Acrylic Overdrive bias

Crispy

Member
i was wondering whats your method of biasing these Jfets in this Plexi Drive circuit. do you use your ear or do you have a set value you aim for? if you use your ear what are you listening for?
 
Bias each one to about half voltage. Then tweak them by ear until the sound “snaps” together to your taste
Hey Buddy. I tried that and set all to 4.5.
Pedal sounds great but it's REALLY bass, even in the standard (bass cut) mode. I find myself running tone and gain on max... Just wondering if I should try to bias it colder or hotter to remedy this bass thing and produce a bit more gain or would increasing add more low end (like a bigger capacitor)
 
Thanks for the info.
I'd like to do something to alter what I'm getting, as I find the non-cut setting almost unusable on most of my guitars... It sounds pretty good with the tone cranked into my tele, but mostly I run the bass cut setting and the tone around 9-10... The gain is also pretty lacking, imo... but I just attributed it to replicating a legit Plexi, which isn't a really gainy amp... and most old Marshall's are bass heavy amps (they are based on a bassman circuit after all)...
I've not really explored much on if B Wampler was trying to make a legit "amp in a box" with this pedal or if it was supposed to be a distortion circuit that emulated the character of a cranked plexi... kinda like the "Bluesbreaker" pedal.

If I could add a bit more treble/less bass and bump up the distortion a touch, that'd be killer. I'll recheck my bias again but I biased it once, let it "cook" for a bit by leaving it on, then checked bias again, running it 4.5v at each drain.

When I built it, I used thru-hole J-201's I had bought from Mouser, and they were FRIGGIN EXPENSIVE... Probably should have bought smds on the adapters to be sure, but I had them already and decided to roll with them. Mouser is reputable, and had they been fleabay J201's I probably wouldn't have used them... but I digress...
 
There are things we can do to increase gain and alter the tone.
What are the voltages on Q1-S, Q2-S and Q3-S? Not all J201s are created equal and first we need to see whatcha got.
I'll wager that Wampler's production pedals do not have trimmers inside. Smart builders like Brian Wampler will hand-pick the JFETs to obtain the desired performance. If you've read my rants in Chuck's Boneyard, then you'll know that JFETs (even the ones we buy from Mouser) are not plug-n-play.
 
There are things we can do to increase gain and alter the tone.
What are the voltages on Q1-S, Q2-S and Q3-S? Not all J201s are created equal and first we need to see whatcha got.
I'll wager that Wampler's production pedals do not have trimmers inside. Smart builders like Brian Wampler will hand-pick the JFETs to obtain the desired performance. If you've read my rants in Chuck's Boneyard, then you'll know that JFETs (even the ones we buy from Mouser) are not plug-n-play.
I biased all at 4.5v on the drains (I had seen that information on another thread) It was my first time really messing with trim pots other than a Treble booster from BLAMMO! Electronics I was having trouble with... I have a Walrus Audio Defcon 4, which has directions for adjusting trim pots, but they just tell you "set the pointers here for factory default, but move them anywhere they sound good", which seems like TERRIBLE advice, given you aren't monitoring any voltage whatsoever and counting on EVERYTHING component-wise to be perfectly in spec.
 
There are things we can do to increase gain and alter the tone.
What are the voltages on Q1-S, Q2-S and Q3-S? Not all J201s are created equal and first we need to see whatcha got.
I'll wager that Wampler's production pedals do not have trimmers inside. Smart builders like Brian Wampler will hand-pick the JFETs to obtain the desired performance. If you've read my rants in Chuck's Boneyard, then you'll know that JFETs (even the ones we buy from Mouser) are not plug-n-play.
 

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You're not gonna measure the voltages on the JFET source pins then?
Lol, sorry Chuck... It's been a long hot day at work and I had a brain fart. I actually just cracked the pedal open on my bench to check just that...
I totally missed the -s on your last message
 
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