Aleph J 2SJ74 Idss question

[markvarga91]

I know similar questions have been posted before, but I was not able to derive an answer to my question specifically.

I am trying to replace the most likely fake JFETs in my Aleph J build, and I found the following two options:
Toshiba 2SJ74 MATCHED QUAD to 0.03mA / 4mV 6.5mA - 7.9mA Idss Range | eBay

Toshiba 2SJ74 MATCHED QUAD to 0.03mA / 4mV 10.0mA - 10.7mA Idss Range | eBay

Upon contacting the seller, I am told that the first option would have an Idss of 7.x mA for all four of the JFETs. Based on the listing, the second option would have 10.x mA for the four JFETs. Which one is better for my Aleph J? Does it matter at all? These are all BL grade, so it should work with both options, but I am wondering which one is the more optimal match.

Thanks.

 

[Zen Mod]

in this case , doesn't matter

they're biased at approx. 5mA via CCS

 

[markvarga91]

Follow up

in this case , doesn't matter

they're biased at approx. 5mA via CCS

Thanks. Let me ask a follow-up question. I found this thread:
Acceptable Idss Range for 2SK270-BL and 2SJ74-BL

In the thread, Nelson Pass says the following:
"Idss of 6 mA will work on these amplifiers, sometimes with small adjustment.

8 mA is a more desirable, and is the presumed default most of the time."

Not sure if this applies to Aleph J specifically, but I thought of the 8mA as a rule to follow, and I was wondering if it's better to go slightly below or a bit more above it. Or does it still not matter in the case of Aleph J?

Thanks.

 

[Zen Mod]

well , if you re-read my previous post and think little more , you'll get that everything is replied

 

[6L6]

What ZenMod is getting at is in the Aleph J, there is a CCS (constant current source) that controls the current through the Jfets - the self Idss is not anywhere as important in this amp as it is in others.

As long as the J74s are BL or B grade and reasonably well matched (10%) they will be fine in-circuit. Choosing any particular Idss over another in this case will not make any meaningful nor measurable difference.


In other circuits, for example the input stage of the F5, the Idss of the Jfets has a lot to do with the current that will be flowing through the circuit when it’s biased properly, hence the requirement for matching and a particular Idss range. Although, it’s worth mentioning that the acceptable range of Idss in the F5 is still pretty wide.

 

[markvarga91]

Right, I understood your answer. Just wanted to double make sure about the "doesn't matter" part in light of Nelson Pass' general answer and that there is no contradiction.

Thanks!

 

[markvarga91]

What ZenMod is getting at is in the Aleph J, there is a CCS (constant current source) that controls the current through the Jfets - the self Idss is not anywhere as important in this amp as it is in others.

As long as the J74s are BL or B grade and reasonably well matched (10%) they will be fine in-circuit. Choosing any particular Idss over another in this case will not make any meaningful nor measurable difference.


In other circuits, for example the input stage of the F5, the Idss of the Jfets has a lot to do with the current that will be flowing through the circuit when it’s biased properly, hence the requirement for matching and a particular Idss range. Although, it’s worth mentioning that the acceptable range of Idss in the F5 is still pretty wide.

Thanks, that clears it all up! So then I guess the specific mA answers apply to other amps, but not really Aleph J. I will probably go ahead with the 7.x mA version, then.

One more question if I may. How much improvement will I get by replacing the fake JFETs with genuine Toshiba? Since I know that this particular JFET is very-low noise with relatively high transconductance, and a fake might be a completely different part with much lower noise/transconductance performance, I assume that the difference will be substantial, but I was wondering if anyone have experience with this.

 

[Zen Mod]

difference between TV screen not being cleaned for 7 years , and same TV being clean

 

[Dennis Hui]

I am trying to replace the most likely fake JFETs in my Aleph J build...

Actually I'm curious what led you to think you may have fake jfets?

Thanks,
Dennis

 

[markvarga91]

I bought them on eBay from a Hong Kong seller for a low price of $5.83 for ten pieces. This is very low compared to what I linked in above, and also very low compared to the LSJ74 JFETs (unmatched) found on diyaudiostore.com. When I tried matching them more than a year ago, I did not really compare my numbers to what they should be with original parts, but based on other posts I am relatively confident that these are fake for this price.

 

[6L6]

The difference between fake and genuine will be much, much more than you’d expect.

 

[markvarga91]

I compared my current Aleph J with an ICEPOWER Class D amplifier, and the second sounded much better. Now, this ICEPOWER is one of the good ones among Class D (don't think of the quality of commercial AV receivers, this is much better than those), but the difference still did not make sense to me. So I started thinking and realized that the JFETS are probably the problem.

I will go ahead and buy those genuine JFETs, and hope that it will beat the Class D with those, thus restoring the proper balance of nature

Thanks for all the replies!

 

[Eric]

thus restoring the proper balance of nature

Ha! Thanks for the laugh!