# Trimpot Adjustment - Son of Ben



## Jovi Bon Kenobi

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 long, short, or common. Ha. Here ya go..."Benson preamp"


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## Mourguitars

Think you could post pic's of the process in your other thread for us newbie's JBK ?


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## Chuck D. Bones

With the power off, connect Q1 drain to Vcc.  Now you have the pot in parallel with 10K and nothing else.  2K in parallel with 10K = ... now let's not all raise our hands at once... 1.67K.

Unless otherwise specified, all voltages are measured w.r.t. ground.

After that, I'd give it a listen and tweak as req'd.


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## Jovi Bon Kenobi

Mourguitars said:


> Think you could post pic's of the process in your other thread for us newbie's JBK ?


Id like to keep that thread as basic as possible


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## Jovi Bon Kenobi

Chuck D. Bones said:


> With the power off, connect Q1 drain to Vcc.  Now you have the pot in parallel with 10K and nothing else.  2K in parallel with 10K = ... now let's not all raise our hands at once... 1.67K.
> 
> Unless otherwise specified, all voltages are measured w.r.t. ground.
> 
> After that, I'd give it a listen and tweak as req'd.


This is slightly out of my grasp of understanding. I've tried googling where the VCC function is on the multimeter but came up more confused.
What I've done is set my multimeter to the "v" mode then ran a probe to ground. Then measured the drain pin on Q2 and Q3. Both trimpots were then dialed to 4v. I still don't understand how to read trimpot 1 to 2k


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## Jovi Bon Kenobi

I should specify, I think I successfully biased Q2 & Q3 to 4v with the pedal plugged in to my power supply but not engaged. I'm at a loss on how get the trimpots value to 2k while it's in the enclosure. What I _did _do is just turn trim 1 until it sounded good. So, I guess I did the opposite of what you suggested. Anyway. It sounds GREAT!


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## Chuck D. Bones

At the end of the day, that's what matters!  

FYI, here's more specific instructions on setting TRIM1 to 2K.  Assume that you are viewing the component side of the board with the DRIVE & VOLUME pots at the top.
1) Disconnect power from pedal.
2) Install jumper (clip lead or tack solder) from Q1 drain (left lead) to D100 cathode (right lead).  D100 cathode is the point in the schematic named Vcc.
3) Set meter for Ohms, 2K or 2,000 scale.
4) Connect meter one meter lead to one end of R5, connect the other meter lead to the other end of R5.
5) Adjust TRIM1 until meter reads 1.67K.  You don't have to hit it dead-on, just get between 1.5K and 1.8K and you're close enough.
6) Disconnect meter & jumper lead.


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## Robert

For prototyping I just adjusted so the resistance measured from VCC (Cathode of 1N5817) to Q1 Drain was 12K.

12K total (Trim1 + R5) is what you're really shooting for (this was a fixed resistor in the original), the trimpot was added in the event that further adjustments were necessary.


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## Jovi Bon Kenobi

Ah. Ok I get it now. Thanks thanks for clarifying! ?


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## Chuck D. Bones

Robert said:


> For prototyping I just adjusted so the resistance measured from VCC (Cathode of 1N5817) to Q1 Drain was 12K.
> 
> 12K total (Trim1 + R5) is what you're really shooting for (this was a fixed resistor in the original), the trimpot was added in the event that further adjustments were necessary.


What about the sneak path from Vcc to ground thru all the FETs?  Even if you dial the other two trimmers up to max resistance, they still influence the measurement.  I guess it gets close enough, hmmmm?


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## Jovi Bon Kenobi

I just ran it through again with that method, Chuck. 



I dialed it in to exactly 1.67k. Boy oh boy does it sound great now! Trim 1 is interesting. It goes from no sound to a broken crackling starved voltage sound to perfection. I can see it being tricky for people. Thanks for the excellent method.


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## Robert

Chuck D. Bones said:


> What about the sneak path from Vcc to ground thru all the FETs?  Even if you dial the other two trimmers up to max resistance, they still influence the measurement.  I guess it gets close enough, hmmmm?



It worked fine for me, but if it hadn't I would have just tweaked it by ear.     There's really no reason _not_ to take the extra time and do it the way you suggested though...  (or just measure it out of circuit before installing)

The original used a fixed 12K with no adjustment possible, which really baffles me considering the other two stages have trimmers.

I'd be curious to know what the VCC -> Drain measurement reads (with all FETs installed) after adjusting to 1.67K.


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## Chuck D. Bones

Had I know that Q1 was in a socket...
1) Remove Q1 from socket.
2) Measure from D100 cathode to the bottom end of R2.
3) Adjust TRIM1 for 2K Ohms on meter.


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## Mourguitars

Very cool...now i get a grasp on how its done, a picture is worth a thousand words !

Glad you got it JBK..

Mike


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## CanadianDave

This info is really helpful. The Benson is next up on my work table.


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## cooder

Excellent, just adjusted the populated pcb it this way, all good! Thanks for this explanation!


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## cooder

For what it's worth, I measured after adjusting trim 1 to 12kohm the drain measurements on Q1 as 5.1 V / Q 2 and 3 adjusted to 4 V each, input voltage 9.6 V.


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## Vinnyallen

Can someone explain busing q2 and q3? I ran a probe to ground and a probe to the drain on both with the pedal plugged in. They both read 9v and that value doesn’t change when I adjust the pot. What am I doing wrong? I successfully adjusted q1 to almost 1.67


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## music6000

They look like SILICONIX J201's by the Dot on Top, where were the sourced ?
Do they have Markings like this:


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## Bobb Lobblaw

Vinnyallen said:


> Can someone explain busing q2 and q3? I ran a probe to ground and a probe to the drain on both with the pedal plugged in. They both read 9v and that value doesn’t change when I adjust the pot. What am I doing wrong? I successfully adjusted q1 to almost 1.67



Yes, Co-sign this request! 
Please can some one explain this process as if you were talking to a sixth grader raised by wolves in a national forest?


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## Chuck D. Bones

Step 1. Check your spelling before hitting send.
Step 2. Verify your JFETs are actually good JFETs.
Step 3. Inspect your board for workmanship.

At one end of the trimpot travel, you should get close to 9V.  At the other end, you should get substantially less, maybe something close to zero.  If that isn't happening, there is something wrong with your pedal.


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## Chuck D. Bones

Bobb Lobblaw: do you know how to use a DVM?

I have to ask, because none of the wolves I know even savvy what a DVM is.


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## music6000

Bobb Lobblaw said:


> Yes, Co-sign this request!
> Please can some one explain this process as if you were talking to a sixth grader raised by wolves in a national forest?


Hope this will get you there :


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## Bobb Lobblaw

I'm learning as I go, Chuck. The Wolves are pure analog, baby.

Thank you, M6K.
This is only my third build (not counting my failed Echo Nightmare), and I am definitely a newbie when it comes to metering and such, so I'm basically trying to learn it all at the same. But that's how I thought the process was supposed to go.
HOWEVER..
Here's the thing. Well, two things.
1) According to the datasheet of the installed "InterFet J201" the pins go 1) source, 2) drain 3) gate.(https://www.mouser.com/ProductDetail/InterFET/J201?qs=/ha2pyFaduh6EYvyYo4gV6ZjdGsoMaCrkP1C2KUJ4mQ=)

2) Unfortunately, that doesn't seem to matter because when I do as you so awesomely illustrated, (Meter set to 20 volts)  I get no reading at all from any pin.  Nada, zilch. Be it plugged in and (true) bypassed , plugged in and on. Unplugged...I just get nuthin'.
If it's plugged in and bypassed & I put the ground on the "source" (1)  and the other on the drain (2)  I get a reading that I can dial to "4." I have no idea if that is actually 4v or if it's just teasing me.

The pedal works and sounds pretty good. . But of course, if I can reach for the ULTIMATE TONE, man... I'd like to.
Interestingly, with the drive all the way down, I get no sound at all even with the volume cranked. I don't think that's supposed to happen.


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## music6000

Bobb Lobblaw said:


> I'm learning as I go, Chuck. The Wolves are pure analog, baby.
> 
> Thank you, M6K.
> This is only my third build (not counting my failed Echo Nightmare), and I am definitely a newbie when it comes to metering and such, so I'm basically trying to learn it all at the same. But that's how I thought the process was supposed to go.
> HOWEVER..
> Here's the thing. Well, two things.
> 1) According to the datasheet of the installed "InterFet J201" the pins go 1) source, 2) drain 3) gate.(https://www.mouser.com/ProductDetail/InterFET/J201?qs=/ha2pyFaduh6EYvyYo4gV6ZjdGsoMaCrkP1C2KUJ4mQ=)
> 
> 2) Unfortunately, that doesn't seem to matter because when I do as you so awesomely illustrated, (Meter set to 20 volts)  I get no reading at all from any pin.  Nada, zilch. Be it plugged in and (true) bypassed , plugged in and on. Unplugged...I just get nuthin'.
> If it's plugged in and bypassed & I put the ground on the "source" (1)  and the other on the drain (2)  I get a reading that I can dial to "4." I have no idea if that is actually 4v or if it's just teasing me.
> 
> The pedal works and sounds pretty good. . But of course, if I can reach for the ULTIMATE TONE, man... I'd like to.
> Interestingly, with the drive all the way down, I get no sound at all even with the volume cranked. I don't think that's supposed to happen.


Here is the Correct pinout for the J201 Transistor:


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## Robert

Bobb Lobblaw said:


> 1) According to the datasheet of the installed "InterFet J201" the pins go 1) source, 2) drain 3) gate.



Do you by _any chance_ have one of those that is _not_ installed in your pedal?   I'd be curious to know what sort of resistance measurements you get between the Source / Drain pins and the Drain / Gate pins, as indicated in that datasheet.

It's certainly not impossible that InterFet changed the pinout, but this is the first I've heard of it.


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## music6000

It appears Interfet have reinvented the J201 with their own Pinout if you follow the same basic shape, 3. Gate 2. Drain 1. Source
In order for it to work with the Original Standard J201 Pinout, 1. Drain 2. Source 3. Gate , It has to be flipped 180 Degrees!
The Drain & Source are interchangeable so it should work after this Mod!


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## Bobb Lobblaw

Ahh, crap on toast..
So I have to remove them from the PCB, flip them and resolder "backwards?" 
Well, there goes my pretty wiring job.
OK, I'll give that a shot, thanks very much.


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## Chuck D. Bones

No, don't remove any parts.  If your pedal is working then there is no problem with the FETs.  You don't need a DVM to fine tune the FET bias, it's only used to get you in the ballpark.  Adjust by ear.  Start with the one next to Q3, then Q2, then Q1.  Make at least two passes.  The adjustments can be pretty subtle, so take your time.  The Q3 trimmer also affects the TREBLE control tuning.

To respond to your other questions & comments:

The sound should go away if you dial DRIVE down to zero.  Some pedals do this, some don't.  The SoB was designed to kill the signal when DRIVE is set to zero.  

Sounds like you need to practice using a DVM so you can use it with confidence and believe your readings.  We're not really running a school here, try YouTube.


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## Bobb Lobblaw

Yeah, I know, Chuck.
"Electronics for Dummies" is sitting in my Amazon cart.
One more thing. Just out of curiosity, you say Q3 affects the treble control, what do the trimmers for Q1 and Q2 do?
And, Thanks again, my solder sucking aren't great.


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## Chuck D. Bones

I said the Q3 trimmer also affects the treble control.  The purpose of the trimmers is to set the drain voltage on their respective FETs.  The trimmers are necessary because JFET specs are pretty loose and most of us don't cherry pick our JFETs.

Hey, we all had to start somewhere.


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## Thewoodenone

I can't get over 1.1V and I tripple checked components and solder. I did change the value of trim 1 so I could get 2k easier. What am I missing?


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## Thewoodenone

Here is the gut shot


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## Chuck D. Bones

The good news is your JFETs are in sockets.  Try pulling them out and measuring Vp and Idss.


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## soothsayer86

music6000 said:


> Here is the Correct pinout for the J201 Transistor:
> 
> View attachment 4342


I just checked the datasheet for the interfet j201 https://www.mouser.com/datasheet/2/676/jfet-j201-j202-interfet.r00-1649170.pdf, and it is definitely labeled 1)Source 2)Drain 3)Gate. Did anybody figure out whats going on there? I have a SoB on the "to-do" list.


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## Thewoodenone

So my DMM that was able to quickly check transistors got fried when I let a friend borrow it. Long story but he is lucky to be alive. I'm not sure what this is "supposed" to sound like but as it is it sounds JUST like an Orange amp. After tweaks its no different voltage wise but I'm hesitant to change anything. What is it supposed to sound like? BTW I got 2 J201s from NOS and 1 from Tayda.


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## Chuck D. Bones

Unless you measure Vp & Idss, you don't know if they're the real deal or not.  If you can't dial in the drain voltage, then something's amiss.  All that doesn't really matter if you like the sound.


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## soothsayer86

If anyone has used SMD j201's for this project would you mind posting a picture so I can see your method of socketing them?  Would really appreciate it since its the first time I will be using the adapter boards. Thanks


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## bassmancharlie

soothsayer86 said:


> If anyone has used SMD j201's for this project would you mind posting a picture so I can see your method of socketing them?  Would really appreciate it since its the first time I will be using the adapter boards. Thanks



I have. I’ll post a picture soon ?


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## soothsayer86

bassmancharlie said:


> I have. I’ll post a picture soon ?



I just went for it and it worked out ok. But feel free to post your pictures still as a resource to others who might need it!


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## bassmancharlie

soothsayer86 said:


> I just went for it and it worked out ok. But feel free to post your pictures still as a resource to others who might need it!


Great to hear!

For anyone else curious, here's a guide on how to socket the SMD J201s, using the SOT23 through hole adapters.

Solder the SMD J201s to your SOT23 through hole adapter boards (not needed if you bought them pre-soldered)
Solder angled headers* to the adapter boards - the longer pins go through the adapter board, sticking out on the front side (J201 side, in this case)
After the headers are soldered to the adapter boards, cut off the excess length of the soldered pins
Once that's good, solder to the main PCB (see last photo for orientation)

*angled header used was Tayda part A-199: https://www.taydaelectronics.com/40-pin-2-54-mm-angle-single-row-pin-header.html but any 2.54mm right angle single pin header will work


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## soothsayer86

bassmancharlie said:


> Great to hear!
> 
> For anyone else curious, here's a guide on how to socket the SMD J201s, using the SOT23 through hole adapters.
> 
> Solder the SMD J201s to your SOT23 through hole adapter boards (not needed if you bought them pre-soldered)
> Solder angled headers* to the adapter boards - the longer pins go through the adapter board, sticking out on the front side (J201 side, in this case)
> After the headers are soldered to the adapter boards, cut off the excess length of the soldered pins
> Once that's good, solder to the main PCB (see last photo for orientation)
> 
> *angled header used was Tayda part A-199: https://www.taydaelectronics.com/40-pin-2-54-mm-angle-single-row-pin-header.html but any 2.54mm right angle single pin header will work
> 
> 
> View attachment 5400 View attachment 5401
> 
> View attachment 5402


Nice and clean! I just used the cut legs from some electrolytics and made my own "headers"   Next time I will be doing it your way because it is much cleaner and probably more durable.  One question, will those header pins fit into a socket? I prefer to socket my jfets in case there is an issue I can easily swap them out without the desoldering hassle. That was the benefit to my method as the legs from the caps easily fit into the sockets.


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## bassmancharlie

soothsayer86 said:


> Nice and clean! I just used the cut legs from some electrolytics and made my own "headers"   Next time I will be doing it your way because it is much cleaner and probably more durable.  One question, will those header pins fit into a socket? I prefer to socket my jfets in case there is an issue I can easily swap them out without the desoldering hassle. That was the benefit to my method as the legs from the caps easily fit into the sockets.


Thanks! I like your method too - resourceful 

The pins on the headers I used won't fit into SIP sockets. There are headers that do, but I've only found straight ones, not angled, before. For example: https://www.ebay.com/itm/Single-Row...757572?hash=item4641916204:g:rM4AAOSw37tWAUnp






You'd need angled ones to fit in the Son of Ben pedal. Angled ones exist but I'm not sure where they can be purchased.

If anyone else knows of a source for them, feel free to chime in 

*Update*: it's possible to use the headers I linked in this post and carefully bend the long end to make them angled! Then they can be used just like the angled ones in my earlier guide, but with sockets.


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## mrlobo

music6000 said:


> Hope this will get you there :
> View attachment 4334


I've just assembled this pedal, sounds great, great PCB. I biased the JFETS Q1, Q2 and Q3 by ear and then measured the voltages: they were all around 4.5V in my case.


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## Boba7

cooder said:


> For what it's worth, I measured after adjusting trim 1 to 12kohm the drain measurements on Q1 as 5.1 V / Q 2 and 3 adjusted to 4 V each, input voltage 9.6 V.



Yes, I believe someone on the fsb forums measured 5v for Q1 too.


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## jnash85

Stupid question, but thought I would ask. To get close on TRIM1, I want to set it to 2K before I install it. Which side do I set to 2K? Do I measure from pin 1 to 2 or from pin 2 to 3?


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## music6000

jnash85 said:


> Stupid question, but thought I would ask. To get close on TRIM1, I want to set it to 2K before I install it. Which side do I set to 2K? Do I measure from pin 1 to 2 or from pin 2 to 3?


Legs 1 & 2 are linked so adjust so Legs 2 & 3 in Red circles measure 2K prior to fitting :


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## jnash85

music6000 said:


> Legs 1 & 2 are linked so adjust so Legs 2 & 3 in Red circles measure 2K prior to fitting :
> 
> View attachment 11492


Thanks!


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## jnash85

Ok. I finally got around to finishing this pedal and I have two more questions on biasing.

I got Trim 1 done. I used the method in this post where I connected Q1 drain to D1 cathode, and adjusted the trim until I got to 1.6k measured across R5. I am using 50k trimmers. I barely moved the trimmer to get vastly different values. Is this normal? If the actual resistance around this trimmer needed is around 2k, why specify a 50k trimmer? Why not a 5k and have more flexibility to dial it in?

The second question is for Q2 and Q3. I am getting ready to connect power and bias them. Do I need to have the foot switch and LED wired up as well? Right now the pedal is complete, but no LED, footswitch, or jacks have been connected. Can I leave those off until I have biased Q2 and Q3?


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## Chuck D. Bones

Good question about the trimmer.  The 2K setting is just a starting point.  Yes, the trimmer is touchy.  There is a LOT of variation with J201s, so some people will need more adjustment range.  Whether they'll ever need the full 50K is another question.  My understanding is Benson does not put trimmers in the production pedals, they select the J201s.

You don't need the stomp switch, LED or IN/OUT jacks to set the bias.  All you need is 9V power to the board.


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## jnash85

Great. Thanks!


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## caiofilipini

Chuck D. Bones said:


> My understanding is Benson does not put trimmers in the production pedals, they select the J201s.



Mine does actually have trimmers for both Q2 and Q3, but not Q1. And just as an additional data point, with 9.26V coming from the power supply, I read 3.85V in Q2's drain, and 3.89V in Q3's. Not very far off from the 4V recommended in the build doc. Q1's drain read 6.78V.


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## Chuck D. Bones

I stand corrected.


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## caiofilipini

By the way, I just played my original Benson side by side with the Son of Ben after adjusting the trimmers in mine to match what I measured in the original, and they sound pretty damn close! Mine sounded a little flubbier until I pulled C10 out. Now they're head to head. Great job on this one @PedalPCB!


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## Chuck D. Bones

Two questions:
Does your original have C10?
Can you compare the Source voltages between OB & SOB on all three transistors?


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## caiofilipini

Chuck D. Bones said:


> Does your original have C10?
> Can you compare the Source voltages between OB & SOB on all three transistors?



@Chuck D. Bones my original doesn't have C10, no.

Here are the source voltages:

Benson:
Q1: 0.206V
Q2: 0.180V
Q3: 0.196V

SOB:
Q1: 0.130V
Q2: 0.184V
Q3: 0.122V


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## Chuck D. Bones

Hmmmm.  Benson must have made more than one version of the Preamp.

Q1 and Q3 are hotter in the Benson; their Vp and Idss are higher than the SOB based on your measurements.  

Thanks for the data.


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## almondcity

building one of these in a few days

it sounds like maybe Q2 and Q3 require the 50K trimmers but perhaps Q1 could use a smaller trimmer to dial it in easier?  I'm considering a 20K that I have


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## jcrhee

almondcity said:


> building one of these in a few days
> 
> it sounds like maybe Q2 and Q3 require the 50K trimmers but perhaps Q1 could use a smaller trimmer to dial it in easier?  I'm considering a 20K that I have


I used a 10k trimmer per a member of this forum's recommendation.


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## gheorge77

caiofilipini said:


> @Chuck D. Bones my original doesn't have C10, no.
> 
> Here are the source voltages:
> 
> Benson:
> Q1: 0.206V
> Q2: 0.180V
> Q3: 0.196V
> 
> SOB:
> Q1: 0.130V
> Q2: 0.184V
> Q3: 0.122V


Just wanted to pass along some additional data here. The first set of J201s I used I could get biased above 3.8v on the drain. I bought that set from Pedal Parts Plus probably 5 years ago. I ended up using some that I bought from Guitarpcb that I was able to bias to exactly 4v, with a max of around 9v on the trimmer. Interestingly my source voltages were all higher. Q1 was around .3v, Q2 was .24v, and Q3 was .27v. The correctly biased set of J201s sounded substantially better, which makes sense. I'm guessing the first set were either fakes or just totally out of spec.


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## music6000

I have 20 Fairchild J201's from Pedal Hacker that are all out of Spec, Just left over rubbish!.
Paypal reimbursed as he wouldn't respond!


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## gheorge77

music6000 said:


> I have 20 Fairchild J201's from Pedal Hacker that are all out of Spec, Just left over rubbish!.
> Paypal reimbursed as he wouldn't respond!


I'm glad I never ordered from that guy. I was close on some GE transistors but I ended up just buying a DCA 55 and a bunch of transistors from Ukraine and Russia instead.


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## MichaelW

music6000 said:


> I have 20 Fairchild J201's from Pedal Hacker that are all out of Spec, Just left over rubbish!.
> Paypal reimbursed as he wouldn't respond!


Well crap, I have some from him. The ones I used in my SOB were from GuitarPCB but I think I grabbed some from PedalHacker at one point. Unfortunately, I mixed them all in one bag so I don't which is from where. I guess I better go check them.


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## fig

I suggest _always_ measuring NOS components ( or all ) before use. I’ve gotten OOS pieces from both reputable vendors and questionable ones. Transparency in their QC would be refreshing (I.e. spot-checking batches).


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## Chuck D. Bones

music6000 said:


> I have 20 Fairchild J201's from Pedal Hacker that are all out of Spec, Just left over rubbish!.
> Paypal reimbursed as he wouldn't respond!


How did you measure them and how far out of spec were they?  Some pedal circuits will accommodate OOS parts and a few, like the Fairfield Unpleasant Surprise require OOS J201s.


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## gheorge77

Chuck D. Bones said:


> How did you measure them and how far out of spec were they?  Some pedal circuits will accommodate OOS parts and a few, like the Fairfield Unpleasant Surprise require OOS J201s.


Better question is how should we all measure our less than ideal J201s...


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## Chuck D. Bones

I believe the DCA measures Vp & Idss.  The Cheap Chinese Transistor Testers do not measure Vp & Idss.  A DMM & a 9V power supply wil measure Vp & Idss.  Search these forums to see how.


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## music6000

Chuck D. Bones said:


> How did you measure them and how far out of spec were they?  Some pedal circuits will accommodate OOS parts and a few, like the Fairfield Unpleasant Surprise require OOS J201s.


They were all measured on my Greatly Improved JFET Tester back when I built the PAL M800 after I couldn't get it to work right, 1.2 to 1.3 from memory!
Some of the Voltages were at 2.5 to 3v or it would Fart out!, Used it for a bit but it never had the Sustain or Push.
Got hold of some *Siliconix J201's *and Tested them, All in Spec! .625 to .675 from memory!.
Replaced the Fairchild's ????? & dialed the trimmers to 4.5v & it was Bliss!!!!!!
I played it again Yesterday & I myself prefer it a tad better than the Golden Falk which is also a Great pedal, just different!

I'm sure Pedal Hacker sells some Legit components!!!
He just didn't do the right after sale service on this purchase!!!


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## Chuck D. Bones

Did I read that right?  Are you reporting Vp in the 1.2 to 1.3V range?


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## music6000

Chuck D. Bones said:


> Did I read that right?  Are you reporting Vp in the 1.2 to 1.3V range?


I believe you told me up to 1.0 max!
These were 1.2 to 1.3, I told you they were out of spec!


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## Chuck D. Bones

Vp spec is 0.3V to 1.5V

Idss spec is 0.2mA to 1.0mA


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## music6000

Chuck D. Bones said:


> Vp spec is 0.3V to 1.5V
> 
> Idss spec is 0.2mA to 1.0mA
> 
> View attachment 26021


Yep, It was 1.2 to 1.3 Idss.


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## Chuck D. Bones

you sure now?  

Those should be fine in 95% of the pedals that call for J201.


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## music6000

Chuck D. Bones said:


> you sure now?
> 
> Those should be fine in 95% of the pedals that call for J201.


It sounded like SHIT in the Pal M800!!!


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## Chuck D. Bones

That would be one of the 5% then.

Idss a little on the higher than spec is not what made those J201's perform poorly in the PAL800.  What made them perform poorly was the circuit needed JFETs with Vp near the low end of the spec range and yours had Vp at the high end of the range.

I've said it before, and I'll say it again: when it comes to JFETs, having the right part number is no guarantee that you'll get the right tone.


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## DAJE

Chuck D. Bones said:


> That would be one of the 5% then.
> 
> Idss a little on the higher than spec is not what made those J201's perform poorly in the PAL800.  What made them perform poorly was the circuit needed JFETs with Vp near the low end of the spec range and yours had Vp at the high end of the range.
> 
> I've said it before, and I'll say it again: when it comes to JFETs, having the right part number is no guarantee that you'll get the right tone.


I'm not seeing any Vp on my Peak Atlas. I really don't understand what the figures mean, I'm much more of a music guy than a numbers guy, but here's one of my MMBFJ201s. I've compared this J201 to known good ones and I believe it's good, and the numbers seem pretty typical to me.

Vgs(off)=-0.56V at Id=5.0µA
Vgs(on)=0.43V at Id=1.18mA
gfs=1.9mA/V at Id=0.7mA to 1.2mA
Idss=0.43mA at Vds=3.00V
Rds(on)=607.6Ω at Id=1.2mA and Vgs=0.0V

Not seeing a Vp there, and the manual says nothing about it.


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## giovanni

The SOB is also on my todo list. Do I need to measure SMD JFETs as well? Or are they more likely to be in spec? Where do I get an adapter to measure them using my transistor tester?


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## DAJE

giovanni said:


> The SOB is also on my todo list. Do I need to measure SMD JFETs as well? Or are they more likely to be in spec? Where do I get an adapter to measure them using my transistor tester?


I solder them to adapter boards then test them. Fig (of course) has some kind of SMD tester rig thing.


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## giovanni

DAJE said:


> I solder them to adapter boards then test them. Fig (of course) has some kind of SMD tester rig thing.


Ah yes, @fig to the rescue!


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## Chuck D. Bones

DAJE said:


> I'm not seeing any Vp on my Peak Atlas. I really don't understand what the figures mean, I'm much more of a music guy than a numbers guy, but here's one of my MMBFJ201s. I've compared this J201 to known good ones and I believe it's good, and the numbers seem pretty typical to me.
> 
> Vgs(off)=-0.56V at Id=5.0µA
> Vgs(on)=0.43V at Id=1.18mA
> gfs=1.9mA/V at Id=0.7mA to 1.2mA
> Idss=0.43mA at Vds=3.00V
> Rds(on)=607.6Ω at Id=1.2mA and Vgs=0.0V
> 
> Not seeing a Vp there, and the manual says nothing about it.


Vp is the same as Vgs(off).  With N-channel devices, Vgs(off) is always a negative number.  Since that's understood, we frequently refer to Vp and leave off the minus sign.

Your Peak Atlas is not measuring/reporting Vgs(off) correctly.  Vgs(off) is usually specified for Id << 1uA.  In the datasheet above, it's specified at Id = 10nA.


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## DAJE

Chuck D. Bones said:


> Vp is the same as Vgs(off).  With N-channel devices, Vgs(off) is always a negative number.  Since that's understood, we frequently refer to Vp and leave off the minus sign.
> 
> Your Peak Atlas is not measuring/reporting Vgs(off) correctly.  Vgs(off) is usually specified for Id << 1uA.  In the datasheet above, it's specified at Id = 10nA.


Thanks. I may be able to change the setting, I'll look into it next time I boot up Windows. I'm mostly a Linux user but the Peak Atlas software only runs on Windows. It's entirely possible that I've messed up the settings, since I don't really know what I'm looking at all I can do is measure known good examples and compare the numbers.


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## Chuck D. Bones

It's also possible that there are typos in your reported data.  You report Idss = 0.43mA.  Idss is the maximum current that the JFET can flow, it can't get any higher except in a fault condition.  Then you report Rds(on) at Id = 1.2mA.  They can't both be correct.


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## DAJE

Chuck D. Bones said:


> It's also possible that there are typos in your reported data.  You report Idss = 0.43mA.  Idss is the maximum current that the JFET can flow, it can't get any higher except in a fault condition.  Then you report Rds(on) at Id = 1.2mA.  They can't both be correct.


It's copied directly from the Peak Atlas software, so it can't be a typo.

I'm using Windows now, so here's a screenshot (and graphs) of the same J201.

I haven't tried this particular J201 in a circuit, but I have similar results from others that do work.

Here are the results for the J201s that are in my SoB pedal, which works.

J201    Ben R
Vgs(off)=-0.61V at Id=5.2µA
Vgs(on)=0.40V at Id=1.20mA
gfs=1.9mA/V at Id=0.7mA to 1.2mA
Idss=0.49mA at Vds=2.99V
Rds(on)=592.6Ω at Id=1.2mA and Vgs=0.0V

J201    Ben C
Vgs(off)=-0.70V at Id=4.8µA
Vgs(on)=0.41V at Id=1.33mA
gfs=1.9mA/V at Id=0.8mA to 1.3mA
Idss=0.60mA at Vds=2.99V
Rds(on)=601.8Ω at Id=1.3mA and Vgs=0.0V

J201    Ben L
Vgs(off)=-0.72V at Id=5.3µA
Vgs(on)=0.39V at Id=1.35mA
gfs=2.0mA/V at Id=0.8mA to 1.4mA
Idss=0.62mA at Vds=2.99V
Rds(on)=583.9Ω at Id=1.4mA and Vgs=0.0V

Again, these are copied directly from the Peak Atlas software, I'm not typing anything so there's no typos.

EDIT: I stuck the same JFET in my LCR T7, and it says: Id= 0.23mA @Vg=164mV.


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## Chuck D. Bones

I can't make sense of those curves. They contradict the manufacturer's datasheet.  JFETs are depletion-mode devices, which means that when Vgs = 0V, they are fully ON and by applying a negative Vgs, they gradually turn off. Vgs(off) and Idss are boundary conditions.  The JFET operates in a linear region when Vgs is between 0V and Vgs(off) and when Id is between 0 and Idss.  I attached the datasheet.  Compare the bottom right graph on page 2 with your GRA1.png plot. Compare the upper left graph on page three with your GRA2.png plot.  Note that they are not even close.  I don't own a DCA Pro, I do not know how the displays are calibrated. What's an LCR T7?


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## DAJE

LCR T7. TL;DR - cheap multi tester thing. Works pretty well as far as I can tell. 

The DCA75 is kind of a mystery to me, too. I got a good deal on it when I had some money to blow, and felt optimistic about my ability to understand electronics properly. The manual doesn't help much. It's possible I have messed up the settings, but I couldn't find any way of adjusting them so I can't see how I'd have done it. 

So, all it's useful for to me is, as I said, comparing the numbers to known good transistors. I know that the crap out-of spec 5457s I have give very different results to the good ones, so it does seem to do something accurately.


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## MichaelW

DAJE said:


> LCR T7. TL;DR - cheap multi tester thing. Works pretty well as far as I can tell.


I bought one of these and also built the kit I got off Ebay. Both are great for mundane measuring chores, (diodes, caps, resistors). Neither are very consistent in recognizing transistors. Like it only works 50% of time. 99% of what's being discussed in this thread is way over my head anyway.......


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## DAJE

MichaelW said:


> I bought one of these and also built the kit I got off Ebay. Both are great for mundane measuring chores, (diodes, caps, resistors). Neither are very consistent in recognizing transistors. Like it only works 50% of time. 99% of what's being discussed in this thread is way over my head anyway.......


Mine is useful for BJTs, in that it gives an hFE number that's simple enough for me to grasp and is very close to the DCA75 result.

It's the JFETs that are confusing. I have no idea what they're meant to do and reading about them does not help. I'd get more information from looking at runes.


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## Jazzwolf

Jovi Bon Kenobi said:


> I just ran it through again with that method, Chuck.
> View attachment 1438
> I dialed it in to exactly 1.67k. Boy oh boy does it sound great now! Trim 1 is interesting. It goes from no sound to a broken crackling starved voltage sound to perfection. I can see it being tricky for people. Thanks for the excellent method.


Just made this pedal and it was my first time doing a bias adjustment. This pic and thread helped me out big time!

Ryan


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## apc42069

just tacking my gratitude on here as well.. this thang (i'm not from philly and refuse to use "jawn") sounds fuckin killllllller!!


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