# Mod Benson Preamp for lower gain?



## fgb1967

Hey,

hoping to get some ideas on how to mod the Benson Preamp for lower gain.

I'd like to get a bit more cleanish range before the Drive knobs goes into dirt.

thanks

Frank


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## phi1

1st, make sure you adjusted the trimmers according to the note on the schematic page.

for lower gain, you could try removing C5 and/ C10


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## Flying

Certainly remove C10, it was only used in the first 100.


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## fgb1967

Thanks for replies,

clipped C10 out, no noticeable difference, but big gain change with C5 clipped. Just what I was hoping!!!

thanks again!


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## Leftovernoise

fgb1967 said:


> Thanks for replies,
> 
> clipped C10 out, no noticeable difference, but big gain change with C5 clipped. Just what I was hoping!!!
> 
> thanks again!



So do you dig the lower gain sounds without the c5 removed? Thinking about making the Benson but also interested in the lower gain side of the pedal.


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## junderwood

Could you put a switch on it to remove these from the circuit? That seems like it might be a nice little addition.


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## Leftovernoise

junderwood said:


> Could you put a switch on it to remove these from the circuit? That seems like it might be a nice little addition.


Actually that would be sweet. Too much of a noob to know off the top of my head how you'd wire that up. But I'd be interested in that


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## fgb1967

Leftovernoise said:


> So do you dig the lower gain sounds without the c5 removed? Thinking about making the Benson but also interested in the lower gain side of the pedal.


Yeah, I do, more range on the Drive knob before it gets real dirty.


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## Paradox916

How clean can it get now? This is very interesting and might be what I’m looking for. With this mod and going a little on the “cold“ side of the bias does it clean up pretty well?


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## fgb1967

maybe an extra 30% on the drive knob before it gets dirty


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## Leftovernoise

fgb1967 said:


> Thanks for replies,
> 
> clipped C10 out, no noticeable difference, but big gain change with C5 clipped. Just what I was hoping!!!
> 
> thanks again!


Just to clarify since I'm interested in doing this also when I build a Benson, by clipped do you just mean you don't have anything in the pads for that capacitor? Or do you jumper the pads?

Also do you re-adjust the trimmers after? Sorry for the noob questions!


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## phi1

Empty, not jumpered.


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## fgb1967

Leftovernoise said:


> Just to clarify since I'm interested in doing this also when I build a Benson, by clipped do you just mean you don't have anything in the pads for that capacitor? Or do you jumper the pads?
> 
> Also do you re-adjust the trimmers after? Sorry for the noob questions!


No probs with the questions, we all gotta learn one way or another.

I built it with the capacitors soldered in, I 'clipped' them with wirecutters to remove them from the pcb.
In your case just leave those capacitors out, I didn't need to adjust the trimmers after I removed the caps.

Good luck, it's a great sounding pedal!


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## phi1

Here’s some background on what’s going on with these.

the reactance from jfet Source to ground does two things:
1. Affects the DC bias of the jfet
2. Affects the gain of the transistor (less resistance means more gain).

DC can’t flow through the cap. So, the DC bias is set by the resistor, regardless of cap. That’s why the trimmers didn’t need reset when the cap was clipped out.

audio signal will flow right through the cap. Basically, audio signal sees the cap as no resistance from Source to GND, resulting in higher gain. Clip it out, and now audio has to go through the resistor.

caps will affect different frequencies differently. I think the C5 and C10 caps are large enough to pass all guitar frequencies (haven’t calculated though). If you use a smaller cap value, you can get higher gain on high frequencies, lower gain on low frequencies.

it’s the same concept as ‘cathode bypass cap’ on a tube amp.


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## Leftovernoise

phi1 said:


> Here’s some background on what’s going on with these.
> 
> the reactance from jfet Source to ground does two things:
> 1. Affects the DC bias of the jfet
> 2. Affects the gain of the transistor (less resistance means more gain).
> 
> DC can’t flow through the cap. So, the DC bias is set by the resistor, regardless of cap. That’s why the trimmers didn’t need reset when the cap was clipped out.
> 
> audio signal will flow right through the cap. Basically, audio signal sees the cap as no resistance from Source to GND, resulting in higher gain. Clip it out, and now audio has to go through the resistor.
> 
> caps will affect different frequencies differently. I think the C5 and C10 caps are large enough to pass all guitar frequencies (haven’t calculated though). If you use a smaller cap value, you can get higher gain on high frequencies, lower gain on low frequencies.
> 
> it’s the same concept as ‘cathode bypass cap’ on a tube amp.


Thank you! I really appreciate this info! This forum is amazing and I'm learning a lot!

Would you happen to know a good resource for learning how to measure the voltage to adjust the bias trimmers?

I have a meter and know how to use it, just not sure exactly where to measure. Is it just from the drain leg on the jfet to ground?


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## phi1

Yep that’s it exactly.


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## Leftovernoise

phi1 said:


> Yep that’s it exactly.


Awesome! And then it sounds like for the first trimmer it's easiest to just set the resistance before installing it.


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## caiofilipini

A lot of good info on biasing the Son of Ben in this thread:






						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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## Leftovernoise

junderwood said:


> Could you put a switch on it to remove these from the circuit? That seems like it might be a nice little addition.


I'm about to build one and I think I'm going to attempt to mod it with a switch so I can have the option of using the c5 cap and not using it.

Don't have much if any modding experience but I think if I put the positive of the cap in it's pad route the negative from the cap to the outer lug of an on on toggle, and then the center lug to the negative hole for the cap, it should be switchable for higher or lower gain.

Not 100% sure if it'll work because I'm semi new at this but I don't see why it wouldn't.


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## phi1

Yep that would do it


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## junderwood

Oh that would be awesome. So when you take that c5 cap out, do you not replace it with a direct connection, or is it just absent? My apologies, I'm not super familiar with the design. If you can just simply remove it for a lower gain mode, then that would be super cool to do the above mod. I'm thinking of building one of these as well, and would totally include that switch as an user accessible option. Looking forward to seeing/hearing your results @Leftovernoise !


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## phi1

For taking out C5 for lower gain, leave those pads disconnected. Don’t link them.


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## Hutzbordello

So just to be clear, its totally okay to just twist and pull those two capacitors out? The two bottom of the three? And I shouldn't need to do anything else?

I honestly don't know anything about modding pedals, however I just got a clone the other day and would like less gain. Hoping this is as simple as I'm thinking it is.


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