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GB "long and skinny" FirstWatt J2 clone PCBs
GB "long and skinny" FirstWatt J2 clone PCBs
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Old 22nd October 2019, 04:49 AM   #191
astromo is offline astromo  Australia
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Quote:
Originally Posted by JeffYoung View Post
Did anyone buy 2SJ109s from tommygun_nz on eBay? How did they turn out?
Yes.

Refer FirstWatt J2 Post #1453.

Look ok?

Note that I haven't put them to use yet. So, yet to be road tested.
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Old 22nd October 2019, 07:24 AM   #192
vasillis is offline vasillis  Greece
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Thanks for reply Jeff.
i have them in hand soon.i test them and i post the result.

BR
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Old 22nd October 2019, 08:00 AM   #193
blackdod is offline blackdod  Europe
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Quote:
Originally Posted by astromo View Post
Yes.

Refer FirstWatt J2 Post #1453.

Look ok?

Note that I haven't put them to use yet. So, yet to be road tested.


Interesting. I don't know if I should take a chance
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Old 22nd October 2019, 02:01 PM   #194
JeffYoung is offline JeffYoung  Ireland
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GB "long and skinny" FirstWatt J2 clone PCBs
@astromo, yep, they look OK to me.

@blackdod, based on astromo's tests I think they're worth a gamble. But everyone's level of risk tolerance is different....

@Vasilis, if they do turn out good, does the source have more of them?
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Old 22nd October 2019, 02:32 PM   #195
Dr_Strangelove is offline Dr_Strangelove  Netherlands
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I looked in my stash of parts. But I only seem to have V grade 2sj109...
Can I use a single V grade instead of BL grade?
What are optimum Idss values of 2sj109 and 2sk170?
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Old 22nd October 2019, 02:57 PM   #196
JeffYoung is offline JeffYoung  Ireland
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GB "long and skinny" FirstWatt J2 clone PCBs
As far as I understand it the only requirement for the 2SJ109s is that they're comfortably over the LTP current. The more over they are the less linear the operating point is, but I've been lead to believe this isn't the end of the world. If this is correct then V grade will work fine, but isn't strictly optimal.

I calculated the LTP current (ie: the two 2SK170s added up) to be optimal at 8.5mA, but again, the sweet spot is pretty shallow so I'm not sure it matters much. This means you want GR parts. See Long skinny builders thread.

Sadly I didn't get any feedback on my analysis, so I don't really know if it's correct or not....

The other thing I'm curious about (but don't know) is the relative effects of hitting the LTP current sweet spot vs. the distortion reduction of using two CCS JFETs. In other words, is a single 8.5mA 2SK170 better or worse than a pair of 5mA 2SK170s?

Cheers,
Jeff.
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Old 22nd October 2019, 07:38 PM   #197
Dr_Strangelove is offline Dr_Strangelove  Netherlands
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Thanks Jeff!
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Old 23rd October 2019, 02:44 PM   #198
JeffYoung is offline JeffYoung  Ireland
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GB "long and skinny" FirstWatt J2 clone PCBs
The boards do indeed make music.

For a more detailed report see: Long skinny builders thread.

Any volunteers to organize a second run?
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Old 23rd October 2019, 10:51 PM   #199
gary s is offline gary s  Australia
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Hi Jeff,

Good to hear your J2 is working and playing music. Also read your report in the other thread with regards comparison to your F5 build - especially interesting seeing these 2 amps use totally different output devices as well. I wonder if the sonic character on the J2 will change after it has been "burned in" after some hours use?

A question on the J2 for you - did you keep a record or measure the ID in the long tailed input pair and the individual Idss figures on both the 2SK170 GR and 2SJ109BL fets? Is it close to the 8.5mA you worked out in your modelling?
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Old 23rd October 2019, 11:00 PM   #200
Dennis Hui is offline Dennis Hui  Canada
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GB "long and skinny" FirstWatt J2 clone PCBs
Quote:
Originally Posted by JeffYoung View Post

Any volunteers to organize a second run?
If it's just ordering boards and sending them out (and no cutting MDF )
then I can volunteer.

Dennis
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