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Old 7th November 2012, 09:13 PM   #11
Zen Mod is offline Zen Mod  Serbia
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Rs= 0R for Idss
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Old 8th November 2012, 07:19 AM   #12
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hi!

thank you all!

ok, now I know how to get Idss from them.
I will check that today in the afternoon.

Is it possible to change the value from P1, too? I thought, I read that in the preamp-thread...?

best regards,
matthias
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Old 8th November 2012, 09:36 AM   #13
juma is offline juma  Serbia
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Quote:
Originally Posted by GentryBa View Post
... Is it possible to change the value from P1, too? ...
Of course it's possible but I wouldn't do it before we know what are we dealing with. So, what's the Idss of JFETs that you have ?
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Old 8th November 2012, 11:14 AM   #14
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Hi juma,

I have BL ones installed in my circuit. I didn't match them.
In the moment I can't do the measuring, because I'm at work.

I saw a pair of matched GR-types, will they do the job as good as the BL-types?

Thank you for your time to answer me!

Best regards,
Matthias
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Old 8th November 2012, 11:29 AM   #15
AndrewT is offline AndrewT  Scotland
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post11
replace the Ammeter with a 100r resistor.
Measure the voltage drop across the resistor to determine the Idss.

eg.
Vr= 100.8mV, then Idss = 0.1008V/100r = 10.08mA +-accuracy of DMM, +-accuracy of resistor, +- the effect of error in Vds & error in Tj.
i.e. 10mA is close enough.
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Old 8th November 2012, 12:14 PM   #16
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by AndrewT View Post
post11
replace the Ammeter with a 100r resistor.
Measure the voltage drop across the resistor to determine the Idss.
you are suggesting to measure across a 100R drain resistor for both k170 and j74 ?
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Old 8th November 2012, 12:40 PM   #17
juma is offline juma  Serbia
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Quote:
Originally Posted by GentryBa View Post
...I saw a pair of matched GR-types, will they do the job as good as the BL-types?...
Not without changing some resistors.
There is no use in watering the subject down before you do the measurements...
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Old 8th November 2012, 01:05 PM   #18
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Originally Posted by tinitus View Post
you are suggesting to measure across a 100R drain resistor for both k170 and j74 ?
yes.
I don't understand why the question. Do you see a problem that I have missed?
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Old 8th November 2012, 01:07 PM   #19
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
post11
replace the Ammeter with a 100r resistor.
Measure the voltage drop across the resistor to determine the Idss.

eg.
Vr= 100.8mV, then Idss = 0.1008V/100r = 10.08mA +-accuracy of DMM, +-accuracy of resistor, +- the effect of error in Vds & error in Tj.
i.e. 10mA is close enough.
Even Bigger OOPS than usual !!!
1008mV across 100r for 10mA of Id.
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Old 8th November 2012, 05:57 PM   #20
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hi!

measuring (maybe correct ) done:

my "high quality" chinese devices have following values for Idss:

2SJ74 BL : from 4.7 to 6.2mA (3 pcs.)
2SJ74 GR: 6.2mA (1 pc.)
2SK170 BL: from 13.8 to 13.9mA (5 pcs.)

the N-types are from china (3pcs.) and from a german distributor called "reichelt" (2pcs.)
their values are very close together (if measuring was correct, then they would be higher then BL specification, I think?)

three P-types (guess which ones- the chinese ) have 4.7 to 4.85mA and should be "BL", the reichelt one has got 6.2mA and is a "GR" graded one.

changing one of the chinese devices gave me an DC-offset of 160mV, much better than before, but not really changeable if I turn the pot?

so, where can I get GOOD jfets for this preamp?

best regards- and as always- THANK YOU for helping!
matthias

Last edited by GentryBa; 8th November 2012 at 05:59 PM.
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