Aion EP-3 preamp: JFET Idss testing

[anechoic]

I bought a batch of 2N5485 JFETs for matching as required by the Ares EP-3 preamp and am testing for Idss using my DMM in series with the +V of the supply and D of the JFET with the G&S tied to ground. I'm using a plain vanilla wall wart 9 VDC supply.

The instructions for the EP-3 preamp state that the Idss should be btwn 7 and 8 mA for Q2 2N5485 but I'm getting consistent readings of 2.9mA for all ten of the 2N5485 JFETs I have -- which is closer to the readings of 3mA expected for Q1 the 2N5484 JFET.

I tried using the test jig (the one without the opamp) as suggested in the instructions but it gives erroneous readings as verified by others in the comments below the schematic.

it is posted here:

http://tagboardeffects.blogspot.com/2012/07/greatly-improved-jfet-matcher.html 


My questions:
===========
- do the EP-3 instructions state the correct values for Idss for a 2N5485 JFET?
- is there something wrong with the way I'm testing for Idss?

any help is appreciated!
ciao!
kc

[thermionix]

Build an EP-1 preamp instead.

[Mattnezz]

Idss for 1N5485 should be in the range of 4 ~ 10 mA according to the datasheet.
Your test-setup should work though. The opamp setup measures Idss in quite the same way. It measures the voltage drop over a 10R resistor (and buffers that voltage?).
If you have a DMM with high (>1M) input impedance you might skip the opamp and try to connect G & S to GND through a 10R resistor and measure the voltage drop across that R and do the math.

[diydave]

Or set your multi-meter to measur milli-amps, connect Gate en Source to ground, connect you multi-meter between Drain (black probe) and powersupply (red probe) en read the milli-amps on your display.
Easy peasy. You get Idss when voltage between Gate en Source is 0 Volt.

[aion]

Did the 2N5484 test out OK?

The main issue with the tester schematic (IIRC) is that the "center off" position didn't actually turn anything off. I did use the non-op-amp version for mine, but didn't try the manual method.

[anechoic]

Did the 2N5484 test out OK?

The main issue with the tester schematic (IIRC) is that the "center off" position didn't actually turn anything off. I did use the non-op-amp version for mine, but didn't try the manual method.

since the schem shows the switch in the Idss position connects to only the amp meter and the JFET - I simply connected the G&D to gnd and the D to one side of the amp meter and the other to +V (9 v)...this is how I got the 2.8 mA readings...any help is appreciated  

[Gus]

this thread might be of help. 
http://www.diystompboxes.com/smfforum/index.php?topic=107397.msg977022#msg977022