Website update.

[René Schmitz]

Hi Jürgen!

>Nice ! A Borg Assimilator Cube ! - sorry:  Borg Assimilator Tube.

:-)

>I'm sure it sounds better than the various Korg versions with
>diode limiters.
>Which tube performs the limiting at high resonance ? Or
>is it the transformer ? Have you tried a transformerless version
>as well ? 

As of yet, I didn't try a transformerless version. The limiting I mentioned
isn't a hard DC-limiting in the sense of the korg circuits, its just that
the grids become conductive with Vgk > 0 (Vin > Vk), and the resulting
current charges the grid caps, thus shifting the bias towards less
distortions! In which tube this happens depends on input amplitude and
resonance settings. The transformer is not getting saturated, because of
the low currents that flow. The DC bias of 0.75mA doesn't do any harm.

>ECC83's for the triodes would work as well ?

I think they ought to work, but I didn't check. 

>Ah ... reading on ... there is some text as well ... making things
>clearer. An ordinary 50Hz transformer ? I flinched, but then I read
>about the Moog-like HP feedback - that's it! One of these tiny
>pcb mount transformers ? 

Right, though it should be possible to use "more iron". The point is its
"only" in the resonance loop, and this is a musical circuit, not some
audiophile preamp. The trouble with power transformers is they're made for
stiff sources (the 230V net). They have a high stray capacitance and stray
inductance which leads to oscillations (at 10kHz or so), when driven from a
high impedance, like the EF86. Because they don't have intersected
windings, the treble response is actually more critical than the bass,
which is just a matter of the inductance of the transformer, bigger ones
would be better in this respect. However there is a damper resistor, which
reduces the overshooting greatly. 

>Final question: as you have a transformer anyway, you can do
>phase inversion with the transformer and might get along with
>one tube stage ? Would that be possible ?

It might work with a different turns ratio. I got roughly enough gain with
a single stage, but wanted to have hefty overdrive from the resonance
circuit back to the filter core. You would have to do the resonance setting
with a low-ohm pot on the transformers secondary. And you would have the
transformer-highpass within the normal signal path. -> poor bass. 

>Congrats - that's the most interesting VCF I've seen for a long time !

Many thanks! 

Bye,
 René


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uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159