# Why are JFETs such a Pain in the Ass?



## cooder (Dec 6, 2020)

Chuck you're amazing, thanks for that and you have a way with words too. Like your humour "_Well, first you go to college for four years and get a degree in electrical engineering."_
Very enlightening and yes we do need someone (wink wink....) to enlighten us which circuits on this site need special attention, what measurements are required and how to keep sane. Having that in the build docs aside from the schematics (cough cough...) would be really good.
So I'll ask someone (that is you...) "_someone who can analyze the circuit and recommend a range for Vp and Idss_" what would I need in for example Catalinbread DLS MK3 and 5F6 (Covert and Tweed Man) and Formula 55 (Drive 55)? How about the Fairfiled Barbershop (Chop Shop)?
Cheers man. You should write a book. Well you're actually doing that here kinda.


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## Barry (Dec 6, 2020)

cooder said:


> Chuck you're amazing, thanks for that and you have a way with words too. Like your humour "_Well, first you go to college for four years and get a degree in electrical engineering."_
> Very enlightening and yes we do need someone (wink wink....) to enlighten us which circuits on this site need special attention, what measurements are required and how to keep sane. Having that in the build docs aside from the schematics (cough cough...) would be really good.
> So I'll ask someone (that is you...) "_someone who can analyze the circuit and recommend a range for Vp and Idss_" what would I need in for example Catalinbread DLS MK3 and 5F6 (Covert and Tweed Man) and Formula 55 (Drive 55)? How about the Fairfiled Barbershop (Chop Shop)?
> Cheers man. You should write a book. Well you're actually doing that here kinda.


Really enjoying the articles Chuck's posting, many thanks to him for taking the time to share some of his infinite wisdom


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## Danbieranowski (Dec 7, 2020)

I really believe that buying anything on ebay is just a bad idea all-around. There are so many good reputable vendors that deserve your money and can provide genuine parts (SmallBear, DigiKey, stompboxparts, even big boys like Tayda and Mouser, etc etc). I've purchased dodgy ICs before (clearly salvaged) and it was a good reminder to me to just spend the extra few bucks and support a good business while you're at it. 

Additionally, you're a king as usual Chuck. Thank you for breaking this stuff down in human terms for us. I haven't had time to dig into pedal projects lately, but I still read through each of these posts and try to soak up as much info as I can.


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## Chuck D. Bones (Dec 7, 2020)

Thanks for the kind words, guys.

Cooder, I'll address your questions in an upcoming article on biasing JFETs.  Warning: I'm going to subject y'all to some high-school algebra, it's unavoidable.


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## cooder (Dec 7, 2020)

Hit us with your rhythm stick Mr. Chuck, bang in some algebra. Looking forward to it, my pidgeon brains hurts already...


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## phi1 (Dec 7, 2020)

Great article Chuck! Ever since my first misadventure buying J201s from eBay... I started buying exclusively smd jfets and using adapter boards.

I built the simple (non-IC) version tester from this page. I’m not sure if this method produces perfect accuracy (Chuck could probably weigh in), but for my purposes of finding consistency it seems to work great. I’ve read the background articles from runoffgroove that were also interesting.








						Greatly Improved JFET Matcher
					

Collection of vero (stripboard) & tagboard layouts for 100s of popular guitar effects, with over 500 verified designs. DIY your own boutique effects!




					tagboardeffects.blogspot.com
				




since I was using exclusively smd, and some boards have spots to solder smd directly, I designed and ordered this pcb with through hole socket and smd pads, following that tagboard circuit. Gotta hold the jfet down on the pads with tweezers which is annoying, but I’m getting better at it haha. I printed some info and some target values. These are MY approximate averages from several smd parts I’d measured. So don’t take these numbers as text book truth. I have extras of these boards if anyone is interested (I won’t charge, maybe just shipping).

my experience has been pleasant in that most land pretty close to the numbers printed (within 0.1-0.2 or so). This includes smd jfets from tayda, pedalpcb, and diyguitarpedals. I’ve only had a few that were way off from this (though I think they were still technically within the spec).

I’m really curious if others have had similar experience with smd, and what method you all use for measuring.


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## HamishR (Dec 7, 2020)

For whatever reason I have had good results in builds which specify 2N5457, 2N5458 or 2N5952. It's those bloody J201s which drive me crazy. Some wonderful pedals, such as many by Menatone, use J201s (as Chuck suggests, in place of tubes) and they are so fussy about biasing it takes the fun out of building them. The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s. I have a ton of J201s bought from Small Bear but until I have a streamlined process for using them they stay in their little bags.


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## untamedfrontier (Dec 7, 2020)

Always a great read, thanks for this.


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## cooder (Dec 7, 2020)

Yes it would be great to see what other methods and devices peeps use and recommend for jfet testing and also the fiddly smd breed ones that I also like to use. Thanks phi1 for your input there!


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## jubal81 (Dec 7, 2020)

HamishR said:


> For whatever reason I have had good results in builds which specify 2N5457, 2N5458 or 2N5952. It's those bloody J201s which drive me crazy. Some wonderful pedals, such as many by Menatone, use J201s (as Chuck suggests, in place of tubes) and they are so fussy about biasing it takes the fun out of building them. The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s. I have a ton of J201s bought from Small Bear but until I have a streamlined process for using them they stay in their little bags.


People will always prefer the J201 because it's the part number originally used in all those designs. I have a hunch it became popular because its |vp| and IDSS lead to source and drain resistor values that can be closer to the values you commonly see in typical guitar tube preamps when used as common-source amplifiers.

There are a good number of other JFETs available with typical |vp| values less than 1 volt that can handle a lot more current - often designed with audio or RF signals in mind - that I've made some great sounding gain circuits with. In fact, I just ordered a new Aion PCB for the DLS MK2 that I'm going to 'hotrod' with RF JFETs.

Personally, I've never heard any 'magic' out of any particular JFET part number, and in my experience they all break up similarly and work great as long as they're set up to operate correctly. This also includes my personal experiments with the ROG 'Fetzer Valve' setup, which I never heard any particular 'magic' from.

I'm a certified JFET junkie. They make great audio amplifiers and give great sounding distortion. But like Chuck says, you design for the actual values, not the part numbers, to get them working right.


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## Chuck D. Bones (Dec 8, 2020)

phi1 said:


> Great article Chuck! Ever since my first misadventure buying J201s from eBay... I started buying exclusively smd jfets and using adapter boards.
> 
> I built the simple (non-IC) version tester from this page. I’m not sure if this method produces perfect accuracy (Chuck could probably weigh in), but for my purposes of finding consistency it seems to work great. I’ve read the background articles from runoffgroove that were also interesting.
> 
> ...



That method is very good, it's as accurate as your DMM.  I have used that method numerous times.  I plug stuff into a protoboard to make the Idss & Vp measurements, but it's the exact same method.


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## Chuck D. Bones (Dec 8, 2020)

I agree with Jubal81.  JFETs with low Vp tend to have higher gain, but at the expense of lower headroom because you can't drive the gate beyond 0V or Vp without getting much nastier distortion.  With a little finesse, we can have any value source or drain resistance we want; we don't have to pick a JFET with a particular Vp.  About the only restriction we have w.r.t. Vp is it has to be less (a lot less) than the power supply voltage.

HamishR, you have been lucky so far, or you're building pedals that are tolerant of Vp variation.

The Vp range of the PF5102 overlaps the J201 and with careful selection, PF5102s can be used in most pedals that call for J201.


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## Chuck D. Bones (Dec 9, 2020)

HamishR said:


> The King of the Britains is a great overdrive despite the ungrammatical name. The 6-knob version is outstanding but difficult to build because of the J201s.



I just looked into that pedal and yes, it depends on the low Vp of the J201.  Not all legit J201s will work on that board; their Vp needs to be near the low end of the spec.  I think I can see a way to make it a little more FET-friendly and still retain the sound.  Basically, it comes down to adjusting the source resistors to get it into the ballpark and then fine-tuning it with the drain resistors.  I ran a sim with PF5102s and some resistor changes... looks like it works, only problem is the overall gain goes up by about 20dB.  Oh wait, that's not a bad thing!  Another project for the breadboard.


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## Chas Grant (Dec 9, 2020)

@Chuck D. Bones I would like to say Thank You! The knowledge you have been sharing has been invaluable! Especially on the JFET's! Your explanations are easy to understand and quite pertinent to what we are using them for and what we are trying to accomplish with them. Nothing is requiring you to share this knowledge, yet you choose to do so. This choice is helping all of us to gain more knowledge in a "Hobby" that we are all passionate about! So again, Thank You!


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## HamishR (Dec 10, 2020)

I would be extremely interested in what you make of the King of the Britains. I'll just say it once - it should be called King of the Britons. The current name is ungrammatical and plain wrong.


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## Chuck D. Bones (Dec 10, 2020)




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## music6000 (Dec 10, 2020)

HamishR said:


> I would be extremely interested in what you make of the King of the Britains. I'll just say it once - it should be called King of the Britons. The current name is ungrammatical and plain wrong.


Are you referring to Menatone.


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## HamishR (Dec 10, 2020)

Of course!


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## music6000 (Dec 10, 2020)

HamishR said:


> Of course!


I think he may have got depleted by it too, It's simply ''The King'' now!


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## phi1 (Dec 10, 2020)

@Chuck D. Bones how low does the Vp need to be? Smd J201s have been fairly consistent in the ballpark of 0.7 for me, so maybe that’s an option?


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## music6000 (Dec 10, 2020)

phi1 said:


> @Chuck D. Bones how low does the Vp need to be? Smd J201s have been fairly consistent in the ballpark of 0.7 for me, so maybe that’s an option?


That's why PedalPCB does a J201 Pre Soldered SMD Adapter Board for *$2.00*, Only option if you can't source reliable NOS J201's $$$$$$$. :








						MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com
					

JFET




					www.pedalpcb.com


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## Chuck D. Bones (Dec 10, 2020)

phi1 said:


> @Chuck D. Bones how low does the Vp need to be? Smd J201s have been fairly consistent in the ballpark of 0.7 for me, so maybe that’s an option?



Depends on the pedal, the Calamity needs JFETs with Vp at the very bottom-end of the range, around 0.2V.  I've seen a schematic on the 'net of the King of the Britains with measured voltages indicated; the Vp on those parts was in the 0.4V range.


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## phi1 (Dec 10, 2020)

Ah gotcha, yeah I was asking about the KOTB for HamishR, but my post wasn’t clear. So maybe not ideal for that.


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## Chuck D. Bones (Dec 10, 2020)

I think if you tweak the *drain *resistor on the 1st FET and the *source *resistors on the others, you can get there.  Use your lowest Vp FET for the 1st one.  Don't bother trying to dial in the 4th FET, you get what you get when the 3rd one is set.  Here's the annotated schematic, I'm assuming it's correct.  Hit those drain voltages within ±10% and you should be good.


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## benny_profane (Dec 10, 2020)

I have a question about source resistor selection, but I think I'll hold off until you post specifically about biasing. (Placeholder so I don't forget.)


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## Chuck D. Bones (Dec 10, 2020)

Go ahead and ask your question, it will help me write the biasing article.


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## benny_profane (Dec 10, 2020)

Regarding tube emulation circuits, most of the JFETs seem to be self-biased through the establishment of a Q-point by applying the voltage dropped across the source resistor to the gate. This isn't the only JFET biasing method, but it seems to be the one least dependent on the drain resistor. Why is the trimmer placed on the drain in that case? And why do different circuits using the same JFET device and topology have such large source resistor value differences?


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## Chuck D. Bones (Dec 10, 2020)

If you don't need an immediate answer, I'll address all of that in the Biasing article.  Will take a few days...


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## benny_profane (Dec 10, 2020)

Nope, not at all. Please take your time!


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## Chuck D. Bones (Dec 12, 2020)

music6000 said:


> I think he may have got depleted by it too, It's simply ''The King'' now!


Anyone have a schematic for The King?  Menatone says it'd based on KotB, but with all controls on the front panel.  I'd like to know what the EQ switch and knob do.


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## manfesto (Dec 15, 2020)

Chuck D. Bones said:


> You need to own and understand how to use a transistor tester.  You can get a perfectly adequate one for about $20US on eBay.


Off-topic, I've seen those cheapie transistor testers on eBay and Amazon, but near as I can tell none of the cheapies measure leakage (which would come in very handy for testing germaniums).

Is there anything cheaper than the DCA55 you know of that measures leakage (besides that Geofex circuit on a breadboard)?


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## Chuck D. Bones (Dec 15, 2020)

manfesto said:


> Off-topic, I've seen those cheapie transistor testers on eBay and Amazon, but near as I can tell none of the cheapies measure leakage (which would come in very handy for testing germaniums).



Not true (well, the off-topic part is true).  Here is an MP38A Soviet NPN Germanium under test.




hFE is the DC current gain.
6.7mA is the emitter current during the hFE and Vbe tests.
Vbe is the base-emitter voltage.
ICEO is the collector to emitter current with the base lead open.  This is usually the definition of leakage.
ICES is the collector to emitter current with the base lead shorted to the emitter. 
The in-circuit leakage is somewhere between the two.
I have compared the hFE results on this tester with the results I get on a curve tracer and I can assure you that this tester is accurate and it takes leakage into account when measuring and reporting hFE.


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## manfesto (Dec 15, 2020)

Chuck D. Bones said:


> Not true (well, the off-topic part is true).  Here is an MP38A Soviet NPN Germanium under test.
> 
> View attachment 8442
> hFE is the DC current gain.
> ...


Awesome, thanks! Will pick one up!


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## Chuck D. Bones (Dec 15, 2020)

Got mine on eBay, not everything there is crap.  I bought it assembled, except for clear plastic case.  Putting that together is like building a house of cards.  Hint: painter's tape.  You'll need a *POSITIVE CENTER* 9VDC power source.


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## manfesto (Dec 15, 2020)

Chuck D. Bones said:


> Got mine on eBay, not everything there is crap.  I bought it assembled, except for clear plastic case.  Putting that together is like building a house of cards.  Hint: painter's tape.  You'll need a *POSITIVE CENTER* 9VDC power source.


Ordered the unassembled version to save a few bucks, thanks for the heads up on the power source! Ordered it with a battery adapter because I wasn’t sure, should’ve checked back in the forum before purchasing haha

For anyone who’s looking like I am, make sure when you search eBay you specifically search “GM328” to get the same model transistor tester Chuck has that tests leakage (ICEO and ICES); there are other cheap testers on eBay (most branded “mega328” it looks like) that *only* show hFE and Vf (they have green monochromatic screens and will be a little cheaper)

EDIT: found a pic, this is the one you *don’t* want. The GM328 will be worth the extra $5.


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## cooder (Dec 15, 2020)

Thanks for all this and the hint of the GM328 tester, seems better than my cheapie I got and great to know that it seems to agree with Chcks curvetesting, which I'm sure will be a benchmark.
So I'll be looking forward to see the Chuck version of the King of britons including correct spelling...
So where would I get a good batch of the PF5102 jfets then that you keep popping up? Mouser doesn't seem to have them in stock, did you buy them all up, Chuck?


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## Devoureddeth (Dec 17, 2020)

Chuck D. Bones said:


> I just looked into that pedal and yes, it depends on the low Vp of the J201.  Not all legit J201s will work on that board; their Vp needs to be near the low end of the spec.  I think I can see a way to make it a little more FET-friendly and still retain the sound.  Basically, it comes down to adjusting the source resistors to get it into the ballpark and then fine-tuning it with the drain resistors.  I ran a sim with PF5102s and some resistor changes... looks like it works, only problem is the overall gain goes up by about 20dB.  Oh wait, that's not a bad thing!  Another project for the breadboard.


Do you imagine jfets Vp could br the issue with people trying to recreate the EQD Monarch? Schematic


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## Chuck D. Bones (Dec 17, 2020)

No.

That schematic contains two glaring errors that by all rights should prevent any board built that way from working.  There is also at least one typo.  Other schematics I've seen of this board contain the same errors.  JFETs need to have a stable voltage, even if it's zero volts, present at the gate terminal in order to set the operating point (bias).  The gates on Q2 and Q4 have no DC path to anywhere, therefore their bias point is unknown.  The only way a board built this way could work is if C7 and C14 are leaky or there is enough flux residue on the board to absorb some moisture and provide a DC current path across the surface of the board.  The right way to correct the problem is:

Replace C14 with a jumper.
Replace C7 with a jumper.
The typo is the value of R19.  47K is too low and it loads the GAIN pot.  470K would be a more appropriate value.  I could even make a case for deleting R19 and replacing C15 with a jumper.

For sure, getting the right Vp is important.  Q2 and Q4 both need a fairly low Vp.  Q4's Vp determines Q3 and Q4's drain current.  Q2 and Q3's Vp set the headroom.  I'd shoot for a Vp around 1V on all three JFETs.  Again, part number is not important, Vp and Idss are.  J201 will not work for Q4.

I really must finish that article on biasing JFETs!

The idea of replicating the Orange Amp sound is intriguing, but there is a lot of room for improvement to this implementation.  MOSFETs are noisier than JFETs or BJTs, so using one as the first stage is not something I would do.  JFETs have the same transfer function as MOSFETs, have more gain at these drain currents and are quieter.


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## Devoureddeth (Dec 18, 2020)

Chuck D. Bones said:


> No.
> 
> That schematic contains two glaring errors that by all rights should prevent any board built that way from working.  There is also at least one typo.  Other schematics I've seen of this board contain the same errors.  JFETs need to have a stable voltage, even if it's zero volts, present at the gate terminal in order to set the operating point (bias).  The gates on Q2 and Q4 have no DC path to anywhere, therefore their bias point is unknown.  The only way a board built this way could work is if C7 and C14 are leaky or there is enough flux residue on the board to absorb some moisture and provide a DC current path across the surface of the board.  The right way to correct the problem is:
> 
> ...


Awesome Chuck, love the information but I am still trying to wrap my head around mu-amps. Hopefully I will get time over Christmas break to breadboard some changes. Maybe this could be another Chuck PCB. I know people have been wanting a version of this for sometime.


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## Chuck D. Bones (Dec 18, 2020)

I'll have to prioritize this on my breadboard also.  One more suggestion: we can save a JFET and make the last stage an emitter-follower.  Should sound the same.


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## Chuck D. Bones (Dec 20, 2020)

HamishR said:


> I would be extremely interested in what you make of the King of the Britains.


I just breadboarded KotB using PF5102 FETs. Got the bias dialed-in by tweaking the source resistors.  Lotta gain there.  On headphones it's pretty harsh, even with GAIN, TREBLE and MASTER dialed way down.  Tried running it at 18V; it got louder but the tone didn't change much.  I'll recheck my wiring and plug it into an amp & speakers tomorrow, see if it sounds any better.


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## Chuck D. Bones (Dec 21, 2020)

Found the problem: the gain & EQ settings on my headphones amp were screwed up.  Sounds much better now.


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## Chuck D. Bones (Dec 22, 2020)

OK, today I spent some time with KotB.  Played my Chibson Res Paur thru it and straight into a tube amp.  Sounds pretty good, but not exceptional (to me anyway).  The Marshall tone stack doesn't do all that much, no surprise there.  Having two Gain controls is a plus.  I have two other Marshall emulator pedals: DLS mk 3 (Covert) and ROG Thunderbird.  To me the KotB is similar to the Covert, but the Thunderbird is superior to the other two IMHO.  One thing that the Covert has over the KotB is its mu-amps respond well to increased power supply voltage, whereas the KotB sounds the same at higher voltage.  I'll play thru it some more, but I doubt it will get beyond the breadboard stage.   I made a few minor mods.  Here is the as-built:





I replaced 5 of the J201s with PF5102s and one (Q4) with a 2N5089.  Most of the changes were to adjust the bias for the PF5102s.  D1 was added to correct Q1's bias.  I added R5 because there is no need to dial the GAIN all the way down to zero.  R9 was increased to 4.7K to correct Q2's bias.  R13 was increased to 2.7K to correct Q3's bias.  R14 was 100K, which to me was way too high and could have been a typo, so I changed it to 10K.  Q4 was originally a source follower.  I tried an emitter follower instead and saved a JFET.  Out of necessity (not enough A1M pots), I scaled the impedances of the tone stack down by approx 5x.  Rescaling the tone stack _might _cause some small tonal differences when Q4 is overdriven hard.  Where R18 is, there used to be two 1K resistors with R17 connected in the middle.  To get Q5's bias right, I moved R17 to the bottom of the pair and replaced the two 1Ks with a 2.2K.  R23 was increased to 750Ω to dial-in Q6's bias.  The VOLUME pot was decreased to A100K and C17 was increased accordingly.  Did I build an exact replica of the KotB?  No.  Is it pretty damned close?  Yes.


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## Chuck D. Bones (Dec 23, 2020)

Just for S&G, I cannibalized some pots from another breadboard and assembled the tone stack in accordance with the original schematic.  Sounds the same.  It's a decent pedal, but it's not for me.  I definitely like the Covert & Thunderbird better.  Next up... the EQD Monarch, with the bias issues corrected.


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## djmiyta (Nov 4, 2021)

😁
😁
😁 Thank you Chuck D Bones I have a much better perspective now very cool way of explaining that quantum string theory Jfet mystery I had going on that you were unaware of
1 question is this or will this work for the info needed obviously I'm still not fully grasping all the electronic lingo but I snapped a pic of my tester sadly I dont  understand any of the info it provides maybe someone could explain ? Does what you said about the jfets include mmbfj201 for instance?I mean  should all jfets be auditioned per circuit even SMT ones that are in spec aside from the tolerant circuits? And would the PAL 800 be considered a tolerant circuit?


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## djmiyta (Nov 4, 2021)

Chuck D. Bones said:


> OK, today I spent some time with KotB.  Played my Chibson Res Paur thru it and straight into a tube amp.  Sounds pretty good, but not exceptional (to me anyway).  The Marshall tone stack doesn't do all that much, no surprise there.  Having two Gain controls is a plus.  I have two other Marshall emulator pedals: DLS mk 3 (Covert) and ROG Thunderbird.  To me the KotB is similar to the Covert, but the Thunderbird is superior to the other two IMHO.  One thing that the Covert has over the KotB is its mu-amps respond well to increased power supply voltage, whereas the KotB sounds the same at higher voltage.  I'll play thru it some more, but I doubt it will get beyond the breadboard stage.   I made a few minor mods.  Here is the as-built:
> 
> View attachment 8600
> 
> I replaced 5 of the J201s with PF5102s and one (Q4) with a 2N5089.  Most of the changes were to adjust the bias for the PF5102s.  D1 was added to correct Q1's bias.  I added R5 because there is no need to dial the GAIN all the way down to zero.  R9 was increased to 4.7K to correct Q2's bias.  R13 was increased to 2.7K to correct Q3's bias.  R14 was 100K, which to me was way too high and could have been a typo, so I changed it to 10K.  Q4 was originally a source follower.  I tried an emitter follower instead and saved a JFET.  Out of necessity (not enough A1M pots), I scaled the impedances of the tone stack down by approx 5x.  Rescaling the tone stack _might _cause some small tonal differences when Q4 is overdriven hard.  Where R18 is, there used to be two 1K resistors with R17 connected in the middle.  To get Q5's bias right, I moved R17 to the bottom of the pair and replaced the two 1Ks with a 2.2K.  R23 was increased to 750Ω to dial-in Q6's bias.  The VOLUME pot was decreased to A100K and C17 was increased accordingly.  Did I build an exact replica of the KotB?  No.  Is it pretty damned close?  Yes.


Interesting I pulled mine out of the meh box last I built the I believe 1st version 3 knobs 2 toggles when it’s paired it’s nice but yeah overall back to meh pile I’ve only heard great things of ROG designs and yet to build one


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## Chuck D. Bones (Nov 4, 2021)

Unfortunately, those testers do not measure Idss or Vp.  In a few other threads, I described how to measure Idss and Vp using a DMM.  The bottom line is very few circuits will tolerate JFETs over their entire spec range.  The PAL800 is tolerant to the extent that the trimmers have a large adjustment range.  Still, no guarantees every in-spec J201 will work. The pros have to test and select their JFETs if they want every pedal to work.


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