[sdiy] tube ladder

[media.nai at rcn.com]

At 10:37 PM -0700 06/20/02, Tim Ressel wrote:
>
>Nope, it kinda dried up. The ladder filter relies on
>the emitter resistance of a transistor (or the
>impedance of a diode) changing by varying the current
>through the device. There does not seem to be an
>equivenant mechanism in a triode. And while tube
>diodes do show this property, it didn't seem to vary
>enough to be useful.

How about a tube "OTA" filter??  If you look at many filter IC's used in
synths, they use a stack of OTA's with external capacitors.  Many VCA's are
also built using OTA's.  A non-optical tube "VCA" uses a remote-cutoff tube
(ie. "variable mu") which is similar in operation to an OTA.  So I wonder
if cutoff tubes could be used instead of OTA's in a VCF.

Of course, like OTA's, all tubes use transconductance.  The bias of a
triode amplifier is set by the plate and cathode resistors.  The voltage
gain of a regular op-amp is set by the input and feedback resistors.  In
either case, the gain is set by a voltage divider which isn't variable, so
you are "back where you started".

However, unlike a regular op-amp, in an OTA, Iabc controls the current
gain, (so by adding resistors, Vabc controls the voltage gain).  A
remote-cutoff tube, has control grid with wires that are farther apart at
the center than the ends, so the amplification of the tube so the
amplification does not vary in direct proportion to the bias, such that the
grid voltage controls the plate current.

Can a VCF be made with variable mu pentodes??

>Then the rain started in my life, and all work on this
>end has halted. I think it needs to be further
>hammered on as an intellectual discussion. I for one
>need to grok tubes in fullness.

Try sharing water with Eric Barbour ;)