Author Topic: More probably fairly trivial jfet questions  (Read 578 times)

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slashandburn

More probably fairly trivial jfet questions
« on: January 01, 2018, 03:36:21 PM »
Sorry to pester you all again during the holidays with trivial questions you're probably mostly sick of answering. If I wasn't a total cheapskate I'd just order some j201s and 5457s and be done with it but they seem less than readily available at what I'd call acceptable prices for the though hole variety. I almost finished an order with smallbear but the shipping just seemed crazy when all I really need are a dozen old trannies. Chinese ebay outlets, maybe but with the stories of fakes I'm not so confident.

My main options seem to be:
A: Cough up the dough and buy legit, through-hole  201s and/or 5457s and the required postage. For the sake of an easy life.
B: Entertainment the idea of buying smd components and melt my fingers attempting to give them legs and bend them around to fit (smd part j201 seems to have a different pinout?)
C search through the supposed j201 substitutes. Bit of a minefield for novices. Farnell recommends this part. PF5102  JFET Transistor, JFET, -40 V, 4 mA, 20 mA, -1.6 V, TO-92, JFET

I realise that in many situations the actual jfet used isn't critical, such as with source followers and the like. Identifying these is out of my remit, and even then, I don't know in which scenarios I'd have to look at re-biasing the substitute jfet or whatever.

Help out a cheapskate hack? What's my best option here?

EBK

Re: More probably fairly trivial jfet questions
« Reply #1 on: January 01, 2018, 03:53:54 PM »
I'd go B-ish.
Buy SMD equivalents, and either:
1) try to solder wires to them,
2) buy adapter boards to solder them to,
3) make your own board that has SMD pads for each FET,
4) solder the SMD transistor to veroboard, between rows and across a break.

Personally, I like the last option.  I'll dig up a pic...

 (that is a pic I found -- not my own work).
« Last Edit: January 01, 2018, 03:56:08 PM by EBK »
No affiliations. If I glowingly mention specific merchants or products, it is because I like them without having to be paid to like them.

Kipper4

Re: More probably fairly trivial jfet questions
« Reply #2 on: January 01, 2018, 04:20:27 PM »
If it's just followers I'd try subbing with.
2n3819
2sk30a
Fake j201. Never had any problems with them.
I'd even go so far as to say the last batch I got where fairly close in tolerances.
2n5458 ?
Just off the top of my head.

YMMV
Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.

stonerbox

Re: More probably fairly trivial jfet questions
« Reply #3 on: January 01, 2018, 04:28:01 PM »
SMD adapters are quite a hassle to build but worth in the end (financially) and they can also look like some sort of weird scrapyard caravan if viewed at the right angle! ;D

"JFETs are like people.... similar,  but different from one another."
- BubbaFet

R.G.

Re: More probably fairly trivial jfet questions
« Reply #4 on: January 01, 2018, 04:47:35 PM »
Let's go back and study the First and Second Laws of Frogs.

The First Law of Frogs is - if you find you have to swallow a frog for some reason, don't look at it too long.

The Second Law of Frogs is - if you find you have to swallow a number of frogs, swallow the biggest one first.

This brings up the Zeroth Law of Frogs - do you REALLY have to swallow a frog?

So let's evaluate your options within the Laws of Frogs.

It seems that you are wondering if you have to swallow one or more frogs. This is based on your thinking that [through hole JFETs of a certain type] are
(1) moderately difficult to find
(2) prices are "unacceptable" for these
(3) shipping prices are crazy.
It could well be that you're trying to avoid the First Law of Frogs by looking at this way too long.  :icon_lol:

In any case, your question involves an assortment of frogs.
(1) Work harder and pay the price, including shipment; but then you get to get on with your life and do what you started to do in the first place, build effects.
(2) Order SMD parts and (... gulp ...) just learn to deal with them, because the entire world is going SMD. A little unpleasant at first, but it might lead to your sidestepping frogs in the future, or making the apparent size of future frogs smaller.
(3) As a side, and smaller frog, deal with the fact that pinouts are ... well, different with different parts. We've all swallowed that frog before, and will do so again. Sigh. If it were me, I'd accept this one as an inevitable, and modestly sized frog. Gulp.
(4) Recognize that either you will have to learn to sort through substitutes, or fall back on Frog #1 above. You're only a novice until you decide to stop being one. Gulp. This is much the same size and shaped frog as the question about identifying what JFET is good for what application.
(5) A subtly hidden frog is that if you are going to live with Frog #1, you don't need to worry about Frog #4, as long as you're willing to live with building with no substitutes and paying for the few JFETs of the right type that you need.

This gets us down to your final question:
Quote
Help out a cheapskate hack? What's my best option here?
There is no one here who can tell you what your >best< option is. Only you can do that, once you've sorted through the frogs and thought which is largest, most ungainly, seems to smell worst, is hardest to hold, etc.

I personally would advise thinking about your self description as a cheapskate hack. There is a decision tree lurking there. If you are a cheapskate, you cheap about money, time, exasperation or some mix of all of these? If it's only money, you're likely to spend a lot of effort and time getting things to work. But that may be an acceptable outcome for you. You may prefer to spend the hours, days, months, etc. to get cheap parts. It depends on whether what you want to do is to >use< the effects you make, or >build them as cheaply as possible< or >work on building them just as long as it takes<. Think about this set of tradeoffs. There will be a quiz, and you will administer it to yourself.  :)

I would also examine that second word, "hack". If you're a hack, why is this so? If you don't have the learning to be a not-hack, you >could< buy the neato, known working right parts, and then spend the excess time you would have spent on figuring out and adapting substitutes, etc. to learn about the background so pretty soon you're no longer a hack. It's there, and such learning is so very much cheaper now than it was in the past that sometimes I am awestruck by the availability of learning.  Just sayin'

Finally, there is what is either the Fourth Law of Frogs, or the First Law of Princes, depending on how you look at it. This was told to me by a female-type person, who also told me that males and females have different cultures and outlooks. In the cultural canon of female persons, it's widely held that if you want to marry a Prince, you will need to kiss a lot of Frogs.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

iainpunk

Re: More probably fairly trivial jfet questions
« Reply #5 on: January 01, 2018, 04:52:04 PM »
Most Jfets in pedals are used as source followers, in such case, most jfets of the same polarity (n or p channel) can be substituted, as the signal gain is independent of the jfets transconductance, always being ≈1

Otherwise you could @#$% around with the bias resistors and use a bjt as an emitter follower. I personally prefer this method, since i have around 50 bc547's lying around.

Iain
If we don't study the mistakes of the future, we're bound to repeat them for the first time.
-Ken M

thermionix

Re: More probably fairly trivial jfet questions
« Reply #6 on: January 01, 2018, 05:41:09 PM »
There are still good deals to be found on Ebay.  I bought a lot of 50 2N5485s hoping to match up a quad or several for the Phase 90.  I ended up with 6 nicely matched quads, 7 matched pairs, and of course the rest as singles.  Very happy.

My approach to Ebay component buying:

-Look for "western" sellers, not China or Hong Kong.  Tayda is cool of course.

-Look for 100% positive feedback.  Maybe there will even be feedback on the particular item you're looking at.

-It's a bonus to me if the seller doesn't advertise items as being for guitar pedals, otherwise you might be looking at someone's rejects, after they bought a lot and kept all the good ones for themselves.  The prices might also be lower if they don't know of that desirability.

mac

Re: More probably fairly trivial jfet questions
« Reply #7 on: January 01, 2018, 05:59:06 PM »
BF245A
2SK246

I'd say 2SK117 but I'm not sure if they are still in production.

Quote
and they can also look like some sort of weird scrapyard caravan if viewed at the right angle! ;D

Or "The War of the Worlds" fighting machine  :icon_lol:

mac

Harry Muff

Re: More probably fairly trivial jfet questions
« Reply #8 on: January 03, 2018, 11:56:50 AM »
There are still good deals to be found on Ebay.  I bought a lot of 50 2N5485s hoping to match up a quad or several for the Phase 90.  I ended up with 6 nicely matched quads, 7 matched pairs, and of course the rest as singles.  Very happy.

My approach to Ebay component buying:

-Look for "western" sellers, not China or Hong Kong.  Tayda is cool of course.

-Look for 100% positive feedback.  Maybe there will even be feedback on the particular item you're looking at.

-It's a bonus to me if the seller doesn't advertise items as being for guitar pedals, otherwise you might be looking at someone's rejects, after they bought a lot and kept all the good ones for themselves.  The prices might also be lower if they don't know of that desirability.

yeop that chros kyrokeos or whatever his name is from LIC pedal ripped me off on what turned out to be some fake or reject germanium transisitors..got me for 40 bucks ..

I ended up getting some really nice ones from Kaary of Plum FX ..that made a few awesome tonebenders for myself with
« Last Edit: January 03, 2018, 12:00:27 PM by Harry Muff »

slashandburn

Re: More probably fairly trivial jfet questions
« Reply #9 on: January 03, 2018, 04:29:17 PM »
Awesome! Thanks for all the replies, some real food for thought there. I really like RGs frog analogy and I guess in the time I've been deliberating my options I could've drank the entire pond. I also could've spent the same time educating myself and finding ways to achieve similar results without resorting to swallowing frogs.

In the end I've ordered some dirt cheap 201s and 5457s from China. If and when they arrive I'll fire them through my jfet matched and brace myself for disappointment.

I've managed to finish and box up quite a few unfinished pedals over the last couple of week and looking at my parts box and my lack of a phaser I'm considering using this new found confidence to attempt to put together a Bad Stone, which will probably keep me occupied for most of this year. No rush for the jfets then.

One thing that really catches my imagination was other Iain's suggestion for source followers. I also have a whole load of Bc547 so if they're an option in some situations I should really just do some reading and learn to re-bias or redesign around the plain old general purpose NPN then I should get reading.

Thanks again everyone!

reddesert

Re: More probably fairly trivial jfet questions
« Reply #10 on: January 03, 2018, 09:44:09 PM »
The JFET when used as a buffer, for example in the RAT, is not usually very sensitive to the specs of the JFET. (The Rat buffer is a source follower.)

However, when used as a gain stage, as in a lot of "amp simulation" type pedals, or as a variable resistor as in phasers etc, then the specs matter more, because you have to get the stage to bias correctly, ie it has to have a reasonable voltage drop in its quiescent state. The V_gs and I_dss parameters usually are what matter and you can look the spec ranges up in datasheets. If you sub a different type (for example a 2N5458 for a J201), or if you have parts that are out of spec, then they may not bias correctly and you may get no sound, or no effect, or the wrong level of gain. It helps to read up - look for the simple Runoffgroove JFET tester - to understand how you can measure these specs.

If you get parts that are slightly out of spec but still actually JFETs, you might be able to get them to work, and it could be a useful learning experience. OTOH, debugging circuits with questionable parts is annoying and you also have to ask what your time is worth.

equatorjbl

Re: More probably fairly trivial jfet questions
« Reply #11 on: January 12, 2018, 04:36:07 AM »
Grab some smd's and use an adapter board. Diyguitarpedals sell them but i think you could also grab some from Ebay, check this vid

https://www.youtube.com/watch?v=IjcsL6tHdl8

slashandburn

Re: More probably fairly trivial jfet questions
« Reply #12 on: January 13, 2018, 02:41:46 PM »
>> Grab some smd's and use an adapter board.

Thanks. Sounds like a decent plan B!  I can etch and droll so no worries there. I'll definitely manage to burn my fingers a pot doing this though. Here's hoping the Chinese thru-hole do the job.

anotherjim

Re: More probably fairly trivial jfet questions
« Reply #13 on: January 13, 2018, 04:37:49 PM »
I'm pretty sure the smd pinout difference isn't going to be the problem you think it is.
SMD or Thru-hole, the J-FET source and drain are interchangeable. That is, of the two, the one you call the drain is whichever pin you want to be the drain.
The SMD package puts the gate pin (the only one you can't swap or call anything other than the gate) on it's own in the middle of one side. Essentially, the gate is one corner of a triangle which can be turned one of three ways on the layout. So the gate can be placed at either end or in the middle. S'that simple.
"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

DIY Bass

Re: More probably fairly trivial jfet questions
« Reply #14 on: January 13, 2018, 08:24:29 PM »
You won't burn your fingers soldering surface mount.  Get a third hand with a magnifier to hold the board.  Put a small blob of solder on one pad.  Hold the component with fine tweezers, melt the solder blob and position the part.  Once it is help in place by one pad, you can solder the rest.  Once you've done it, it will take less time to do than to read that description.  A fine soldering iron tip and temperature control help.

slashandburn

Re: More probably fairly trivial jfet questions
« Reply #15 on: January 13, 2018, 08:54:07 PM »
Thanks Jim! I'd heard about this source/gate being interchangeable but since I still manage to gets my pinouts wrong so often I figured it couldn't be that straight forward. You mean all this time all I needed to do is find the gate and not really worry about the others?

Diy bass, thanks for the tips and the misplaced confidence. I'm pretty sure I'll still manage to burn my fingers though. And probably my leg.


anotherjim

Re: More probably fairly trivial jfet questions
« Reply #16 on: January 14, 2018, 05:05:18 AM »
Quote
You mean all this time all I needed to do is find the gate and not really worry about the others?
Yes, for all the audio/general purpose/switch types it don't matter.

See the note by the SOT-23 version? It's the same bit of silicon as the TO-92 so.....

I think you really need a sticky-tip thingy to handle them with. I find tweezers (including the cross grip ones) have a secondary function as catapults - I've lost a lot of small things that way.
« Last Edit: January 14, 2018, 05:08:39 AM by anotherjim »
"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

Kipper4

Re: More probably fairly trivial jfet questions
« Reply #17 on: January 14, 2018, 05:21:53 AM »
the J-FET source and drain are interchangeable.


I'd heard about this source/gate being interchangeable

Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.

EBK

Re: More probably fairly trivial jfet questions
« Reply #18 on: January 14, 2018, 07:20:47 AM »
The Dr rain and source are generally interchangeable.

Some JFETs are asymmetrical, with a non-uniform gate along the channel length, I think.  Luckily, if I remember correctly, it would only make a difference at much higher frequencies than we are concerned with. 

Because I am a chronic rule-follower, sometimes to the annoyance of those around me, I always make sure I have the source and drain correct, even though I know perfectly well that this is unnecessary.  :icon_rolleyes:
No affiliations. If I glowingly mention specific merchants or products, it is because I like them without having to be paid to like them.

antonis

Re: More probably fairly trivial jfet questions
« Reply #19 on: Today at 12:15:19 PM »
JFETs manufactures could make our life easier by Gate middle pin orientation..
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