[sdiy] Re: did someone say "tube"?

[René Schmitz]

jhaible wrote:
> If LDRs are allowed for frequency control, it cannot be that difficult.
> I would think two ECC83's would do the job to buffer the RC's
> and provide  a loop gain of 4.

You could merely cascade two of my filters.
(Btw, the ECC83 doesn't work too good in that circuit, it calls for a 
medium mu device. ECC85, 81 and 82 are the best candidates if you want 
to use common european types.)

> If you want to use tubes for frequency control, however, I wouldn't know
> how to do it for 10+ octaves.

That indeed gets a bit tricky. The space charge law doesn't permit as 
drastic changes in dynamic resistance as the exponential behaviour of 
semiconductor diodes. So VT-Diodes would have a more limited sweeprange.
The most promising candidate in my opinion is to use the 
transconductance control that the screengrid does on a pentode.
Then there is also so called remote cutoff tubes, where the 
transconductance depends on the negative grid bias. And can be smoothly 
controlled over a large range.

Cheers,
  René

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