Cgs synth

On 3/24/07, Richard Brewster put forth:

>I find a very, very small output being passed when the Gain and CV pots
>are at zero. I didn't try adjusting the trimpot, though, and this might
>be why a small signal is passed. Initially I set the trimpot by ear.
>
>The CGS-65 really cannot do double duty as a regular VCA for a couple of
>reasons. One is that a unity gain response is distorted for much over a
>2 volt p-p input. You don't want any distortion for regular VCA use. A
>second reason is that the control voltage feeds through significantly.
>It thumps when there's an envelope but no input signal.

Thanks, that gives me a much better idea of what it can do. Although
2Vpp isn't that small of a signal. Perhaps I could try different
capacitor values in the LPF for the control voltage.

Have you tried sending it the inverted output of an envelope
follower?? It might make a nice sustain distortion unit.

I have been wondering why no tube compressors use pentodes as VCA's
or why I have never heard of a tube gate. Although it might be
possible to design a tube VCA where the control voltage is inaudible
using a pentode with a different configuration or with a tube with
more than three grids, I do not think anyone has ever done it.

On 3/25/07, sasami@... put forth:

>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.

Oh, I'm not looking for it to be a "perfect" VCA, I'm just trying to get a
better idea of what I can do with it. The amount of space and
modules I can build is limited.

> >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.

Thanks, that is very good advice, considering that 1/2W and 1W
resistors are available in E24 values.

>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.

OK, with two 1K5 resistors in parallel and say another 100 Ohms for
the heater itself, V/R = i, 11.8/850 = .013mA

So if didn't use any diodes, and instead used two 2K or one 1K
resistor, V/R = i, 15/1100 = .013mA, P=iV, .013*15 = .195W, would
that work??


Has anyone tried adding a cathode bypass cap?? Admittedly, I have
very little knowledge and no experience when it comes to
directly-heated cathodes or using a "negative" cathode voltage, but
I guess I am just used to seeing them.


Also, as an aside, I'm guessing that these tubes use some sort of
coated tungsten, so their heater resistance would be only about 12
Ohms upon start-up, as tungsten filaments have only about 1/8 their
normal operating resistance when cold. While that difference is
negligible when using the resistance needed for 11.8V or 15V, for
anyone thinking of using a lower heater-cathode voltage with these
tubes, for example the 5A supply for their modular, the increased
power dissipation upon start-up might make a difference when choosing
resistors of the correct wattage.