Tube Moog (Was:Re: Website update)

[René Schmitz]

At 08:45 30.09.00 -0700, patchell wrote:
>
>
>Paul Perry wrote:
>
>> At 06:47 PM 29/09/00 -0700, Malcolm "(still crazy)" mcintos at onvoymail
wrote:
>> >I actually tried one.... It had 3 ladder rungs (thus a 12 pole filter, I
>> >hoped).  I could not get the thing to sweep very much at all, maybe from
>> >50Hz to 500Hz.
>>
>> I suppose the limiting factor would be the ratio that the transconductance
>> sweeps over, which I doubt would be enough for the full audio range..
>> those semis have spoilt us hackers ;-)
>>
>> paul perry melb aust
>>
>> PS stay crazy, nobody sane ever made a 'new' circuit!
>
>    I don't know.  I was always convinced that the screen grid could provide
>quite a few decades worth of transconductance change.  I just don't know that
>much about tubes.
>

I'd too say that the transconductance sweep is larger. Its just not
possible to vary the transconductance via the standing current of the
ladder, like it is possible with transistors. (Would be equally difficult
than a FET-ladder filter...) 
With pentodes you could vary the transconductance over a larger range, just
you can't stack them like transistors. Malcolm, you used triodes, right?
What did your arrangement look like? This might be a totally different
story than what Jim had in mind.

Staggering the tubes creates some side problems, you have to have a higher
supply and you must take care of the maximum allowed filament to cathode
voltage which is +/-100V usually. One barely sees amplifiers with more than
two systems in series for these reasons. 

IMO the best way would be to fold the ladder so that every tubes cathode
returns to GND, then one can use pentodes whose screengrids are steered
from the same source refered to GND. It should be a stiff source as the
screengrid draws some current.
I drew a sketch of such a thing, I've put it up to:
http://www.uni-bonn.de/~uzs159/tubelad.gif

Bye,
 René





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