Suitable Jfet substitute for Boss FA-1 ??

[bluelagoon]

Just wondering how the 2SK209 GR would fit as a substitute for the 2SK246 GR in the Boss FA-1 circuit.
Main concern was the mismatch in the VGS off figures being so low in the 2SK209 compared to that of the 2SK246.
Aside from that they seem to match reasonably well.
There is also the 2SK208, which might be considered a better match aside from the IDSS being a bit of a lower  mismatch on the 2SK208
Any Thoughts?
Thanks

2SK246
2SK209
2SK208

[GGBB]

Look at the Idss note in the datasheet. They all have the same Idss when the Idss classification codes are the same - "GR".

[bluelagoon]

Yeah thanks Gord, My mistake, I see that all the IDSS specs match up same across all of them. Just were wondering and not knowing what the other difference meant and how if at all it would affect the FA-1 circuit if subbed for the original 2SK246 GR
Main quandary was the very low spec figures for VGS (off) in the 2SK209 compared to higher values in the other 2.
What implications does a lower VGS off have??
If anyone could shine a light on that for me. Thankyou.

PS: Fixed links above to Data sheets on 2SK Jfets

[GGBB]

Vgsoff specifies what gate-source voltage shuts off the FET (no sound). Since you don't want any sound cut off in this circuit, a larger negative number is better. So a smaller negative number means that the FET is easier/more likely to shut off. Remember that the AC audio input signal will swing the gate voltage slightly, but the DC gate voltage in the FA-1 should be about 1/3 V+ or +3V with a +9V supply. I'm not sure exactly where the source voltage would sit but I think it's somewhere near +4V making Vgs around -1V which means either unit could cut off in this circuit, but it's near the end of the spec range so likelihood is low. You can adjust gate voltage (bias) of the JFET by playing with R14 and R15 values - you could even use a 10k trimpot (connected as a voltage divider with wiper going to R2) for R14 and 12k for R15 if you want to be able to dial in your JFET. You might need to adjust R3 (probably lower).

I think the 2SK208 is probably a better and good enough match, but I'm not really the guy you want to ask about that. I saw an FA-1 schematic that used an MPF102. Assuming that worked, I don't see why other units in the ballpark wouldn't also work even if they're a bit off. JFET Vgsoff values are notoriously inconsistent anyway, so the likelihood that your 2SK208 is outside the range of a 2SK246 _AND_ will mess up the circuit because of this is probably very small.

[bluelagoon]

Thanks again Gord, for explaining that to me, makes better sense now. I think the 2sk208 is a very close match to the original 2sk246, its just that the pcb fab house I use only has a stack of the 2sk209's on hand, so just needed to see where they would sit. They are a good match apart form the VGS off, but like you explained they should still fit the bill. Probably will give them a try and find the outcome.
Cheers.

[amptramp]

The voltage divider that supplies the input common mode voltage to the op amps supplies 3.08 volts to the gate resistor of the JFET.  If you use a different FET, you may have to juggle values for R13, R14 and R15 to get a different value for a different JFET.