From obxwindsurf@yahoo.com Mon Oct 06 09:38:45 2003
Subject:Re: Next Generation Vibrato/Chorus Clone

Au contrere!

In a tube circuit, it's all about bias differential whether or not a
signal gets through. You can pinch off a signal completely from
getting through a tube (valve) circuit or allow it through depending
on the voltage differential between the grid and plate. Without
doing an exhaustive analysis of the effect of the resistor between
the VDL's common to be in the circuit or bypassed, my gut feel is
that by allowing the common of the VDL to float you are changing the
bias potential and thus whether the mixing takes place (chorus) or
not (vibrato). By driving from a higher source impedance any bias
on the signal dominates and the signal's effect on the tube's grid
(s) are more pronounced.

With transistors (whether it's an IC op-amp or an op-amp made from
discretes), signals are brought to the summing junction of an op-amp
which sits at a virtual ground (if the + input is grounded) or a
virtual bias voltage (of the + terminal). The relationship between
input and output signals are then -Vin1 * Rf/Rin1 + -Vin2 * Rf/Rin2
(assuming two inputs), where Vin1 is the source signal #1, Rin1 is
the input resistor for this signal and likewise for Vin2. Rf in
this case is the feedback resistor.

The design approaches are entirely different because the
amplification devices work on entirely different principles - tubes
are heated which causes electrons to boil off the cathode and
accelerate towards the plate due to the plate being more positive
than the cathode. The grid has the signal applied to it to either
retard the flow of electrons from cathod to plate, thereby providing
modulation and amplification.

Transistors are normally cut-off and when a current is applied which
forward biases the base/emitter junction a large collector-emitter
current flows which is a function of the base-emitter current times
the gain of the device.

Hope this helps,
Kevin

> the ground and the LC junctions.
>
> James