k3yPr0gg3r
New member
Hi,
Just wondering what the bias settings are for the Indiana builds? Tweed has 3, Deluxe has 4.
Thank you,
Just wondering what the bias settings are for the Indiana builds? Tweed has 3, Deluxe has 4.
Thank you,
Re: Indiana Tweed / Deluxe Bias
- Thread starter k3yPr0gg3r
- Start date
4-4.5V on the drains.
I can't find the source so it's entirely possible I'm misremembering, but I swear I read somewhere that because the originals didn't have trimpots, Wampler would basically spot check (and swap if necessary) JFETs to make sure they were all biased at 4V or higher, anything more "exact" than that probably wasn't super critical to the circuit's sound (and I'm sure he had batches of JFETs pre-selected for certain specs so the variation wasn't all that huge compared to what we hobbyists deal with).
I can't find the source so it's entirely possible I'm misremembering, but I swear I read somewhere that because the originals didn't have trimpots, Wampler would basically spot check (and swap if necessary) JFETs to make sure they were all biased at 4V or higher, anything more "exact" than that probably wasn't super critical to the circuit's sound (and I'm sure he had batches of JFETs pre-selected for certain specs so the variation wasn't all that huge compared to what we hobbyists deal with).
1stbsmn
New member
FWIW, I own a reissue of the Black 65 and I had this question as well. Every JFET in the Black 65 is biased to 4.9V from the factory.
Last edited:
darwin999
Well-known member
Here's my comments on this Indiana Tweed (Wampler Tweed 57) bias topic, hopefully helpful to others.
On Guitar FX Layouts, lvlark said this about biases:
tagboardeffects.blogspot.com
Having read this beforehand, when I first installed the J201s in my Indiana Tweed, I naturally adjusted the 3 trimmers to get bias = 4.5V. But the box still didn't have much gain, and the waveform didn't look that distorted on my scope, even at max gain. I knew my J201s were legitimate, as they were Fairchilds that I'd bought from Mouser ~25 years ago. So when I measured the FETs on a DCA75 tester, I noticed that the ones I had randomly selected had gm values at the lower end of specs (thus lower gain) plus the FETs had a ~30% variation in Vgs(off) - especially for the Q2-Q3 and Q4-Q5 pairs. That concerned me, as those pairs use shared trimpots. So I went back through my J201 stash and found some with higher gm & higher Vgs(off) values, plus I closely matched Vgs(off) for the Q2-Q3 and Q4-Q5 pairs. Wow, did that make a huge change in the overall gain! And the waveform looks like I'm finally cooking with gas, too! I'll post an update on how it sounds later, as I didn't have the time to plug it in today. But I'm psyched.
On Guitar FX Layouts, lvlark said this about biases:
You may want to measure the drain of all the FETs. Brian has said that the drain voltage of Q1 to Q5 should be as close to 4.5V as possible. He also said it was important that none are under 4.5V so I don't think this requires critical biasing, just make sure you're getting at least 4.5V from those FETs, or swap and change as necessary.
Wampler Tweed '57
Collection of vero (stripboard) & tagboard layouts for 100s of popular guitar effects, with over 1000 verified designs. DIY your own boutique effects!
tagboardeffects.blogspot.com
Having read this beforehand, when I first installed the J201s in my Indiana Tweed, I naturally adjusted the 3 trimmers to get bias = 4.5V. But the box still didn't have much gain, and the waveform didn't look that distorted on my scope, even at max gain. I knew my J201s were legitimate, as they were Fairchilds that I'd bought from Mouser ~25 years ago. So when I measured the FETs on a DCA75 tester, I noticed that the ones I had randomly selected had gm values at the lower end of specs (thus lower gain) plus the FETs had a ~30% variation in Vgs(off) - especially for the Q2-Q3 and Q4-Q5 pairs. That concerned me, as those pairs use shared trimpots. So I went back through my J201 stash and found some with higher gm & higher Vgs(off) values, plus I closely matched Vgs(off) for the Q2-Q3 and Q4-Q5 pairs. Wow, did that make a huge change in the overall gain! And the waveform looks like I'm finally cooking with gas, too! I'll post an update on how it sounds later, as I didn't have the time to plug it in today. But I'm psyched.
