Setting trim pots for Argentum 1212 Preamp
- Thread starter PunkAsFunk
- Start date
I think most of us shoot for 1/2 Vcc (so about 9V) at the drains of the J201s (which is nice and easy to do with that SMD footprint, you can just put your DMM probe right in the hole).
Of course there's the "use your ears to decide what's best" crowd too, but 9V is a good starting point for that.
Of course there's the "use your ears to decide what's best" crowd too, but 9V is a good starting point for that.
Also, check this thread for info on what to do in case you can't hit 9V (you might either have to up your trimpot size or modify a few resistor values)
forum.pedalpcb.com
Argentum 1212 200k trimpot for Q2
Built a couple of these and while Q1 and Q3 biased to 9v with room to spare, Q2 on the other hand would only go to 10.35v and 10.65v. MMBFJ201’s from Mouser all same batch. Wondered if anyone else experienced this and if the solution would be a different jfet for Q2 or perhaps a 150k-200k...
forum.pedalpcb.com
PunkAsFunk
Member
Thanks! I got Q1 & Q3 at about 9V. I couldn't get Q2 any lower then 11V because the trim pot was turned all the way.Also, check this thread for info on what to do in case you can't hit 9V (you might either have to up your trimpot size or modify a few resistor values)
Argentum 1212 200k trimpot for Q2
Built a couple of these and while Q1 and Q3 biased to 9v with room to spare, Q2 on the other hand would only go to 10.35v and 10.65v. MMBFJ201’s from Mouser all same batch. Wondered if anyone else experienced this and if the solution would be a different jfet for Q2 or perhaps a 150k-200k...
PunkAsFunk
Member
I'll probably just leave it at Q2 at 11V for the time being, It sounds good as is. Thanks for your help, it is much appreciated!
Based on this teardown video from Gray Bench Electronics, the original Jackson Audio version biases all the JFETS around 11-11.2V.
Too bad, a bunch of people in this thread and the other one are trying very hard to bias all of them down to 9V. I wonder why PedalPCB does not include biasing information in the build doc? Would have cleared up a lot of the above confusion and saved people some trouble.
Too bad, a bunch of people in this thread and the other one are trying very hard to bias all of them down to 9V. I wonder why PedalPCB does not include biasing information in the build doc? Would have cleared up a lot of the above confusion and saved people some trouble.
It's generally a rule of thumb to bias at 1/2 the voltage supply but it ain't written in stone.
A lot of Members will adjust up & down by ear to what sounds best If the transistors are out of spec???
A rule of Thumb is to Ground the PCB to the Enclosure, apparently Jackson failed to do this, the Members may have to disconnect their Ground to enclosure build to make it the same???
PunkAsFunk
Member
Thanks for sharing this. Ya I thought mine sounded good at 11V and if it sounds good then I'm happy. But also glad it matches the OG pedal's bias.
PunkAsFunk
Member
I saw this comment (see attachment below) in the comment section of the video. Thought it would be worth sharing here.
Attachments
I suspected this if no ground was through the plastic jacks, Gray Bench is human & is allowed to miss this if he didn't see a pot leg ground to pot chassis.
A few members here don't ground the Output jack wire, relying on the enclosure from the Input ground to ground it instead???
Other companies use a Spring from the Ground plane on the PCB that makes contact with the enclosure wall.
Last edited:
Laundryroom David
Keyboard Cowboy 🤠
If it sounds good, it is good. Biasing voltages are a ballpark. Chill chill, my good people.
Edit: not meant to be harsh. Just don’t want people stressing about biasing if it sounds good.
Edit: not meant to be harsh. Just don’t want people stressing about biasing if it sounds good.
PunkAsFunk
Member
Ya agreed, I wasn't losing any sleep over it either. like I said I'm happy with the way it sounds. But it is nice to have all the information in one thread post for anyone looking for similar information in the future.
Yes yes, but I still want to know the OG pedal measurements, even if it’s only a starting point to say “This is how the real pedal does it, for better or worse.” I just don’t see an argument for omitting this information; why not let the builder decide to use it or ignore it. Fwiw Madbean, Dead End FX and others usually include such details.
I’m aware of the rule of thumb to bias at half the pedal voltage. But I’ve encountered so many pedals that don’t follow this rule, it’s become kind of meaningless
That's great info. Makes me wonder how many people having biasing issues were doing it with power applied, but the circuit bypassed.
Regardless, it doesn't really change my grievance lol...I'd love to see that sort of information in the build doc, rather than finding it by chance in a Youtube comment. In this case the default assumption of 1/2 voltage is partially true to the builder's process, but in many circuits that rule does not hold true (Chop Shop/Barbershop to name just one).
Regardless, it doesn't really change my grievance lol...I'd love to see that sort of information in the build doc, rather than finding it by chance in a Youtube comment. In this case the default assumption of 1/2 voltage is partially true to the builder's process, but in many circuits that rule does not hold true (Chop Shop/Barbershop to name just one).
Last edited:
Kroars
Well-known member
The more I play around with this guy, the more I like Q2 & Q3 around 9v and Q1 at 10.50v. The pedal sounds good anywhere between 9-11v. 9v for all three is just a bit more compressed and 11-12v for all three has more headroom. Putting Q1 at 10.5v and the other two around 9v seems to be a happy medium between 9v & 11v. I would guess that Jackson biases them around the voltages that GB found. Not really sure what he was talking about with the drastic changes between voltages depending on “the circuit being turned on”, they stayed exactly the same regardless of it being in bypass or active. When you put a signal through it and test the voltages they fluctuate a bit as the signal is passing through, but not between 11v-12v and 7v-9v. In reality this circuit is fairly forgiving with biasing anywhere between 9v-11v.
Robert
Reverse Engineer
I wish I could tell you but mine arrived with all the trim pots at full CCW.... brand new, bias never adjusted before being shipped... seriously.
The 3PDT footswitch was defective, out of the box, so maybe that's why they didn't set the bias, they couldn't get it out of bypass.
Then there was this:
Last edited:
Kroars
Well-known member
For those of us in the know, we appreciate all that you do for us!! Hope you at least took Thanksgiving off!
. You’ve been kickin ass, seems like every week I get a notification of more pcbs incoming. Godspeed, my man!Got this thing finished out to the point of only needing to pop the J201s in and get to trimmin'.
Just for the sake of keeping a massive fluctuation in the JFET specs to a modest minimum, I've pulled 3 from my indexed J201's (all of which have the general measured values taken from from DCA75)
Q1:
VGS[off]: -0.96V @ ID 5.1uA
VGS[on]: 0.32V @ ID 1.45mA
RDS[on]: 578.8 ohms @ ID 1.4mA
IDSS: 0.84mA @ VDS 3.01V
Q2:
VGS[off]: -0.96V @ ID 4.9uA
VGS[on]: 0.31V @ ID 1.47mA
RDS[on]: 569.7 ohms @ ID 1.5mA
IDSS: 0.85mA @ VDS 3.01V
Q3:
VGS[off]: -0.97V @ ID 4.9uA
VGS[on]: 0.31V @ ID 1.5mA
RDS[on]: 556.1 ohms @ ID 1.5mA
IDSS: 0.87mA @ VDS 3.01V
In general I've got (far) less experience putzing around with FETs than conventional BJTs in regards to building circuits.
I understand the need to run curve traces to really get the characteristics of individual FETs, but based on the values I pulled as "close enough," in spec-
am I close in a pseudo "pre-screening" method sort of way?
I only ask so any JFET based circuit builds moving forward can have potential transistor sets pulled as easily as I did for this build.
Thanks in advance
Just for the sake of keeping a massive fluctuation in the JFET specs to a modest minimum, I've pulled 3 from my indexed J201's (all of which have the general measured values taken from from DCA75)
Q1:
VGS[off]: -0.96V @ ID 5.1uA
VGS[on]: 0.32V @ ID 1.45mA
RDS[on]: 578.8 ohms @ ID 1.4mA
IDSS: 0.84mA @ VDS 3.01V
Q2:
VGS[off]: -0.96V @ ID 4.9uA
VGS[on]: 0.31V @ ID 1.47mA
RDS[on]: 569.7 ohms @ ID 1.5mA
IDSS: 0.85mA @ VDS 3.01V
Q3:
VGS[off]: -0.97V @ ID 4.9uA
VGS[on]: 0.31V @ ID 1.5mA
RDS[on]: 556.1 ohms @ ID 1.5mA
IDSS: 0.87mA @ VDS 3.01V
In general I've got (far) less experience putzing around with FETs than conventional BJTs in regards to building circuits.
I understand the need to run curve traces to really get the characteristics of individual FETs, but based on the values I pulled as "close enough," in spec-
am I close in a pseudo "pre-screening" method sort of way?
I only ask so any JFET based circuit builds moving forward can have potential transistor sets pulled as easily as I did for this build.
Thanks in advance
Similar threads
