Phase 45 2N5952 Orientation Confusion

[Phend]

Hi:
Mr dyslexic here.
Looking at the pinout for 2N5952 on the net I am mixed up with the Drain and Gate.
See GGG Phase 45 picture, lower right Q2 with labled D S G.
On the net I see the opposite, G S D, see picture.






Comparing the GGG picture with it's accompanying schematic they do coincide.

Does 2N5952 come in both orientations ?

[Govmnt_Lacky]

Pretty sure the letters are correct (D, S, G) but the transistor pics on the 45 are reversed. Phase 90 looks correct.

Think you just need to install the 5952s backwards from what the picture illustrates.

EDIT: Looking at the build doc, the PCB was designed with 2N5485s in mind and they are reverse pinout of 5952s. So, just rotate the 5952s 180 degrees.

[antonis]

Both JFET Gates are connected to 470k respective resistors, as they should be..
(you don't need to flip anything)

edit: Lacky is right..!!
(flip 2N5952 180^o)

[Phend]

"EDIT: Looking at the build doc, the PCB was designed with 2N5485s in mind and they are reverse pinout of 5952s. So, just rotate the 5952s 180 degrees"
That explains it. For a second there I thought my brain was getting 135 out of wack.
Thanks Lacky and antonis.

[Mark Hammer]

According to this, the Drain and Gate pins are switched around in the GGG drawing/layout.  If you simply turn the JFETs around you should be OK. (this is why I always use sockets for JFETs - 3 pins but apparently 827 different permutations of pinouts!)

[ElectricDruid]

(this is why I always use sockets for JFETs - 3 pins but apparently 827 different permutations of pinouts!)

LOL! Yep! Surprising how many components there are with similar characteristics!

[Phend]

Yes LoL, now addressing the ambidextrous dyslexic trifocal diyers, why is the Jfet pinout 123 vs schematic symbol 132 made to confuse us.? Who came up with that evil standard ? Like driving on the wrong side of the road. Sounds to me like the electrical engineering  group missed the meeting with the mechanical department.

[Rob Strand]

You can easily identify the gate using the diode test on your multimeter but you have to test in both directions.   A JFET looks like a silicon diode between G and D and G and S.   On an N-channel JFET the gate looks like the anode.  If you know you are dealing an N-channel this quickens the process because you fix the anode lead then see if you get a reading to the other two pins. The voltage is about 0.7V or so, slightly higher than a silicon diode.   You need to check you get no reading in the reverse as the 0.7V might be a false D and S measurement. 

A BJT is open across e and c in both directions which makes it easy to identify c and e.   However, the D and S measurements on a JFET aren't consistent.  It depends on the DMM and the JFET; you might also see different results between diode and resistance ranges, even which resistance range.   You may or may not get a reading across D and S.  Whatever the case it's usually the same in both directions.    I posted some details some time back where you do measurements with the gate shorted to the drain or source to work out what type of JFET it is.

[Rob Strand]

Sounds to me like the electrical engineering  group missed the meeting with the mechanical department.

I blame the mechanical people.   Look at drawings of switches and connectors.  They have drawings which look OK until you actually try to use them.  They don't say if you are viewing from the top or bottom.   Don't have a pin 1 indicated.  Terrible.

A good example is the Bourns "professional" series blend pots.  It has a funky blend taper but you can't work out which gang had what taper.  There was a youtube video from Bourns showing how to wire it up and I think he even wired it up wrong!

[Phend]

Thanks Rob,

[antonis]

There was a youtube video from Bourns showing how to wire it up and I think he even wired it up wrong!

[Phend]

Sorry to disagree but this part just doesn't seem to fit into the circuit without some contorted bending of the wire leads.



This mechanical assembly makes more sense to me.



However, I hope to get the Phase 45 assembled and working soon.

[Rob Strand]

Sorry to disagree but this part just doesn't seem to fit into the circuit without some contorted bending of the wire leads.



This mechanical assembly makes more sense to me.



However, I hope to get the Phase 45 assembled and working soon.

I'm fairly sure the 2N5952 did have a different pinout to common JFETs like the 2N5457.

If you look at the GGG phase *90* project the 2N5952 pinout looks like the datasheet.

When we get the GGG phase 45 PCB the PCB says 2N5952 but the PCB doesn't match.  The PCB matches
the more common JFET pinout.  When we go the phase 45 project,

http://www.generalguitargadgets.com/effects-projects/phase-shifters/mxr-phase-45/

The build docs reveal the problem,

http://www.generalguitargadgets.com/pdf/ggg_p45_instruct.pdf

"The JFETS need to be matched. You can use the information at the geofex.com site
to do that. The kit comes with a great sounding set of matched 2N5485 transistors.
The original units had 2N5952 JFET transistors. Note that the pin out of these
transistors may very. The 2N5952 will probably have to installed backwards from
what the layout shows"

So what's happened is the PCB layout is for the 2N5485, which has the common JFET pinout,
but the text on the PCB shows 2N5952 <--- that's where things have gone wrong.

So the 2N5952's have to be installed flipped.   (That's assuming you do in fact have real 2N5952's.
Fakes are likely to have the common pinout.)

[Mark Hammer]

Sorry to disagree but this part just doesn't seem to fit into the circuit without some contorted bending of the wire leads.



This mechanical assembly makes more sense to me.



However, I hope to get the Phase 45 assembled and working soon.

1) Sometimes, you need to strip some insulation off a piece of wire and slip it over the FET leads so you can flip them around to fit in those kinds of sockets.

2)  I have slowly come to the conclusion that matching is important in some contexts and not in others.  The key aspect of matching is that one wants all JFETs to change their drain-source resistance in response to the current feeding the gate.  You set the bias voltage and current, and the LFO output sums with that to move the drain-source resistance around.  Ideally, one wants the same bias to work for all JFETs involved, such that none of them crap out and stop changing at as they approach either the "top" OR the "bottom" of the sweep.

That said, if the objective is NOT to produce wide slow majestic sweeps, but rather medium to fast limited "bubbly" sweeps (i.e., faster but narrow width), then one should be able to locate a suitable bias for even unmatched JFETs.  And if you limit the current of the LFO output (i.e., something a little higher than 3M9 for the various MXR pedals) to limit the sweep width, you may be able to "get away with" skipping the matching.

Again, matching is important IF you plan on being able to aim for wider sweeps that bring all stages along for the ride.  If you're not going to ask any given JFET to do very much, then matching may be much less of a requirement.  Worth noting that MXR made the 90 and 45 (and even some later models) ONE-KNOB pedals.  Perhaps that move was to skip the matching part and aim for a one-size-fits-all sweep width that would work at all speed settings.

[Rob Strand]

FYI, there's a pic of a real phase 90 PCB here (which uses the 2N5952's),

https://www.mylespaul.com/threads/mxr-block-phase-90-help-ident-pcb-r28-question.188296/

We can see two traces which go to all the JFETs.  That's the Gate and Source.

Find the 3M9 resistor (top right on component side).   That goes to the gate and the track nearer to the PCB edge.

Check out the top left JFET (component side pic):
- top = gate  (g)
- middle = source (s)
- bottom = drain (d)

Which translates to:
\ d s g /   Top View
\ g s d/    Bottom View
and that matches the pinout extract from the 2N5952 datasheet.

[Phend]

Rob, yup that is where my confusion came from.
"So what's happened is the PCB layout is for the 2N5485, which has the common JFET pinout,
but the text on the PCB shows 2N5952 <--- that's where things have gone wrong."

Thanks for directing me to the explanation.
However, in General, circuits with Jfets have been confusing to me because of their Mixed up pins compared to the way they are drawn on the schematics.
I need to really cifer it out, but that is my problem.
Transistors EBC, I have no problems,  but yes, there are CBE somtimes.
Take for instance the EHX Muff Fuzz 1970'S version using 2 BC239 transistors point to point Upside Down in the enclosure.
Like wow talk about tough visualization for Me. But that is my difficulty,  hence my complaining about JfetS pins vs symbols.
Thanks all for the helping the helpless. Lol
(I ordered matched Jfets from GGG) can't wait to get 45 going.
Also matched the numerous 10k resistors, being a fan of blue printing the circuit,  got the time to measure,  if it is worth anything?..?
Will post results and or any more questions.
Thanks again, Gunsmoke in B and W is almost over, good night.

[Rob Strand]

However, in General, circuits with Jfets have been confusing to me because of their Mixed up pins compared to the way they are drawn on the schematics.
I need to really cifer it out, but that is my problem

It's no easier for me.  That symmetrical JFET symbol is a pain, s and d are ambiguous.  For some circuits it's possible to work it out from the design but for things like JFET switch circuits there's no way to work it out.   The symbol is also a pain in CAD work.  In old packages I would write s and/or d in text on the schematic.   I tend to use the asymmetrical JFET symbol as much as possible.

Like wow talk about tough visualization for Me. But that is my difficulty,  hence my complaining about JfetS pins vs symbols.
Thanks all for the helping the helpless. Lol

There's no easy way.   For that MXR Phase 90 trace I went through small steps going from the pic to flipping the part.  I wrote each step on paper with picture.   I do that a lot because it helps get it right but when you're low on coffee some days even that manages to get screwed up.

Also matched the numerous 10k resistors, being a fan of blue printing the circuit,  got the time to measure,  if it is worth anything?..?

Matching the JFET's VGS_off (VP)  is important because small mismatches have a big impact on performance.   Within reason, the other JFET parameters have minimal impact.   Things like the resistor and cap tolerance even less of an effect.

The JFET resistance looks like,

rds  = rds0  / (1- Vgs/VP)   ; Vgs and VP treated positive

rds0 and VP are parameters of each JFET.

Suppose JFET1 has an rds0 of 200 ohm and VP is 1V, at Vgs = 0.99V,  rds1 = 200/(1-0.99/1) = 20k ohm

Now a second,  JFET2,  has a VP of 1.05V, at Vgs = 0.99V, rds2 = 200/(1-0.99/1.05) = 3.5k ohm

Vgs is common in both cases because the gate voltage is fed by the same LFO.

What you can see is when the JFET is in the high resistance region, the low frequency part of the phaser sweep, a small mismatch in VP causes an enormous mistracking in the JFET resistance rds.    (That happens because you are subtracting a number close to 1 (ie. Vgs/VP) from 1, the error gets amplified.)

If on the other hand rds0 was 10% higher on one of the JFETs then rds would only be 10% higher.   Everything scales in a proportional way.

Something weird happened to the text in my post.  Hopefully it's OK now.

[antonis]

The key aspect of matching is that one wants all JFETs to change their drain-source resistance in response to the current feeding the gate.

May I rebuke you for newbies discomposing, Mark..?? 

[Mark Hammer]

The current AND voltage.  But really, what one hears is a function of the current limiting resistors.  Indeed, I always make the 1meg resistor coming off the trimmer 510k in series with a 500k pot to move the range of the sweep upwards and downwards, and make the LFO resistance one normally sees as 3M9 a little higher, and a little lower, via toggled resistance values, to adjust the sweep width.  Both resistances vary the currents being summed.  This is true for both the P90 and P45.