AW: [sdiy] Using Tubes in SDIY

[jhaible at debitel.net]

I've started to focus (for a while) on tube circuits myself a few
months ago (it's always nice to learn something "new" instead of
staying on well-known ground), and while my intention was making
outboard FX devices rather than synth circuits with tubes, the two 
topics are so related that they can hardly be separated. 

For instance, I started to build a variable Mu compressor that uses
a pair of semi remote cutoff triodes for gain control. And I wonder
if this wouldn't make an awesome synthesizer VCA as well. 
I've used a 6BC8 dual triode (just like the UA175 compressor), connected 
between two transformers, and this produces some of the sweetest
soft distortion I've ever heard (with a high input signal), and it's
quite clean with smaller levels. I don't know how much of this comes
from the tube, and how much comes from the output transformer (some
vintage UTC interstage type). And I haven't checked how the circuit 
sounds when the gain is fully set to zero. But results were very promising 
before I had to pack up everything. (Can't wait to get the electronics lab
running again, but the studio comes first.)

Can anybody give a comparison on VCA behaviour (CV feedthru, nonlinearities,
dynamic range, etc.) for single pentode vs. dual triode circuits?

I also wonder if a VCF would be possible with a couple of remote cutoff
triode pairs as control elements. I guess most trouble would be at low
cutoff frequencies (attemting to sweep over the whole audio range), but
so far this is only guesswork from my side.

JH.




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