Cgs synth

>Firstly, while I've heard some very nice demos of its waveshaping
>abilities, I am curious how well it performs as a VCA. If the input
>signal is kept below the level at which it distorts, how well does it
>function as a VCA?? How well does it mute an incoming signal if no
>envelope is applied?? Has anyone here used it as a conventional VCA??

It works as one, yes, however if you want a "perfect" VCA, use the CGS64
instead. People chose tube designs when they add tonal color. A tube designe
that is "perfect" would be of little point as it would deliver the same
performance as a solid state one at a worse cost.

>Secondly, I am quite confused about the cathode circuitry. I do not
>understand the inclusion of either the regular diodes (which are
>described as optional) or the LED's (other than their ability to
>illuminate the tube).

There is a voltage drop required to power the heater. This power has to be
dissipated somewhere. A dirty great resistor could be used. Alternately, the
power could be dissipated as light, hence the LEDs. Besides, DIYers are
notorious for illuminating tubes with LEDs, so why not make it easy for them?

The redular diodes are to prevent the heater from being damaged in case
during experimentation/assembly a too high a voltage is applied to the
heater, e.g. via the installation of the wrong value current limiting
resistor, or a stray wire causing a short etc..

>it looks like the recommended heater current is ~13mA, so its
>resistance would be ~100 Ohms. V/i=R 15/.011 = 1.4K 15/.015 = 1K
>So I'm thinking a resistor between 900 Ohms and 1.3K is all that
>would be needed to limit the current within its recommended range,
>and the power dissipated by that resistor would be close to a 1/4W.

Never run a resistor at its rated dissipation - they die really quickly,
and/or burn the PCB around them.

>It says, "These LEDs are using waste power that would otherwise need
>to be dissipated in the current-limiting resistors for the heater, so
>even if you are hiding the tube, the LEDs must still be included."
>I'm sorry, I don't understand that, than other than the change in
>voltage due to the diode drop, how the diodes would effect the amount
>of current running through the resistors. Could someone please
>explain that??

If there is a 3.2 volt drop across the LEDs, it effectively reduces the
voltage across the current limiting resistor by 3.2 volts. It doesn't change
the current running through the resistors, but it DOES reuce the power they
need to dissipate: P = V x I.

_______________________________________________________________________
Ken Stone sasami@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>