Gotcha, so did your unit have a cap there? Only Q2 had a bypass cap in my friend's unit.bugg wrote:Ahh yes, you are correct. It would have been C11 on your PCB.
Benson - Preamp [traced]
- bugg
- Breadboard Brother
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)
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)
PedalPCB.com - http://www.pedalpcb.com/
Interesting, out of curiousity, is the SMD code on the FETs the same as mine (P1HYE)?bugg wrote:Yep, there was a source bypass cap on the second and third JFET stages.
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?
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: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.
Thanks, I'll update my schematic.bugg wrote:Your cap values are correct, except C8 is 22nF and C5 is 10uF. (The second source bypass cap is also 10uF)
- bugg
- Breadboard Brother
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.
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.
PedalPCB.com - http://www.pedalpcb.com/
Gotcha, I think that's ON's code.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.
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."
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.
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 wellFlyingWild 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.
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.
- Manfred
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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.
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.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.
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Hi Atalas, hi Bugatalas wrote:Yes, they were measured out of circuit by Bugg at 10uF.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.
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
- Nocentelli
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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.
On this schem, Benson uses trimmers, has the drains at around 4.5v and drain resistance is close to 15k.
modman wrote: ↑ Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...
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.Manfred wrote:Hi Atalas, hi Bugatalas wrote:Yes, they were measured out of circuit by Bugg at 10uF.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.
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
Yep, as always the Fetzer Valve is relevant for any JFET based drive pedals.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.
Everytime I read through that, and I'm on probably my fourth or fifth read now, I get at least one eureka moment.
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.
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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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).FlyingWild wrote: I've got some MMBFJ201 on order, but don't relish the idea of sorting through them to find a matched trio!
modman wrote: ↑ Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...
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.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.
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.
- andy-h-h
- Breadboard Brother
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Here's a few options to play with. Thanks for tracing this, much appreciated.
- andy-h-h
- Breadboard Brother
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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...
- plush
- Cap Cooler
Yes, but mind the pinout.knottypine wrote:Could I use 2N5457 in lieu of J201?
No. You can't just simply substitute Jfet with a Mosfet.knottypine wrote: How about BS170 MOSfet?