Tube vcf

[Goddard, Duncan]

I have a phattytron- the tube vcf in it is a bandpass; maybe this is the
only practicable approach with a single tube. it doesn't sound anything like
any vcf I've ever heard before, though I haven't been able to poke around
with a 'scope to see what's going on in there- I suspect that the gain
structure is doing strange things too. it goes into self-oscillation at
about 9 o'clock on the resonance control and gets louder thereafter, while
the frequency shift does more or less what you'd expect, depending how hard
the input is driven. 

the oscillator (also tube) has a very peculiar and quite non-linear response
to volts, which is why he adopted the midi-interface with a voltage
adjustment for each note of the two octaves it actually travels. getting any
greater range out of this particular configuration would mean a lot of work
adapting the rest of the box to the characteristics of a particular
bottle...... it's decidedly non-linear, anyway.
I think I'll probably go with plan B, which is to tune the device for each
note over a few octaves and record each one onto tape (not sampler!) before
getting them put onto mellotron tapes. after that, the pt-1 will be used for
generating non-pitched sounds, which it's very good at.

duncan.



> ----------
> From: 	Malcolm B McIntosh[SMTP:mmcintos at onvoymail.com]
> Sent: 	30 September 2000 02:47
> To: 	patchell
> Cc: 	synth-diy at node12b53.a2000.nl
> Subject: 	Tube Moog (Was:Re: Website update)
> 
> 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.  Now mind, I was just hacking at it....  Since I was
> interested in seeing if the pentodes could be used as a filter core in
> the ladder configuration, I just used an opp amp back end as the
> differential amp section.  One time, (and one time only), I got a really
> nice sounding resonance feedback.
> 
> Tried a 4 pole with 5 twin triodes.... Same result.
> 
> Malcolm (still crazy)
> 
> patchell wrote:
> > 
> >     Hmmm.....what about a pentode ladder filter......that might cut
> >down on the number of diff amps.  That would be a sight, a moog ladder>
  >filter built out of tubes....(slap..) well, it was just a thought.
> > 
> > René Schmitz wrote:
> > 
> > > At 20:13 27.09.00 -0700, patchell wrote:
> > > >    Maybe this is a pretty naive question, and please remember, I 
> > > >have never used tubes in anything, so I might be ignorant......
> > > >
> > > >    Correct me if I am wrong, but doesn't the screen grid on a 
> > > > pentode tube vary the transconductance of the tube?  Seems like 
> > > > there should be someway of exploiting that to tune a tube based 
> > > > filter.
> > >
> > > Right. That should be possible. Varying the screen grid voltage does>
  > > shift the DC-quiescent point. It could lead to a differential design,>
  > > with plenty of diffamps. Looking at my VTVCA, imagine a cap from 
> > > plate to plate of the pentodes whose screengrids are steered. Several>
  > > of these two pentode stages could be cascaded.
> > >
> > > Bye,
> > >  René
> > >
> > > --
> > > uzs159 at uni-bonn.de
> > > http://www.uni-bonn.de/~uzs159
> > >
> > >
> > 
> > --
> >  -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *If you didn't buy a home in Santa Barbara,
> > * You didn't pay enough!
> > ------------------------------------------------
> 


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