[sdiy] Tube Synth web page update

[Marc Bareille]

Hello René

> my filter also can be set for pretty wild behaviour. The filter is
> pretty sensitive to the input levels, the sound hugely depends on that.

Yes , it is! I have a couple of VCO the signal on the filter input is quickly
very high if i use both... Playing with VCO drive pots produces very interesting
sound . The response of tubes is really different from solid state on this point.
Much more sensitive !

>
> The example file is a rather tame one, there is more screaming possible.

I also have no doubt on this point, it can scream incredibly, may be too much on
some tunings :) Reminding me some Korg MS  filters... !  In fact i have put 3
trimers. One replace the 1M resonance pot. I control the Q factor with CV on the
6AU6 second grid  like in the Eric's VCA , same method.  Another (220k)  is
connected between the leg of the transformer primary , instead of the 200k
resistor, and act like a L-R "filter" ajusment I plan to replace it with a larger
value , 470k or 1 M.... The last one is like on your schematics between the
cathode and ground to set the grid bias of the penthode. Depending  settings of
those 3 trims  i can reach drastics screaming sounds, very  interesting for
effects or sampling. But when tuned like this the range of front panel pots  is
reduced too.  There is a compromise position for  the 3 trims combinated with the
gain of tubes and the transformer who produce the good  filter control range.
Another point :  I have forgoten the 5 min on/test/off operations usually used
with most of the discrete designs...Tubes must be very "hot" before tuning, to
let the filter stabilize itself , approx 20 min of work before trials, else it
produce unusual wild sounds... Interesting for sampling anyway...

> However, for tweaking the performance, you might consider other transformers,
> or changing the plate resistor of the pentode.

 I have a 220/18V transformer who fit well in size, so i plan tro try it too. I
just need  to build a socket. I am also a bit curious of the linearity of psu
transformers driving audio. My experience with real audio ones, mostly vintages,
has learned me that they are often unlinear and passive RC compensation networks
are welcome on them....

May be i am a kind of perfectionist, never satisfied ...Anyway, like Ken Stone, i
admit that i love this filter ! I have no other one sounding like this ! Thanks
again :)

Best Regards!

Marc B.