Switching between JFET pairs

[adambzzz]

Is there a simple way to switch between pairs of JFETS, for reference it would be for a BSIAB type, to switch Q2/4 from 2N5457 to J201.

Was looking at this  board http://www.guitarpcb.com/apps/webstore/products/show/3186804 and it looks like it will work, just unsure on the how to part of connecting it to where Q2/4 used to be.

[R.G.]

Any electronic device can be switched in or out of a circuit by a switch of one less poles than the thing has pins. This is because there are no monodes.

So switching a three-terminal device, like a JFET, needs 3-1 = 2 poles of switching, as you've correctly surmised.

A DPDT switch will, to a first order, allow alternating two different JFETs. If you're switching two different JFETs in two different positions, you need four poles. If you're switching one of two JFETs, you need two DPDTs, and if you're switching two JFETs at the same time, you need a 4PDT.

The pole of the switches goes to the circuit nodes where the JFET terminals would otherwise be. The throws go to the two switched terminals of the JFETs, and the third terminal of the JFETs being switched stay connected to the circuit all the time.

You'll need to come up with solutions for the issues of JFETs changing the bias point suddenly on switching, causing huge pops, and the issue of what the changing bias does to the sound, as separate from the issue of what the JFETs themselves changing does.

And the switch parasitics, in JFET circuits. 

[GGBB]

If Q2+Q4 refer to one (1st?) stage/mu-amp, you might want to consider switching in/out the entire stage. Obviously a lot more components on the board, but it will simplify the actual switch, and allow you to address some of the issues RG mentioned. Would probably help with pops a lot too.

[adambzzz]

Thanks, I hadn't considered the bias issue, guess thats why its not an already pretty common mod.  Just happened to stumble on a pic of someone who did it but I could see the popping and bias issue making it more trouble than its worth.

[R.G.]

As noted, it's simpler to make two, one with each type of JFET and switch between entire circuits.