# Benson Preamp (Son of Ben) trimpots



## disthymia (Sep 18, 2019)

What exactly do the trim pots on the SOB do?


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## HamishR (Sep 18, 2019)

Looks like they're for biasing the J201 Jfets.  Could be wrong.


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## Chuck D. Bones (Sep 18, 2019)

This sounds more like a *General Questions* type of post than a *Troubleshooting* issue. You might want to post this question there.

The trimpots are for biasing the JFETs.  Did you read the notes on the schematic about how to adjust them?  Those are guidelines.  You might want to fine tune them for best sound.


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## disthymia (Sep 18, 2019)

Chuck D. Bones said:


> This sounds more like a *General Questions* type of post than a *Troubleshooting* issue. You might want to post this question there.
> 
> The trimpots are for biasing the JFETs.  Did you read the notes on the schematic about how to adjust them?  Those are guidelines.  You might want to fine tune them for best sound.



When I have the trim pots in the middle or less, there's almost no sound, and when I turn them all the way, there's more volume but no clipping or overdrive.


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## Chuck D. Bones (Sep 18, 2019)

You have to dial in the drain voltages.  JFET Vp & Idss specs are pretty loose, that's just the nature of the beast.  Boutique builders might cherry pick them, but we don't have the luxury of buying 1,000 and picking the best ones for our pedals.


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## disthymia (Sep 18, 2019)

Chuck D. Bones said:


> You have to dial in the drain voltages.  JFET Vp & Idss specs are pretty loose, that's just the nature of the beast.  Boutique builders might cherry pick them, but we don't have the luxury of buying 1,000 and picking the best ones for our pedals.



Sorry, I'm pretty new at this. Thanks for the help. How do I know which voltages to set them to?


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## disthymia (Sep 18, 2019)

Chuck D. Bones said:


> You have to dial in the drain voltages.  JFET Vp & Idss specs are pretty loose, that's just the nature of the beast.  Boutique builders might cherry pick them, but we don't have the luxury of buying 1,000 and picking the best ones for our pedals.



Ah, I just looked at the schematic again. So Q2 and Q3 should be 4.0V? How about Q1? Does that just adjust the gain?


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## zgrav (Sep 18, 2019)

If you want to tweak everything, get Q2 and Q3 set properly and experiment using your ears to look for a setting for Q1.  If you find a setting you like for Q1, then experiment with making really small adjustments to one of the other two transistors.  then again to the third one.   then go back to see if you want to tweak the first one again.       

Most of the time your end result won't be very different from what you had after setting the first two and then adjusting Q1 by ear.  But you might get more satisfaction out of tweaking everything along the way.  Your choice.  : ^ )


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## Chuck D. Bones (Sep 18, 2019)

The trimpots are not for adjusting the gain.  They have some influence on the gain, but it is more important to dial in the sound you want.  I'd either set TRIM1 to 2K per the schematic, or set it to 4V and then tweak from there.  To set it by ear, turn the drive up until you get some mild distortion, then tweak TRIM3 for best sound.  Then TRIM2, then TRIM1.  Then do it in that order one more time.  I recommend adjusting the last stage first because it has the largest signal.


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## Jovi Bon Kenobi (Sep 18, 2019)

I just did this. If you haven't seen my post in General Questions here's a link...





						Trimpot Adjustment - Son of Ben
					

The build doc says  Is there a way to test trim 1 without removing is from the enclosure? For Q2&3, is it the same as when biasing a fuzz face? One probe to ground and the other on the drain?  Thanks!  *Edit for future forum searchers* "son of ben" isn't searchable because the words are too...



					forum.pedalpcb.com


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## Jovi Bon Kenobi (Sep 18, 2019)

In my case, once you set Q2 & Q3 to 4v trim 1 really is just dial it in til it sounds good, but I went full ocd and dialed it into 1.67k per Chuck's instructions. Either way it'll sound good.


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