Substituting J201 for 2N5457 in BSIAB2

[hillio]

Hi.  I was just wondering if anyone knows what a good "sweet spot" VGSoff voltage would be for the typical 2N5457 in the BSIAB2 circuit.  The reason I am asking is that I have a bunch of J201's that I'm going to match, and I want to find 3 of them that have VGSoff close to that of a 2N5457 for use in the BSIAB2.

The data sheets list the J201 as 0.3v - 1.5v and the 2N5457 at 0.5 to 6.0, so there is a small window where the two overlap.  Also,  if I'm not mistaken,  J201's are higher gain than 2N5457's, so I would need to be able to lower the gain in the sections where I would be substituting the J201's.  What would be the best way to achieve this?


***One more question:  Is Forward Transfer Conductance synonymous with gain?  If not, what about Output Conductance?
Thanks

[Krinor]

I'm sure some of the more theoretically minded could give you a better answer, but in my experience finding a sweetspot for just about any JFET is simply subject to personal taste.

[soulsonic]

Without getting too theoretical (because I probably can't ), you can tailor the circuit to suit whatever sort of JFET you have by adjusting the Source resistors. I recently reworked a Thor to work with a completely different type of JFET than what they had spec'd and all I had to do was experiment with swapping different Source resistors until the circuit responded the way I wanted. Adjusting the Source resistors is also how I adjust the bias of JFETs. I don't use trimmers for Drain resistors, I just stick with a certain value there and adjust the Source to get the correct current flow through the device. If you are constantly adjusting the Drain, then you're adjusting the load on the Drain and as a result, possibly altering the bandwidth and tonality of the circuit. The main function of the Source resistor is to set the bias point of the device (just like the Cathode resistor in a tube), so it makes sense that is how the bias would be adjusted.