300DC J-Fet Confusion (mine)

[Love Vintage]

I have a 300DC that one of the 2SK109’s legs corroded off, so had a difficult time finding a replacement. The manual says use C or D, in the process of finding one I came across K109-86C, K109-78C, K109A-11D these 3 worked fine but the others I found UPA68H, UPA68HA and individual 2SK170’s worked but caused the DC offset to become 1.25 to 1.5v. I couldn’t adjust any closer than that, so my questions are:

1- Is IDSS the parameter that the C/D refers to, that varied the most on all the fets from 1.82ma to over 12ma?

2- Does anyone know what the other numbers refer to? I assume the letters correspond to idss?

3- Some of the J-fets came from China, some were from the USA NOS. They all worked and all parameters measured appropriately, so when I read about China fakes what does that mean? Will they die sooner, will they not measure right to start with? If they measure appropriate and work ok is it safe to use them?
Thanks
Terry

 

[Love Vintage]

I have a 300DC that one of the 2SK109’s legs corroded off, so had a difficult time finding a replacement. The manual says use C or D, in the process of finding one I came across K109-86C, K109-78C, K109A-11D these 3 worked fine but the others I found UPA68H, UPA68HA and individual 2SK170’s worked but caused the DC offset to become 1.25 to 1.5v. I couldn’t adjust any closer than that, so my questions are:

1- Is IDSS the parameter that the C/D refers to, that varied the most on all the fets from 1.82ma to over 12ma?

2- Does anyone know what the other numbers refer to? I assume the letters correspond to idss?

3- Some of the J-fets came from China, some were from the USA NOS. They all worked and all parameters measured appropriately, so when I read about China fakes what does that mean? Will they die sooner, will they not measure right to start with? If they measure appropriate and work ok is it safe to use them?
Thanks
Terry

Anyone willing to shed some light on my questions? Any help would be greatly appreciated, just trying to learn while doing these repairs,
Terry

 

[Love Vintage]

Can anyone shed some light on this for me?
Thanks
Terry

 

[catrafter]

As I see it, when you get something from a supplier that doesn't deal directly with the manufacturer, you don't know exactly what you are getting. It may be the real deal, it may be a good reproduction or it might be the wrong part with a 'new' label on it. For your own gear, if you put a part in and it works now only to fail later, it's no big deal, you can always fix it again.

The big issue to me is, if you put a counterfeit part in someone else's gear and it fails down the road, you now have a warranty problem.

My 2 cents.

Tom

 

[Love Vintage]

Thanks Tom,
I get what you are saying, I guess I don’t understand what you do if you can’t find the correct nos part, I my case the 2sk109 c or d seems very hard to find when they are gone then what do you do? I am wondering if a different version can be used, how do you/what do you adjust to use an original version that effects the DC offset? Just wanting to learn through the process. Buy the way is IDSS the parameter that the letter refers to?
Thank You
Terry

 

[catrafter]

From my experience, two 2SK170 transistors can be used to replace one 2SK109 dual fet.
Attached is a jpg from long ago listing transistor subs for the 300DC.
Some of the parts may be unobtainium. Many Panasonic EIAJ and Fairchild (KSX) transistors are NLA.

YMMV, I have not vetted every part on this list.

Tom

 

[catrafter]

All about FETs:
https://en.wikipedia.org/wiki/Field-effect_transistor
IDSS
https://www.quora.com/What-is-the-IDSS-in-JFET

Mr. Google is your friend.

Tom

 

[rcs16]

1- Is IDSS the parameter that the C/D refers to, that varied the most on all the fets from 1.82ma to over 12ma?
2- Does anyone know what the other numbers refer to? I assume the letters correspond to idss?
3- Some of the J-fets came from China, some were from the USA NOS. They all worked and all parameters measured appropriately, so when I read about China fakes what does that mean? Will they die sooner, will they not measure right to start with? If they measure appropriate and work ok is it safe to use them?

1) Without a datasheet it is hard to say for sure, but the suffixes usually refer to the IDSS ranges
2) Same as above. Sometimes the other suffixes refer to package options,lead forms etc
3) It could be any of the above. If they measure okay and work I'd say you are relatively safe.

 

[Love Vintage]

Thanks Tom & RCS16 for the help and information you have provided, it helps a great deal. What I don’t understand is how the higher IDSS values change the dc offset, the lower values work fine the higher ones cause the the dc offset to be to high to adjust with the trimmer. Also Tom how do you bond the 2 2SK170’s together, don’t they need thermal contact?
Terry

 

[catrafter]

Thermal contact would be desirable. The 'how' would be up to you.

Tom