About those heckin J201s

[jellyjams]

So I'm new to the DIY pedal game, and as you know by now it's near impossible to obtain (reasonably priced) J201s without fiddling with SMD and adapter boards, which is unfortunate since I see them in a lot of high-gain FET designs *ehemROGehem*. I've stocked up on more common FETs (5457, 5458, 5486, 5952, MPF102, K30A-GR) I can get from my local electronics stores (I live overseas), but based on the datasheets it seems that nothing comes close. My question is: how do you sub the J201 without compromising stuff such as gain and tone and whatever mojo everyone raves about it?

[Rob Strand]

based on the datasheets it seems that nothing comes close. My question is: how do you sub the J201 without compromising stuff such as gain

They kind of stand alone.

Look at JFETS with low Vgs_off.  You could sort through them using a JFET matcher.
Maybe try these:  2N5484, 2N5485, 2N5457, MPF102.

[jellyjams]

I've read up some more and it seems like I_dss only really matters to get a tube-like character and the gain merely comes from the source and drain resistors based on that value; is my understanding correct?

[Rob Strand]

source and drain resistors based on that value

The resistors are definitely linked to IDSS but you have some room to play with.

[PRR]

> the gain merely comes from the source and drain resistors based on that value; is my understanding correct?

Incomplete. There is resistance "inside the Source". The true gain is lower than the ratio of resistors you see.

Knowing "nothing", you can approximate this by assuming the internal source resistance is "around 1K". However this can be 100 Ohms for a fat FET at high current or 3K for a wee one.

[knutolai]

I'd say go for the adapter board solution. I don't really see the problem with it. If you were to buy them through somewhere like SeeedStudio you could get roundabout 100 boards for ~25$. I could draw up a panelized 100x100mm PCB (gerber .zip-file) if the community is interested, though it has probably already been done. Ordering the manufacturing is very easy.

MMBFJ201 is the SOT-23-3 package J201 correct?

[smallbearelec]

A JFET that is similar in VGSoff spec to the J201 is this one:

http://www.smallbear-electronics.mybigcommerce.com/fairchild-pf5102/

It is still available in through-hole, and I will do my best to stock up on them before/if they go obsolete.

I have several thousand of the J201 in TO-92, have had to raise the price and not sell bulk. So I will be able to supply small lots for DIY for some years. Going forward, I will put some $ into the SMD version, already have Surfboards for breadboarding:

http://www.smallbear-electronics.mybigcommerce.com/sot-23-to-to-92-surfboards/

[jellyjams]

I've just measured all the JFETS I have, and fortunately all my 5457s have |V_p| < 1V. Now what's so special about the tiny tiny I_dss of the J201?

[rockola]

The SMD J201 is also a bit worrisome, Mouser is showing delivery dates for backorders well into 2019.

Just received a few SMD J201s from Tayda yesterday - they sure are tiny. I may have to give in and get those reading glasses soon.

[alanp]

http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=6&products_id=52

diygp.au sell SMD J201s, plus adaptor boards so you can use them where the PCB expects a through-hole J201 (The SMD package version has not been discontinued, IIRC.)

[rankot]

I've just measured all the JFETS I have, and fortunately all my 5457s have |V_p| < 1V. Now what's so special about the tiny tiny I_dss of the J201?

According to ROG site, Idss it not of big importance for "Fetzer Valve" circuit.

[slashandburn]

I had this dilemma a month or so ago, in the end I gambled on some 2n5457 and j201 on ebay from China.  I've yet to use the j201s (I put them in the wrong drawer and only found them again yesterday) but I've been steadily working my way through the bag of 5457's and had no problems. Also all of them made it through the jfet Matcher without any smoke or fire and surpringly a lot of them paired up pretty well. They might well all be fakes but so far they're at least functional fakes.

YMMV but the Chinese sellers on ebay could possibly be an option.

[EBK]

Just another friendly reminder that SOT-23 transistors are veroboard compatible.  One example I've found:

[jellyjams]

According to ROG site, Idss it not of big importance for "Fetzer Valve" circuit.

Oh you're right; I must've gotten lost in the sea of equations So I_dss only affects the value of the S/D resistors (while keeping the ratio) and the current draw of the device. Does that mean I have (and only have) to change those resistors to make the 5457 sound like J201, given they have the same V_p?

[reddesert]

J201s don't have mojo in particular. They might very well be from a similar silicon process as the 2N5457 and the ones that come out with smaller Vp are labeled J201s. The issue is that with a JFET amplification stage, you have to both bias the drain somewhere near half of Vsupply, and use the ratio of drain/source resistors to get the desired gain and clipping. So you don't have total freedom to select the gain ratio. With a small Vp like the J201, you can get gains of several times, but with the 2N5457, the stage may have a lower gain. There is an example in Section 11 at the bottom of the Fetzer Valve page, http://runoffgroove.com/fetzervalve.html.

The problem with finding a sub for the J201 is not that I_dss matters in the Fetzer Valve circuit, but that for a given batch of JFETs, I_dss and V_p are highly correlated. To find a JFET with small values of V_p, it also winds up having a small I_dss. The attached image shows the results of measuring small batches of JFETs with the Runoffgroove FET tester shown on the Fetzer Valve page.



The J201, 2N5457, and J113 came from Smallbear; the 2N5458 from Tayda; and the "off-J201" were from a US seller on ebay. You can see that the "off-J201" are close, but probably out of spec.

[PRR]

> J201, 2N5457, J113 2N5458 "off-J201"

The J113 is a Very Big Chip. That's why it runs big current.

The others could very well be the same size chip, just processed differently (channel thinness). And yes the tail-end of "J201" could be true J201/similar but rejected for high Vp (yet sold anyway).

[Rob Strand]

Oh you're right; I must've gotten lost in the sea of equations So Idss only affects the value of the S/D resistors (while keeping the ratio) and the current draw of the device. Does that mean I have (and only have) to change those resistors to make the 5457 sound like J201, given they have the same Vp?

Assuming all other things are equal, if you double Idss you halve Rd and Rs but it's not really the whole story.

Even J201's vary from unit to unit.  The great majority of DIY circuits using J201's have an adjustable Rd.
The adjustment lets you set the drain voltage, which is one of the more important aspects for the right sound.
The Rd adjustment allows you to tune out most of the variables at once.
[Edit:  However, the result usually has a different gain.]

The point where you get to tweaking Rs is when you realize you can't making everything equal with different JFETs.
If Rs is not bypassed by a cap, and you want the same gain, then you can tweak Rd for the correct gain.
If Rs is bypassed by a cap then very roughly want the DC voltage across Rs as least as big as the circuit you are trying to match.
[Edit: Sometimes you can keep Rd the same and adjust Rs to get the correct the source voltage.  This method can often give a better gain match.]

The easiest way to do it is try a few Rs values,  (accurately) adjust Rd so the DC voltage on the drain is right, then choose the one the sounds the best.

[Rob Strand]

Just for fun I put up some results on the effect of variations in J201's.  It shows the effect adjustments in RD and RS has on the preamp behaviour.:
http://www.diystompboxes.com/smfforum/index.php?topic=119707.0

[jellyjams]

What exactly does the bypass cap do?

[Rob Strand]

What exactly does the bypass cap do?

If the bypass cap is big then the preamp will have a higher gain than without it.

However, if the cap isn't that big  then the gain at low frequencies will be small low and the gain at high frequencies will be high.  If the frequencies are in the bass-range then it will act kind of like a bass cut.  If the frequencies are in the mid-range it acts like a presence boost, and if the frequencies are high it acts like a treble boost.   The fetzer page has some comments about this.