Attention Jfet Phaser intellects!

[Rob Strand]

Red - Green : 0.675
Green - Blue : 0.675
Red - Blue : 0.25

OK that's looking much more sensible - well maybe.

N-channel Jfet
Green = Gate
Red, Blue = Source and Drain  (use pin-out as it's often not possible to distinguish the two.)

I believe the Green pin does map to the gate on the K30A.

The hanging "issue" is the fact that the resistance measurement didn't show anything between Red and Blue.

So maybe the next step is the circuit duck_arse posted.

[njkmonty]

ok cheers !  i will get onto it:)

[njkmonty]

ok, using this tester heres what i got



Drain- Blue
Gate - Green               IDSS 0.053v (not multipled)      VP 0.022
Source - red

Drain- Red
Gate - Green               IDSS 2.626v (not multipled)      VP 0.43
Source - Blue

[Rob Strand]

Drain- Blue
Gate - Green               IDSS 0.053v (not multipled)      VP 0.022
Source - red

Drain- Red
Gate - Green               IDSS 2.626v (not multipled)      VP 0.43
Source - Blue

The results aren't "normal" but at least now they are starting to look like some sort of JFET.
The things that still look odd are the Drain=Blue results and the lack of symmetry.

Also if I calculate  Rd = Vp / (2 Idss) = Vp / (2 * VIDSS * 10 *1e-3) = 50 Vp / VIDSS
Blue=drain: Rd = 50 * 0.022 / 0.053 = 20.8 ohms
Red = drain: Rd = 50 * 0.43 / 2.626 = 8.2 ohms

I would have expected those to agree a bit more.  If they were signal JFETs you would expect something around say 200 ohms to 400 ohms.

The Rd value is the same as 1/gm0 which you can see from,
www.kennethkuhn.com/students/ee351/jfet_basics.pdf

The resistance between D and S measured with a multimeter should be in the ball park of Rd.

So at this point:
- it's looking like an N-channel JFET.   
- The low Rds and lowish Vp look more like a switching JFETs.
- The parameters are way off signal JFETs  so you will probably struggle getting
   them to work in most effect pedal circuits.
- The asymmetry could be an indication of a factory reject.

You could check a few others to see how consistent they are.

If it were me I probably try to see if they actually obeyed the JFET square law for Id vs Vgs.   That would be very convincing the devices are JFETs.

BTW,  nice multimeter!

[njkmonty]

i bought these from futurlec  , i was so happy with the results, that few years later i bought some more from them,
similar markings . the first batch worked great in a roland jet phaser clone too.
the new ones identify in the yellow box as jfets
I=0.084a
vgs=0.59v

i think your right regarding switching jfets.
i can make a great phase by bridging the jfet socket source and drain with a pair of tweezers on and off.

i measured the resistance between the source and drain and they vary between 1.6k   to 2.1k
do you suggest any specs or ballpark varieties of jfets i may order to test?
i was having a look at the small signal transistors and jfets guide which i linked  at beginning of thread
to see what examples may be closer to spec required
i think what im looking for is jfets with low vgs (off)?  im not sure   still trying to comprehend your
advanced knowledge!
yes buying that dmm second hand on ebay was one of the best things i did, the continuity tester is fantastic.

[Rob Strand]

i bought these from futurlec

To be honest, you should tell futurelec you have been shipped dodgy parts.
If they are reputable at all they should exchange the parts and carry out their own investigation
to see which of their suppliers is shipping dodgy parts.   In this day and age they have to admit there's
a lot of fakes out there and they should be on top of it.  I don't see why the customer should suffer.

i measured the resistance between the source and drain and they vary between 1.6k   to 2.1k

So that seems inconsistent with the low Rds values.  The bottom line is there's something weird going on
with those parts.

do you suggest any specs or ballpark varieties of jfets i may order to test?

If you get parts with Vp values of 1 to 2V (and Yfs specs in the 2000uS to 8000uS) zone
you should be safe in most circuits.   That's not to say parts with *specs* for Vp of 0.5V to 8.0V won't work,
For one you rarely get the extremes in practice.   Parts with Vp=0.5 can be more finicky to adjust and high Vp's with like 3.5V will certainly work but may benefit from increasing the LFO output.  When you get to Vp's of 5V the bias point has to be set to undesirable values.

The 2SK30A-Y's are fine but the problem is your parts aren't really 2SK30A-Y's!

Some other common ones are 2N5457, 2N5458, MPF102, BF244 (A or B), BF245 (A or B).   People have got J201's to work but they probably aren't the best choice for a phaser.    I don't know what is available these days.  The lists keep shrinking and (fortunately) I'm not buying a lot of parts these days for DIY.

[njkmonty]

To be honest, you should tell futurelec you have been shipped dodgy parts.

the bodgy ones that worked i bought 3 years ago  so i just assumed they were good!

I have most of the other normal jfets for phasing and none work  , it appears the volatges are too low range for their ideal workings. the only other option i think i may have to do is use more common working jfets and adjust the circuit so it lfo cycles better to their values?

[Rob Strand]

the bodgy ones that worked i bought 3 years ago  so i just assumed they were good!

Interesting indeed!

I have most of the other normal jfets for phasing and none work  , it appears the volatges are too low range for their ideal workings. the only other option i think i may have to do is use more common working jfets and adjust the circuit so it lfo cycles better to their values?

That's a bit weird.  What circuit are you using?   Maybe the circuit needs a tweak to help it work with other JFETs.    The voltage at the gate needs to swing down to Vp below the source and up close to the source; hw close determines the range of the sweep.  Normally the bias adjustment sets the Vp condition.  If you don't get the bias right it's pretty much doomed from the start.   The upper limit of the gate swing is determined by the swing of the LFO and the resistor values that combine the LFO and the bias voltage.    Often the resistors affect both the bias and the LFO sweep range so you need to tweak those so both aspects are just right.

[amptramp]

There is a JFET linearization article here as applied to an ST-1000A FM signal generator:

http://ham-radio.com/k6sti/st-1000a.htm

This quote is important:

"A trick to linearize JFET channel resistance may not have been known when the ST-1000A was designed. Applying one-half of the drain signal to the gate makes any variation in channel resistance symmetrical, cancelling even-order distortion. This kills the dominant second-order JFET distortion product, greatly reducing THD."

This is for a JFET used in an oscillator but the principle may apply.