I am replacing all the 2sk68a JFETS in the ka-9100 i'm working on. I noticed that the input pair for the tone amp calls for one L rank and one M rank between the pair. I don't know much about JFET's but I noticed that the gate-source voltage is much lower on Qi3 vs Qi1 so perhaps they call for a higher IDSS part due to the lower voltage differential. The 2sk68a's I purchased all have over double the IDSS rating of the parts that are being replaced. This shouldn't be an issue in this case and I shouldn't need to worry about using different ranks, correct?
KA-9100 mixed 2sk68A JFET rank in input pair
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Walterramjet
New Member
anyone know of a source for a 2SK68A or a substitute? I'm hitting a lot of dead-ends :/
Semanon
Super Member
Nope. That was the point of my question. I have a couple I’ve salvaged, but haven’t seen one I would be comfortable buying in years.
I rebuilt the meter section of a Yamaha M-80 a while ago and used Fairchild J-112 JFETs and they worked pretty well. But they were not in the audio path.
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claneg
Active Member
2sk209 is a perfect substitute...but it's SMD. I made an adapter to easily mount it in place of the original. I'll post a pic later on.
Walterramjet
New Member
I settled on LSK170B from mouser. it's a lot to learn about transistors all at once. and there must have been several generations of transistors since 1970.
This is getting closer to what Im asking so thank you. The schematics specify L and M ranks in the differential pair and I'm hoping to get away with using N ranks for both. From my limited knowledge, higher IDSS parts should not cause an issue. Specifically I'm wondering why they chose different ranks within the same differential pair
I went with this option. Only found out later that they came from Overseas. On the plus side they are JFETS and they meet the promised parameters as tested on my Atlas DCA75. However, the amp is still apart so I don't know how they work in practice. I compared them visually to the old 2sk68a's and they look identical in materials, lettering, shaping, etc...I hope they are not fakes.
Semanon
Super Member
There is a specific reason the engineers used JFETs with different rankings. It does not make sense to me in an LTP, as close matching is usually important, but I suspect it has to do with the downstream circuit of the M ranked JFET. It will require some detailed circuit modeling to understand the intent of the engineers. Regardless, I would make the assumption that deviating from the rankings will impact circuit performance, even if you move to a grade with a higher Idss.
And that was the point of my question. Finding ANY non-counterfeit 2SK68As is a triumph. Being able to select between ranks is a miracle. I’d spend my time looking for an available substitute.
Why are you replacing them? Have the failed?
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The variance in IDSS withon the GR rank of the 2sk209 is up to about the difference in IDSS between the different 2sk68a ranks I have
Thank you, that makes sense. The owner of the amp wants all new transistors in the amp and thinks the amp lacks bass impact. No, they haven't failed but the owner is worried about them eventually failing and going noisy and then having to service the amp again which is intensive due to the disassembly and assembly.
Semanon
Super Member
I get the sentiment. I, however, share the thinking of lots of folks here that transistors, except for some notable and well-known ones, don’t fail regularly enough to risk the impact of a full replacement of transistors or the sound. I think the impact is an incremental one: one transistor is not going to change the sound much, but add 20 or more in series, and the sound is bound to change.
But the customer (and his cash) is always right.
Walterramjet
New Member
i had been searching for a substitute transistor for a long while (2 weeks) and had made 3 orders from mouser, believing I had the right transistor and when they arrived, it was like I had no idea what I was doing and re-researched until I ran into someone who did an overhaul of his Kenwood 9G and he has a list he made 5 years ago. He has a lot of pictures, but if you scroll almost near the bottom, you will see this list. https://audiokarma.org/forums/index.php?threads/project-kenwood-9g-9000g.885716/ anyway, I didn't want you to think i was ignoring. I was just confused and looking at this stuff until my eyes were crossed.
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Thank you, but I do not see any JFETs on this list or in that thread.
Walterramjet
New Member
did you get a picture of that adapter?
claneg
Active Member
Here it comes.
Notice that I made the adapter double face, to make it possible to place close togethere 2 jfet, as per differential pair circuits, where the jfets need to be in touch with each other.
I also made a uPA63 adapter, another difficult to find component.
