Benson Preamp  [traced]

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vtguedes
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Post by vtguedes »

I just realized you are using through-hole J201 fets, did you buy these a long time ago? It's been a while since these have been discontinued, and there are several chinese fakes with the Fairchild logo that just aren't as gainy as the real deal, I have a JCM800 preamp with these fake chinese fets that sounds more like a JTM45 with it's gain range.

The only real substitute for a J201 nowadays is the SMD equivalent.

atalas
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Post by atalas »

vtguedes wrote:I just realized you are using through-hole J201 fets, did you buy these a long time ago? It's been a while since these have been discontinued, and there are several chinese fakes with the Fairchild logo that just aren't as gainy as the real deal, I have a JCM800 preamp with these fake chinese fets that sounds more like a JTM45 with it's gain range.

The only real substitute for a J201 nowadays is the SMD equivalent.
So I'm not sure what his reputation is around here, but I got them from Mike at pedalhacker. I've had good luck from him in the past, so I didn't question the authenticity of these J201. I am aware of the chinese fakes, which is why I went to him.

As it turns out though, I ordered 10 J201 SMD from small bear a couple months ago, as well as a few of their SMD->through hole adapters. I'm gonna try them and see what happens.

atalas
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Post by atalas »

vtguedes wrote:I just realized you are using through-hole J201 fets, did you buy these a long time ago? It's been a while since these have been discontinued, and there are several chinese fakes with the Fairchild logo that just aren't as gainy as the real deal, I have a JCM800 preamp with these fake chinese fets that sounds more like a JTM45 with it's gain range.

The only real substitute for a J201 nowadays is the SMD equivalent.
Wow, huge difference with the SMD J201! If anything the clone now has more gain than the benson. They're pretty close to my ears now, but there's something missing is the low mids in the clone, maybe the coupling cap is a bit small going into the bass control? Anyway I did some more samples, which unfortunately will also be the last round because I need to return this pedal to my friend.

Round 2 soundcloud link

The tracks described in order are:
1) Benson, full everything except volume
2) Clone, full everything except volume
3) Benson vs clone, full everything except volume
4) Benson, manipulating the gain, treble, and bass controls in that order
5) Clone, manipulating the gain, treble, and bass controls in that order

I think it's pretty close now, interested to hear what y'all think!

vtguedes
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Post by vtguedes »

Great to see you getting close to the real thing. I couldn't do any breadboarding last week but I'm glad the SMD fets made a difference.

atalas
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Post by atalas »

So I'm not quite sure where to go from here. I've got a very similiar (IMO) breadboarded clone, three J201 SMD that I can measure, and a bunch of samples. The benson had to go back to its owner unfortunately.

Here are the capacitor values I'm using, same as the second set of recordings:
C3: 330pf
C4: 470pf
C5: 1uf
C6: 22nf
C7: 22nf
C8: 33nf
C9: 2.2nf

I've attached another version of the schematic with the above values shown.
NOTE: the caps in red are still not confirmed! These are educated guesses based on other pedals and recording comparisons, have a listen to my samples and decide for yourself if they're close enough.
benson preamp.jpg
I'd like to describe some good specs for the JFETS.
Can we define a good range of Vp and Idss based on what we know about the JFET operation here?

alexradium
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Post by alexradium »

all in all,a tweaked plexi drive from Wampler.
The power of hype.

atalas
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Post by atalas »

alexradium wrote:all in all,a tweaked plexi drive from Wampler.
The power of hype.
Round and round we go :horsey:

Manfred
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Post by Manfred »

What about the value of R13 it is designated with 47k in the very first schematic, but it is not longer designated in the follwing schematics.
Is the value not securely identified?
Nevertheless, I ran a circuit simualtion of the latest schematic using R13 = 47k.
The frequency response showed no major changes depending on the Treble potentiometer settings.
I changed the potentiometer value in steps of 25 Kiloohms.
Are there audible differences in the real circuit depending on the Treble potentiometer settings?
BensonTrebleControl.jpg

atalas
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Post by atalas »

Manfred wrote:What about the value of R13 it is designated with 47k in the very first schematic, but it is not longer designated in the follwing schematics.
Is the value not securely identified?
Nevertheless, I ran a circuit simualtion of the latest schematic using R13 = 47k.
The frequency response showed no major changes depending on the Treble potentiometer settings.
I changed the potentiometer value in steps of 25 Kiloohms.
Are there audible differences in the real circuit depending on the Treble potentiometer settings?
BensonTrebleControl.jpg
Yes, R13 is 47K, I just forgot to add the value.
You can listen to me manipulating the treble control in one of the samples.
I feel like I can hear the changes, might be a bit of placebo effect.
With the clone, it's definitely the first 10% of the pot which does most of the effect.

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Post by Manfred »

With the clone, it's definitely the first 10% of the pot which does most of the effect.
You are right, here the frequency response with the Treble potentiometer setting range from 225 Kiloohms to 250 Kiloohms.
BensonTrebleControl2.jpg
Sorry, the curves in the first frequency response were incorrectly labelled here the corrected diagram:
BensonTrebleControl3.jpg

chasbenson
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Post by chasbenson »

alexradium wrote:all in all,a tweaked plexi drive from Wampler.
The power of hype.
Heh that would be amazing considering I hadn’t even consciously heard of that pedal before this thread. I do agree that the circuit similarities are striking. Turns out there are only so many ways to make an amp in a box?

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Post by Manfred »

I removed the entire output circuit after the drain node of Q3 and add the Wampler Plexi output circuit.
The circuit simulation showes a better treble control range now.
BensonTrebleControl4.jpg

atalas
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Post by atalas »

Manfred wrote:I removed the entire output circuit after the drain node of Q3 and add the Wampler Plexi output circuit.
The circuit simulation showes a better treble control range now.
BensonTrebleControl4.jpg
Improved by not vastly different. Again, there's also the possibility my DMM just couldn't accurately read the SMD caps in circuit.
I do think a 25K pot would suffice. What is making the biggest difference here? The larger coupling cap or the smaller LPF cap maybe?

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Post by Manfred »

In that case the treble control is not a usual low cut filter but a frequency-dependent gain control of the Q3 stage.
In the AC equivalent circuit the seriell circuit consisting of capacitor C7 and the treble potentiometer which is connected in parallel to the drain resistor
and forms a frequency-dependent load that affects the stage gain.

I ran a ciruit simualtion of the Q3 stage, whereby I kept the capacitor C7 and the treble potentiometer and ommitted the rest of the output circuitry.
BensonQ3_StageHighCut.jpg

atalas
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Post by atalas »

Manfred wrote:In that case the treble control is not a usual low cut filter but a frequency-dependent gain control of the Q3 stage.
In the AC equivalent circuit the seriell circuit consisting of capacitor C7 and the treble potentiometer which is connected in parallel to the drain resistor
and forms a frequency-dependent load that affects the stage gain.

I ran a ciruit simualtion of the Q3 stage, whereby I kept the capacitor C7 and the treble potentiometer and ommitted the rest of the output circuitry.
BensonQ3_StageHighCut.jpg
Eh I'm confused now. I'm pretty confident the treble control is hooked up the way it's shown in the schematic.

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Post by andy-h-h »

chasbenson wrote:
alexradium wrote:all in all,a tweaked plexi drive from Wampler.
The power of hype.
Heh that would be amazing considering I hadn’t even consciously heard of that pedal before this thread. I do agree that the circuit similarities are striking. Turns out there are only so many ways to make an amp in a box?
To be fair, most valve amp circuits resemble each other to varying degrees, so I think any similarity should be forgiven and ignored. It's not like you have a history of putting knock-offs on the market and calling them your own.

Nice pedal by the way Mr Benson - I haven't played one yet, cracking design work on the box. Looks very cool. :thumbsup

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Post by Manfred »

atalas wrote:
Manfred wrote:In that case the treble control is not a usual low cut filter but a frequency-dependent gain control of the Q3 stage.
In the AC equivalent circuit the seriell circuit consisting of capacitor C7 and the treble potentiometer which is connected in parallel to the drain resistor
and forms a frequency-dependent load that affects the stage gain.

I ran a ciruit simualtion of the Q3 stage, whereby I kept the capacitor C7 and the treble potentiometer and ommitted the rest of the output circuitry.
The attachment BensonQ3_StageHighCut.jpg is no longer available
Eh I'm confused now. I'm pretty confident the treble control is hooked up the way it's shown in the schematic.
The power supply source is a short circuit for the AC signal and acts as ground for it.
bensonHighCutCircuit.JPG

bugg
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Post by bugg »

atalas wrote:Eh I'm confused now. I'm pretty confident the treble control is hooked up the way it's shown in the schematic.
I can confirm, the Treble control is correct as drawn in your schematic.

You are missing a Source bypass cap on the third FET stage (it would be C5 looking at your pic of the PCB).

atalas
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Post by atalas »

bugg wrote:
atalas wrote:Eh I'm confused now. I'm pretty confident the treble control is hooked up the way it's shown in the schematic.
I can confirm, the Treble control is correct as drawn in your schematic.

You are missing a Source bypass cap on the third FET stage (it would be C5 looking at your pic of the PCB).
Really?! On the pic of the PCB (not my drawn schematic) C5 is the coupling cap coming out of the 3rd FET stage, at least so I thought.

That trace running to C5 also goes to the treble cap, how can that be a source bypass? Also surely the source bypass cap would be installed in the C11 spot (where none was connected on my unit?)

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Post by bugg »

Ahh yes, you are correct. It would have been C11 on your PCB.

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