stumped as to what the problem could be. Thanks A LOT if you can help!

When the SLO worked, it sounded REALLY cool, and I'm loving the Thunderchief as well. OLC circuits are HIGHLY recommended

 

[HolyDirt]

Re: Pedal building problems... volatile FET bias? :s

My best guess is something is shorting out. When you change the components, you're probably moving leads around causing it to partially or completely short out part of the circuit, thus possibly changing the bias on the FETs. MY best guess is to check all of the connections to make sure nothing is shorting. Changing stuff like input jacks shouldnt change the bias, so I think a short is your best answer.

 

[Vasshu the humanoid typhoon]

Re: Pedal building problems... volatile FET bias? :s

They are a pain to match!!
P.A.I.N. I got something nice for you Pierre by the way
I will hopefully be sending it next week

 

[Pierre]

Re: Pedal building problems... volatile FET bias? :s

Thanks bro! So will the next pickup!
The Thunderchief's FETs balance very well though. But I think I'll rewire the jacks and switch, just for the sake of it, and see what happens. Thanks guys!