chunk chunk biasing/oscillation problems

[eniacmike]

I cannot seem to get my chunk chunk to behave properly. I biased the JFETS to 4.5v using the trimmers and tried adjusting the trim on q1. actually I tried every possible combination of bais on all the jfets and I can't get it to stop oscillating. I also noticed that if I touch the enclosure the oscillation is subject to body contact and the bias of the jfets go up with body contact on the potentiometers or the enclosure.







I made no parts subsititutions, everything is exactly to spec.
I checked for solderbridges and shorts and found none.

[eniacmike]

the only thing I did differently from the build instructions was on the instructions it had the lug wires crossing on all the pots but I mounted my pots upside down so I just wired them straight. They appear to be working in the proper direction though.

[jkokura]

I found that the bias of 4.5v to be too much also. What I did is lower them all to about 1v, and then slowly turn each one up a bit and see what it sounded like (like one at a time a bit, not all of them a bit). I then found that some of the bias points for some J201's was above 4.5v, but some needed to be as low as 2.5. I got mine to sound good, and I'm not sure why this was, because I've been told that's not the way to do it. I tuned mine by ear.

Jacob

[madbean]

the only thing I did differently from the build instructions was on the instructions it had the lug wires crossing on all the pots but I mounted my pots upside down so I just wired them straight. They appear to be working in the proper direction though.

Yes, that is the proper way to do it.

The oscillation can be mitigated by using shielded wire on the input and output of the FX. I do recommend this.

For biasing, you can definitely go by ear. IIRC, I had T2-T4 pretty near the middle and T1 pretty low. This gave the best compromise between gain and noise. As Jacob stated, you may need to go down further than 4.5v.

[eniacmike]

I will try that..

I got some shielded wire from small bear and I planned on using it on my finished build but I was going to wait till I was boxing it up to use it.

I have never worked with shielded wire before it seems really messy to work with.

[eniacmike]

when I am tuning what should the eq and presence be set too?

[jkokura]

Set them to noon I think.

[eniacmike]

I found my issue. I used 1n4001 for the diode instead of 1n4004. I switched it out and it sounds fantastic. extremely quiet almost gated. I love this pedal! I seriously just wrote about 2 albums worth of early metallica/thrash and another album of mid 90's pop punk riffs.

I think every pedal I have built used 1n4001 and I just grab them out of habit without checking. It would have saved me alot of problems.

[oldhousescott]

1N4001 vs 1N4004 in that position shouldn't have made any difference at all, but obviously it did for you, so who am I to disagree? Whatever works, right?

[BassXtreme]

1N4001 vs 1N4004 in that position shouldn't have made any difference at all, but obviously it did for you, so who am I to disagree? Whatever works, right?

I was thinking the same thing. maybe the 1N4001 was bad or the solder joint was cold