tube VCF

[René Schmitz]

Hi Ken and list!

At 21:42 10.01.01 +1100, Ken Stone wrote:
>An unmarked triode that in all likelyhood is a 6ES8/ECC189 and a 6064/CV4014
>pentode. The former was on hand. The latter I bought 3 for $1.

I have also used some other tubes in the circuit, a 6N6P as the dual triode
and an unmarked tube which I believe is a EF89. There weren't
characteristical differences, only the gain changed somewhat. In particular
I don't hear a difference between the EF86 and the EF89. 

>Actually, I have the resonance trimmer set to minimum resonance! The
>behavior described was for this setting too.

Right, that way this trimmer makes sence. 

>I didn't know the voltage ratings. I just used what I had to hand. Assuming
>my measurements were correct, the transformer has a ratio of 60:9 or 6.66:1.
>I back fed it with 15V and got 100v out, with the primary loaded with 1.5k.

So there's the difference in gain, I guess. 

>>Hm, I've got about the same, maybe a little less than in yours. 
>>Most likely the gains of the individual stages V1b and V2 are different
>>too, so that the grid of V2 sees a different voltage. I'd be interested to
>>compare these figures.
>
>I'll have to check this later. All the tubes are out at the moment, because
>I've been working on it again. Hammers, chassis punches and tubes don't mix!

Right. Its always so sad seeing a tube suck (...air, that is).

I'll hook up my filter on the bench on the weekend to make some measurements, 
maybe I'll make some samples also...

>>Btw: A neat addition would be to switch the filament to V2 on and off. (I
>>noticed this when swapping tubes in the hot circuit...)
>
>To ramp Q up and down? Perhaps we could even voltage control it? I'll have
>to try it out.

Hmm, voltage control could be possible. One could use a directly heated
tube, which would allow a faster response. Another option for VC-Q would be
to vary the screengrid voltage of V2 in a manner similar to Eric's double-T
VCF.

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