[sdiy] Ladder filter tubes

[metasonix]

>I'm trying to imagine how the tube stuff would work in a ladder setup. 

I have not tried it yet. Expect it to be very insensitive unless pentodes
are used.

>I'm under the impression that "tubes are voltage devices" and "transistors
>are current devices",

tubes are transconductance devices, like FETs.
Voltage in, current out.

> Would a ladder of cathode-to-plate
>valves with caps as the "rungs" between the sides of the ladder do the same
>thing as a transistor ladder? 

It should.

>Another popular filter type is the cascaded integrator, typically incarnated
>as four OTA's in a row, each set up as a simple R/C filter with the R being
>the OTA. Feedback from end back to beginning allows for resonance. Can a
>tube (voltage device, like a FET) work as a variable integrator here? Or
>would it be better to use the tubes as Hi-Z followers, and use LDR's as the
>R and gang them together for frequency control?
>Then what about a state-variable filter?

Tried it. Multistage feedback, state-variable and biquad circuits 
do not like to use RC-coupled stages or integrators. 
I had all kinds of problems with motorboating due to the 
coupling capacitors. Such circuits were intended for 
opamps and direct coupling, at the most  basic level.

>Perhaps it would be easier for me to visualize this if we started with VC
>filters that use FETs as the controlling element, and then pull them out and
>drop tube circuits in. Anybody have any workable FET designs? I've seen
>FET-based allpass filters in phase shifter schematics, but I don't remember
>seeing any VCF's.

The phase shifters didn't involve a lot of feedback, so they
didn't have motorboating problems. Active filter circuits are
a different area altogether. Especially when you start cascading
sections.


>Darn it, these 1/2 watt resistors don't fit into the plug-type protoboards
>very well :(

Most tube circuits can use 1/4w resistors if you
limit the plate voltage to 150v or less. Using a 100k plate
load on a 12AX7, the maximum power the resistor will
ever dissipate is 225 mW (and then only if the triode shorts out).


E. Barbour

designer, METASONIX (www.metasonix.com)
and senior editor, Vacuum Tube Valley magazine