Salas hotrodded blue DCB1 build

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#2,001
lloop9 said:
Hiya fellows, I'm going to do some jfet matching for some 2SJ103, 2SK170, etc. I would like to know if the below schematic is appropriate for my purpose?

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would these setups work?
 
#2,004
Yes. That little circuit looks like its for matching Vgs(off) spec. Its the cut off voltage of a Jfet. So lets forget that. What we need match here is IDSS. I.e. top current when fully on with Vgs=0. That's the way we use it in this buffer, that's what we look for. So get yourself a 9V battery, a 10 Ohm resistor and some small crock leads. From left to right as you face the flat face of Toshiba 2SK170 you have pins DGS. Catch the two last ones to the right with a crock clip together and connect the other crock end to battery (-).
Now catch the left alone pin D with another crock clip and connect to the 10R on the other end. Connect the free 10R lead to battery (+) in a couple of minutes after you had touched the Jfet to let it lose your fingers heat. Now full IDSS current runs through. Use your DMM on DC voltage mode and autorange. Note down the reading across the resistor after it settles in a bit. Unhook that Jfet and stick it over that reading on paper. Next one ditto. After you got 'em all written down calculate currents from I=V/R. V=your note in V or mV, R=your 10 Ohm resistor or anything in hand up to 50R, I=IDSS. Make pairs of the closest ones. Use two good pairs of a nearby mA region to assemble your DCB1's audio section. Send me cherries.🙂
 
#2,005
Thanks!

Your method of matching Jfet's surprised me at its simplicity. I used it to match LED's also. You've helped me immensely and I am in your debt.
It all fits with something I try to live by." A day where you don'y learn something new is a wasted day. "
 
#2,008
I've just ordered an 80 ma toroid from Richard sumner in Toronto, potted, to start the ball rolling on this build.

Was considering this pot, but it's only 10K and carbon to boot.

KKB1031S28 Precision Electronic Components Ltd | KKB1031S28-ND | DigiKey

Any opinions? Would a goldpoint 20k be that much better?

I'm pretty much a newbie doing these things, having only built a couple items, but my question here is -- the BOM calls for a pot from 10K to 20K. The original B1 was 25K.

I'm a bit confused as to what value to use, and any help would be very appreciated. My readings of the threads are inconclusive, if 10K, 20K or even 25K is the "right" value.

Thanks in advance!
 
#2,011
Ive read that matched LEDs are, and are not necessary. Now is this for Vref? or CCS?
I'm just figuring all this stuff out. And also Does it matter if I have more than the required V all my LEDs measure at between 1.895 Vf and 1.915 Vf. So 5 x any of them will be more than 9V for Vref and 3 x any of them will be more than 5.4V for CCS. Or should I just stick'em on the board and not worry?
 
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