Cgs synth

On 7/7/08, ryan williams put forth:

>a couple of questions and comments: Has anyone noticed the tubes going
>into cutoff with larger input signals? I built the amp as ken's
>circuit twice with 3 different 1j17b's and got the same result. the
>problem was that as the input increased, the voltage at the control
>grid decreased until the tube cutoff. it decreased a few volts. i was
>looking at average voltage of -18 or -19V at the grid with a 10v p-p
>input. mine only accepted very small inputs without clipping most of
>the signal. looking at my archived email, it seems one other person
>has had this problem.

As I built a modified version using a different tube, my circuit is
different, but I did not notice that. It seems odd that the signal
on the input grid would effect the voltage on the control grid. Even
though Ken's circuit has a rather large resistor on the output (which
I included in my version), the control grid shouldn't draw much
current, so unless you are getting capacitive coupling, its voltage
should be determined by the op-amp. Also, it seems odd that you are
getting a voltage higher than the rails. I measured around -14.38V
with the gain potted all the way up.

>so, for the filter version. it sounds nice i think. the thing will
>resonate fine but requires some input to get it started. I have been
>testing and i'm pretty happy with it. I'll be preparing a PCB layout
>this week, and i'll be trying with some 1j24b's soon. i'll post some
>schematics after that. here is a sample of its sound. it starts off
>high resonance, low input. then high input high resonance. then high
>input lower resonance.
>http://www.homebuilthardware.com/tmp/sk-tube-vcf.mp3

That sure sounds like a resonant filter. I would love to see the
schematics, or a description of your circuit.

>for anyone interested there is a really nice article here:
>http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf
>which describes these two types of tube clipping very well. i highly
>recommend reading it!

Thanks :)