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WTB - JFETs for F5 or F6
WTB - JFETs for F5 or F6
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Old 23rd February 2018, 08:43 PM   #1
Metropolis is offline Metropolis  United States
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Default WTB - JFETs for F5 or F6

Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
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Old 24th February 2018, 12:21 AM   #2
jackinnj is offline jackinnj  United States
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WTB - JFETs for F5 or F6
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Originally Posted by Metropolis View Post
Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
For one thing, they don't have to be matched...you'll get very good results with un-matched.

You can pair up 2N5457 and 2N5460 to approach the transconductance of a single K170/J74 pair -- mind you that the pinout is different. Their transconductance is around 5,000mS vs 20,000mS or more for the K170/J74, noise figure is two orders of magnitude worse but input capacitance much better.
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Old 24th February 2018, 03:23 PM   #3
Metropolis is offline Metropolis  United States
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Ok, so it sounds like you are saying I can buy the unmatched packages of K170 and J74 and install randomly pick one of each and it won’t cause any problems? I honestly have no idea
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Old 24th February 2018, 05:17 PM   #4
jackinnj is offline jackinnj  United States
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WTB - JFETs for F5 or F6
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Ok, so it sounds like you are saying I can buy the unmatched packages of K170 and J74 and install randomly pick one of each and it won’t cause any problems? I honestly have no idea
It certainly won't blow up! In later versions of the F5 a 200 ohm trim-pot was used in an attempt to balance the gain of the P and N-channel devices.

The Linear Products variety are available from Trendsetter Electronics.

If you get a package of random devices from a third party, you can match for Idss quite simply -- short the gate and source to ground and measure the current which flows through the device using a 9V battery. This is Idss and matching Idss will start to get you close to ideal matching.

You would need to place in parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74, but this isn't an insurmountable task.
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Old 24th February 2018, 05:42 PM   #5
kinku is offline kinku  United States
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WTB - JFETs for F5 or F6
That was the most specific and useful info I seen in such a long time from this forum.
Two questions remains how will the noise figures add up if you parralel them?
Jack , thank you!
How many of those linear system devices you need to have a matched pair ( typically)
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Old 24th February 2018, 06:36 PM   #6
EUVL is offline EUVL  Europe
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> parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74

2N5457/5460 have a transconductance of about 3mS each and Idss of ~ 3mA.
4x in parallel gives 12mS at 12mA.
4x 2N5460 in parallel has an eq input noise of 30nV/sqrtHz

2SK170 has a Yfs of 45mS at 12mA, 2SJ74 close to 50mS.
Noise is about 1nV/sqrtHz for both.

I guess not quite close ??


Patrick
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Old 24th February 2018, 11:55 PM   #7
jackinnj is offline jackinnj  United States
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WTB - JFETs for F5 or F6
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I guess not quite close ??
Patrick
I've done it, and it works.

The noise is almost irrelevant -- the MOSFETs themselves are at least 2 orders of magnitude noisier than the input stage -- and they are running "hot".
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Old 25th February 2018, 12:15 AM   #8
KevinHeem is offline KevinHeem  United States
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WTB - JFETs for F5 or F6
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Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
It looks like spencer still has NOS Toshibas for sale.
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Old 25th February 2018, 01:15 AM   #9
EUVL is offline EUVL  Europe
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> I've done it, and it works.
Sure, but I would not call 12mS close to 45mS myself.


Cheers,
Patrick
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Old 25th February 2018, 02:07 AM   #10
jackinnj is offline jackinnj  United States
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WTB - JFETs for F5 or F6
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> I've done it, and it works.
Sure, but I would not call 12mS close to 45mS myself.


Cheers,
Patrick
Toshiba 2SK170 data sheet.

(I just matched 40 pieces BF862, fwiw. They are beasts.)
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