Benson Preamp  [traced]

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atalas
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Post by atalas »

bugg wrote:Ahh yes, you are correct. It would have been C11 on your PCB.
Gotcha, so did your unit have a cap there? Only Q2 had a bypass cap in my friend's unit.

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Post by bugg »

Yep, there was a source bypass cap on the second and third JFET stages.

The trimpots are also set up slightly differently. Yours uses 50K trims with a 10K fixed resistor, mine has 100K trims with no fixed resistor.

Otherwise everything else is the same as your schematic.

Your cap values are correct, except C8 is 22nF and C5 is 10uF. (The second source bypass cap is also 10uF)

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Post by atalas »

bugg wrote:Yep, there was a source bypass cap on the second and third JFET stages.
Interesting, out of curiousity, is the SMD code on the FETs the same as mine (P1HYE)?
Ideally I'd like to include a little blurb in the schematic about picking JFETs. Can you get readings on the Vp and Idss for the FETs in your unit?
bugg wrote:The trimpots are also set up slightly differently. Yours uses 50K trims with a 10K fixed resistor, mine has 100K trims with no fixed resistor.
Gotcha, so I might just change my schematic to reflect that. I'll admit I'm not an expert in FETs, is there any danger in the drain seeing all 9V, such that you would want a range-limiting resistor? Current should still be limited through the source resistor correct?
bugg wrote:Your cap values are correct, except C8 is 22nF and C5 is 10uF. (The second source bypass cap is also 10uF)
Thanks, I'll update my schematic.

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Post by bugg »

The FETs in mine were marked 62P, which is still a J201.

If I get a chance I'll measure their specs with my DCA75, but not sure if I'll have the time to crack it back open.

atalas
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Post by atalas »

bugg wrote:The FETs in mine were marked 62P, which is still a J201.

If I get a chance I'll measure their specs with my DCA75, but not sure if I'll have the time to crack it back open.
Gotcha, I think that's ON's code.
From what I read it seems like the J201 SMD are fairly consistent, maybe what's known already should suffice, as in "get your J201 from a reputable source."

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Post by FlyingWild »

Here is another photo of the PCB all be it a little fuzzy

https://i.ibb.co/W6TpN7S/IMG-1970.jpg

It looks likes C6 and C11 are not populated just like the version Atalas traced, what effect will adding or subtracting the bypass cap C11 have, a hazy recollection when I built a tube amp a while back was that it increased gain and bass on that tube. But that was on the cathode of a valve, not sure how that relates to a FET.

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Post by atalas »

FlyingWild wrote:Here is another photo of the PCB all be it a little fuzzy

https://i.ibb.co/W6TpN7S/IMG-1970.jpg

It looks likes C6 and C11 are not populated just like the version Atalas traced, what effect will adding or subtracting the bypass cap C11 have, a hazy recollection when I built a tube amp a while back was that it increased gain and bass on that tube. But that was on the cathode of a valve, not sure how that relates to a FET.
That's so funny, I went back and looked at that pic as well :)

I think the cool thing about building pedals is we get to play around. Installing or leaving out a bypass cap is as simple as that, either install it or don't.
I'm ok with the building process being iterative.
The only reason we'd have some prescriptive reason to use a bypass on Q3 other than "it sounds better" is if benson chimed in and gave his reasoning.

Anywho, here is an updated schematic, with verified values from Bugg.
I kept the source resistance setup on Q2/3 true to my unit, although you could just use the 100K trim and no fixed resistor like Bugg's.
Attachments
benson preamp.jpg

Manfred
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Post by Manfred »

Did you measure C5 and C10?
I estimated the size of the capacitors and therefore I would say that the values of they are greater than 10uF.

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Post by atalas »

Manfred wrote:Did you measure C5 and C10?
I estimated the size of the capacitors and therefore I would say that the values of they are greater than 10uF.
Yes, they were measured out of circuit by Bugg at 10uF.

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Post by Manfred »

atalas wrote:
Manfred wrote:Did you measure C5 and C10?
I estimated the size of the capacitors and therefore I would say that the values of they are greater than 10uF.
Yes, they were measured out of circuit by Bugg at 10uF.
Hi Atalas, hi Bug

I thank you for your measurements and tracings, I appreciated it a lot.

I personally would use the Wambler Plexi Drive output circuity for a better treble control

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Post by Nocentelli »

Interesting - Wampler stated that the desired JFETs for the Plexidrive were ones that would bias to half supply with a 15k drain resistor. He claimed to have bought a shit-tonne of through-hole J201s, and sorted through to find ones that fit that spec: He claimed that others that biased to half-supply but required a much larger or smaller drain resistor value did not sound the same.

On this schem, Benson uses trimmers, has the drains at around 4.5v and drain resistance is close to 15k.
Steven_M wrote: 21 Jan 2011, 17:52
biffa wrote: Sorry,but u r not the only member here

I'm not? Well fuck this site then. I only want meaningful conversations with myself.

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Post by atalas »

Manfred wrote:
atalas wrote:
Manfred wrote:Did you measure C5 and C10?
I estimated the size of the capacitors and therefore I would say that the values of they are greater than 10uF.
Yes, they were measured out of circuit by Bugg at 10uF.
Hi Atalas, hi Bug

I thank you for your measurements and tracings, I appreciated it a lot.

I personally would use the Wambler Plexi Drive output circuity for a better treble control
Fair enough, lots of people will want to build a clone though. Perhaps I could work up a mod procedure for the plexi drive -> benson preamp, and one version could leave the output alone.
Nocentelli wrote:Interesting - Wampler stated that the desired JFETs for the Plexidrive were ones that would bias to half supply with a 15k drain resistor. He claimed to have bought a shit-tonne of through-hole J201s, and sorted through to find ones that fit that spec: He claimed that others that biased to half-supply but required a much larger or smaller drain resistor value did not sound the same.

On this schem, Benson uses trimmers, has the drains at around 4.5v and drain resistance is close to 15k.
Yep, as always the Fetzer Valve is relevant for any JFET based drive pedals.
Everytime I read through that, and I'm on probably my fourth or fifth read now, I get at least one eureka moment.

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Post by FlyingWild »

Any chance SMD FETs are more consistent than their through hole ancestors?

I've got some MMBFJ201 on order, but don't relish the idea of sorting through them to find a matched trio! Wouldn't be so bad if they could be tested as easily at the through hole variety.

Thanks for the link to the Fetzer Value page.

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Post by Nocentelli »

FlyingWild wrote: I've got some MMBFJ201 on order, but don't relish the idea of sorting through them to find a matched trio!
You could just mount a random selection of six of them on SOT23 adapter boards, and see if any of them are close to 4.5v with a 15k drain/1k source resistor: SMD are supposedly more consistent within batches, so you may find they all bias up (in which case you are good), or none of them do (in which case it probably isn't worth testing the rest).
Steven_M wrote: 21 Jan 2011, 17:52
biffa wrote: Sorry,but u r not the only member here

I'm not? Well fuck this site then. I only want meaningful conversations with myself.

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Post by atalas »

FlyingWild wrote:Any chance SMD FETs are more consistent than their through hole ancestors?

I've got some MMBFJ201 on order, but don't relish the idea of sorting through them to find a matched trio! Wouldn't be so bad if they could be tested as easily at the through hole variety.

Thanks for the link to the Fetzer Value page.
From what I've read, they are more consistent. I've talked to Brian from SS/BS, and he's echoed that sentiment as well.
Perhaps there's a way to determine what Vp and Idss should be, given we know what our drain and source resistances should be, and what voltages are present there?
If so, then you could use the JFET matcher to pick them out prior to assembly.

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Post by andy-h-h »

Here's a few options to play with. Thanks for tracing this, much appreciated.
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Benson Preamp 3d.png

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Post by chasbenson »

May as well do it right.
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benson%20preamp.jpg

atalas
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Post by atalas »

chasbenson wrote:May as well do it right.
Thanks for the input, schematic updated.
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benson preamp.jpg

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Post by andy-h-h »

Updated to suit. Thanks Mr Benson, you didn't need to be so kind as to add that missing information. Many others wouldn't and haven't...
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BENSON PREAMP.png

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Post by plush »

knottypine wrote:Could I use 2N5457 in lieu of J201?
Yes, but mind the pinout.
knottypine wrote: How about BS170 MOSfet?
No. You can't just simply substitute Jfet with a Mosfet.

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