Educate me regarding using JFETs as voltage controlled resistors please...

[frequencycentral]

Hi, maybe thats somthing what you are looking for. An Op Amp And Two JFETs Form A Voltage-Controlled Amplifier.
Schematic:
http://electronicdesign.com/Files/29/10116/Figure_01.gif
And how it works:
http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=10116

Thanks, but thats a single ended VCR - I'm looking for floating.

[slacker]

If you want a simple circuit to experiment with try Tim Escobedo's Wobbletron. It should be easy for you to adapt it to use tubes and fit's in with what you're trying to do as it's a phase shift stage, as an added bonus if you remove C1 it makes a nice tremolo.

[Ice-9]

The circuit i had mentioned in an earlier post uses a 555 timer to generate a square wave to drive the fet, It looks more like this will switch on/off the fet rather than controling it like a vr. With some mods you should be able to drive the fet over its normal operating range (hope i've worded this correctly)
If you still want a look at the schematic i will post it up .

Yes, please post it, any examples will help.

[R.G.]

The circuit i had mentioned in an earlier post uses a 555 timer to generate a square wave to drive the fet, It looks more like this will switch on/off the fet rather than controling it like a vr. With some mods you should be able to drive the fet over its normal operating range (hope i've worded this correctly).

The JFET is used as a single-ended resistor to ground.It's a P-channel JFET with its source and drain DC grounded. The gate is pulled up to 1/2 the power supply by the two 1.5M resistors, providing a 4.5V reverse (i.e. off) bias for the FET. The square wave is AC coupled in through C5, so it cannot affect the DC level of the gate. The incoming signal is also low pass filtered by R6/C4 to keep it from being just fully on/off. The time constant of R6/C4 is 100K*1uF = 0.1seconds, giving a rolloff frequency of 1.59Hz. So the square wave from the 555 is substantially filtered toward a Tri-ine/wave-ish thing, not switched on and off.

I would guess that it is is used over its normal range as a tremolo.

[Ice-9]

R.G Thanks for the insight and breakdown of how the circuitry around the 555 timer is working, ie how the square wave is filtered to produce a more tiangle - ish form. I hadn't looked closely at the circuit before and in reality if it had just switched the FET on/off, the effect would not have worked that well.
I am learning quite a bit from all the posts in these forums, great place to learn.