[sdiy] tube ladder

Matt Jones thereminhd at yahoo.com
Sun Jun 23 08:47:51 CEST 2002

--- media.nai at rcn.com wrote:
> 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".

Alternatively, one could consider using 6JH8 
(among others..). These are referred to as 
"beam-deflection" tubes, and essentially use 
electric fields to bend the current passing 
through the tube from one anode to another.
In principle, it should be possible to control
the total current in one with the gate-cathode
voltage, while using the deflectors as signal
inputs. See Eric Barbour's pages for more on the


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