Mod Benson Preamp for lower gain?

fgb1967

New member
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
 
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
 
Thanks for replies,

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

thanks again!
 
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.
 
Could you put a switch on it to remove these from the circuit? That seems like it might be a nice little addition.
 
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?
 
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!
 
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!
 
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.
 
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?
 
A lot of good info on biasing the Son of Ben in this thread:

 
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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