Cgs synth

IT makes that rumbly sound when i tune it low, what can i do to fix
this, maybe something about loading the output, i dont know. And im
kinda curios about something with the tube modules. Im kinda a novice
but im wondering like, i know tubes use much difrent impendences as
solid state. So how does that effect how i need to hook up each module
like the vco to the vcf and then how do i hook that up to my
amplifier. How would i figure what the impedences going in and coming
out need to be on the vcf and the other modules, any one know and it
allready and could give me a rough estimiate id really be glad thanx.

Tubes and transistors are actually more similar than different. Both
have three basic elements (unless diodes) though tubes have lower
input and higher output impedances. Motorboating (low frequency
oscillation) is usually caused by instability at higher gain. A
single voltage amp stage with high (150+) plate voltage can have gains
of over 100! If you're mixing tubes with solid state stages, however,
the solid state device likely has a very high input impedance and
shouldn't load the tube at all. Conversely, driving a tube with the
very low impedance output of at solid state device won't be a problem
either. In fact, the solid state stages will help the tube "fit" into
the circuit.

I would first look at power supply decoupling at the plate of the tube
stage. Put a 50 - 100 mfd electrolytic at the plate terminal to
ground as a bypass to smooth the incoming DC voltage. Mind the
voltage ratings! Then consider the coupling caps. Perhaps a smaller
value at the grid and/or plate would prevent low frequency artifacts
from starting the oscillation. Check your cathode bias voltage, too.
Perhaps the resistor value needs to be raised and the bypass cap
removed or replaced with a lower value.

For more tube synth info, check out Eric Barbour's site. Eric was an
application engineer for a Russian tube company with a US office (my
memory is fuzzy from recent events - forgive me) and not afraid to
experiment! Indeed, he freely shares information and circuits and
sells some very fun audio gear by his company Metasonic. Ken's CGS
site has likes to Eric's site. Also, Dave Wright has some great tube
gear at his site http://notbreathing.com as well as some great music
(I highly recommend his Carrion Sounds cd!).

Tube modules are fun, and I've built a lot of tube gear over the
years, but most can be way outperformed by solid state modules. The
reason is that tubes are big! It would take at least three tube
stages to replicate a basic diff amp - poorly - and simple solid state
circuits like exponential converters would be a nightmare.

On the other hand, using tubes to perform VCA and modulation functions
is a real treat! I've got a pair of pentode VCAs that perform as well
as most solid state units, and when overdriven they create a wide
range of distortion timbres. I've built some of Eric Barbour's cool
non-linear modulators as well. You won't hear those sounds from a
transistor module!

Enjoy!

Chub

I just joined this list after someone on the MOTM list posted about a
dual VCA module he built using two CSG65 PCB's. Although I decided
to do a bit more research first before I start asking questions. I'm
not familiar with directly-heated cathodes, and know extremely little
about 1J24B tubes.

If you could post the design you used for your tube VCF, perhaps I
could suggest what might be causing these unwanted oscillations.

In general, when used as amplifiers, pentodes can oscillate when they
do not have sufficient current, or when the plate capacitor and the
load (such as a speaker) form an RC network with a frequency close to
that of another RC network (such as the coupling capacitor and grid
resistors). Perhaps you could try putting different resistors in
series with the output, or changing the value of the plate capacitor,
and see what happens.

As far as impedances are concerned, adding an op-amp stage to the
input and output to provide a relatively high input impedance (eg.
20K) and a relatively low out impedance (eg. 1K), as well as match
voltage levels, might be a good idea. This way the pentode stage of
your VCF will see the same load regardless of what you connect to the
module.

On 3/10/07, kidkrazy58 put forth: