channel switching

[wizard333]

Anyone have a sample schematic using JFETs for chanel switching?

[lowell]

I don't but can tell you that they can be used quite simply for shunting. Connect the source to ground, drain to the signal line (e.g. tube grid - in parallel w/ grid leak via a capacitor), and a negative control voltage to the gate that will turn the jfet on and off. Make sure there is a cap between the drain and the signal line so the jfet only affects AC. You will need to taylor the cap's value to surrounding resistors using the Fc equation so that ALL the AC signal is shunted. (1/2pieRC) It'll probably actually be fine if you use the same value as whatever coupling cap comes from the previous stage. I believe -15v will suffice for a gate control voltage but not really positive about the control voltage. When the negative voltage is on the gate the jfet is off, therefore your signal is going into the grid of the tube. When the gate has no negative voltage (0v) it will be like a closed switch and the signal will short to ground.

[lowell]

I just reread KOC's info on this. He says to use a series and shunt jfet together. I don't see how this is necessary if you're using a shunt arrangement. Anyone know bout this?

Also, can someone suggest a suitable mosfet for switching needs? They will need to be able to switch an amplified zener in and out of the screen and plate supplies.

Finally, can a simple supply for switching jfets and mosfets on/off be tapped of the bias supply w/ a reverse biased diode, 2 resistors (voltage divider), and 1 capacitor? I'd think this'd work but maybe there are unforseen issues with it's simplicity.

[Jazz P Bass]

N channel FET

Here is a link for N channel fet function.
P channel is the opposite function.
Link:SCEN103 -- n-channel MOSFET switch