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DIY Stompboxes => Building your own stompbox => Topic started by: Athin on July 26, 2004, 12:01:13 PM

Title: ROG 18 will not co-op
Post by: Athin on July 26, 2004, 12:01:13 PM
Hi,
as a breather from my funk-trip I've built the eighteen from ROG, and it kinda doesn't work. I get the sound and all, only it's way too quiet.... I got the bias right (4.5V and 7V) checked the parts - the only thing is that I've used 2n3819's not j201's - those were the only jfets in my garage. Anyways I intend on testing all sorts of jfets - is there a way of getting that 4.5V bias automagically? It takes me a while to bias them fets right and switching 'em and starting all over, well, I'm not that patient... :roll:
Is there something I should watch out for when picking the next jfet type? like drain current or something?
Title: ROG 18 will not co-op
Post by: RDV on July 26, 2004, 12:36:08 PM
The pinout of the 2N3819 is(looking @ the flat side) DGS. The pinout for the J201 is DSG. If you switch the gate and source you may be able to make those work.

RDV
Title: ROG 18 will not co-op
Post by: Athin on July 26, 2004, 01:02:41 PM
I've noticed the difference in pinout... and twisted them pins and triple-checked that they don't connect and still no go :(
Title: ROG 18 will not co-op
Post by: Athin on July 26, 2004, 01:39:46 PM
found something real bad - the source of Q2 is at 6.5V (vs. Drain at 6.8V )... can't figure out why though...
edit 6.8 ) not 6.8)
Title: ROG 18 will not co-op
Post by: RDV on July 26, 2004, 02:48:37 PM
The pinout I described earlier is the Fairchild version. The On Semi & Siliconix versions are SGD.

RDV
Title: ROG 18 will not co-op
Post by: Gringo on July 26, 2004, 03:15:48 PM
You can try with 50k  trimpots instead of 100k. I had a real hard time biasing my thunderchief (sweet spot was around 10k-20k for all the fets i tried).
Title: ROG 18 will not co-op
Post by: Athin on July 26, 2004, 05:01:02 PM
well... I can't tell which I have :( it's got silver lettering and a mercedes-benz-like logo , more of a Y upsidedown (silver). Any way of telling which is the drain and which is the source?
Title: ROG 18 will not co-op
Post by: ildar on July 27, 2004, 01:45:23 PM
Quote from: GringoYou can try with 50k  trimpots instead of 100k. I had a real hard time biasing my thunderchief (sweet spot was around 10k-20k for all the fets i tried).
The trimpots are really bias pots, right? Is there any advantage of using regular pots for these?
Title: ROG 18 will not co-op
Post by: RDV on July 27, 2004, 01:54:31 PM
Quote from: ildarThe trimpots are really bias pots, right? Is there any advantage of using regular pots for these?
No advantage at all to using regular pots unless you are going to use them to set the resistance and replace with a fixed resistor. The trims are a "set it & forget it" type thing.

RDV
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