The Infamous JFET purchasing question

[bassybeats]

So I am trying to decide on some bulk JFET purchases and am after some alternatives. 2N5457, J201 and PN4393's seem to be hard to come by genuine ones. Looking at building some of the 'Amps in a box' style pedals (soldano SLO/Dr Boogie/Laney Superlead) from Tagboard effects.

So does anyone have useable equivalent's (i've heard J112, J111's etc) or a reasonably priced source that can send to New Zealand?

I don't mind eBay if its confirmed to be not fakes like Tayda etc.

Cheers

[nzCdog]

Hey dude, always cool to see another kiwi

Check out these little SMT adapter pcbs from Paul across the ditch... cool idea coz you get to use genuine JFETs in SOT23 package adapted for through-hole.  He sells JFETs as well

[bassybeats]

Those look pretty damn cool, might need to get a bunch! Might make matching them up harder though haha

Is it correct that a lot of those pedal designs 'could' use any N-channel JFET and sound "pretty close".

[Mojo Fandangle]

Yep, definitely check out Pauls DIYGP page.
Soldering the J-Fets to the tiny pcb's is a hassle but when you consider the price and cheap postage compared to other web stores, it's well worth it.
I'm in Australia and it's only $2.50 postage

[alanp]

My position on the whole SMD JFET thing is that it's going to happen with everything, eventually, so get used to it.

[raulduke]

My position on the whole SMD JFET thing is that it's going to happen with everything, eventually, so get used to it.

Yep I'd agree. It will happen eventually.

I got a sh*tload (1000's) of 2N5457's when they announced to be discontinued. Was going to sell em (for a little profit of course  ) but shipping to the US (the main market IMO) for small parts pretty much kills it for cost.

Royal Mail royally screw you over for anything not going to the EU.

[bassybeats]

Luck for some ^^^ haha

Are there any alternative devices to the usual J201/2N5457 etc, other than going SMD?

[raulduke]

I'll sort out some way of selling them so people here can get em.

I've given up on the idea of making and selling pedals in large quantities, so I'm never going to use them all.

[EBRAddict]

I've been very happy with SMD JFETs. A few of the PCB slingers have been building boards with SMD pads in addition to through hole. A couple weekends ago I built a batch of a bunch of different ones MMBFJ201, MMBF5457, 2SK209, MMBF4393.

Has anyone figured out a way to elegantly measure the Vp/Vgs(off) and Idss on SMD? Right now I'm holding the tiny component down on some pads with my finger and trying to measure with my other hand.

[midwayfair]

Are there any alternative devices to the usual J201/2N5457 etc, other than going SMD?

No one can, or should, answer this question generally. If you're stuck substituting parts because you can't get the ones you need, you need to know exactly what each part is doing before you can determine what can be a substitute.

Here's what you need to do. Get all the schematics for the distortion pedals you're building. Look at each gain stage and determine the following:

1) What's the center of the bell curve maximum gain for each FET in this instance, and can the other FETs I can buy achieve the same gain? You'd be surprised how much overlap there is.
2) Does the output impedance and current consumption matter? If you change FETs you change their ideal resistors for their operating point.
3) Does the |Vp| matter? While it's possible to achieve a specific gain factor with a huge range of FETs, if they are selected in the design for their |Vp|, then you aren't faking that with a different choice of resistors/operating points ... it's device-specific. Fortunately, this doesn't matter most of the time, because most of the time it's either (a) "as low as possible" (i.e., a J201) or (b) there is something else determining how much the signal is clipped instead of |Vp|. a can be overcome to some extent by using a higher voltage with a lower |Vp| FET if you need to or adding a source bypass resistor. Sure it'll sound different, but sometimes it doesn't matter. b appears in things like the more recent ROG circuits where there are clipping diodes at the input and the Vp of an individual FET has very, very little effect on the circuit.

You can buy "plenty" of FETs from Smallbear, though. For hobby level building, I mean. If you are building 10 pedals that use 3-4 JFETs each, you can get 30-40 FETs from Smallbear that you know will be genuine and useable for any given part number. This isn't "bulk" ordering, really. If you are building 20 of a single pedal and you need to sort JFETs, then you're building a product and not just some pedals for yourself, and you need to know that you can get the parts in the future, so it's time to suck it up and use SMD. There's nothing wrong with SMD FETs and it really only takes a minute to solder together an adapter board if you can't make a PCB layout using them.

[EBRAddict]

Just as an example, here's the readings I took from my small SMD sample, measurements in V and mA:

             VGS_OFF    IDSS    
Y2   2SK209-Y   -0.38    2.14    
Y1   2SK209-Y   -0.39    2.20    
J1   J201      -0.38    0.22
J2   J201      -0.40    0.25
C1   2N4393   -1.17    10.31    
C2   2N4393   -1.37    12.00    
A1   2N5457   -1.53    3.10    
A2   2N5457   -1.53    3.16    


(Sorry the formatting sucks, not worth it to get it neat...)

[Muadzin]

+1 to Paul's adapter boards. And they're not that hard to solder. Soldering some clippings as pins is a way, way bigger PITA.

[alanp]

If both the adaptor board and the PCB have 0.1" spacing, then right angle pin headers would be a good solution to that.

[nzCdog]

If both the adaptor board and the PCB have 0.1" spacing, then right angle pin headers would be a good solution to that.

:like:

[chromesphere]

If both the adaptor board and the PCB have 0.1" spacing, then right angle pin headers would be a good solution to that.

Pin header is not standard 2.54mm spacing Alan.  This is mainly due to pricing and size of the adapter PCB's.