[sdiy] Vacum tube VCA

[karl dalen]

 --- jhaible at debitel.net skrev: 
> A single tube VCA, pentode or triode, will suffer from
> a high amount of CV feedthru. (It can be done, of course,
> just as you can build a VCA from a single transistor.)

I had that in mind too for a moment, something like the
JX3,JX8P VCA.

> Much better: using a differential pair.
> This works with pentodes and triodes. In order to make it work
> fairly well with triodes, you need remote-cutoff tubes.

I was more into that Dough have done in his tube theremin,
but then summing the diffs into a regular opamp.
  
> This is because unlike a transistor differential pair, you
> control the gain by changing common mode grid voltage, not
> cathode current.
 
> Search for UA175 schematics on the web. There you see how a
> dual semi-remote cutoff triode (6BC8) is used as a VCA
> in a compressor circuit.

Thanks for the tip.

Reg
KD

> > 
> > Quoting karl dalen <dalenkarl at yahoo.se>:
> > 
> > > Hi!
> > > 
> > > I have a new 6SL7GT sovtek tube and would like to use
> > > it as a quality vca, gain cell, but at low voltages!
> > > 
> > > Would it be possible to get this with this tube or 
> > > any tube for that matter?
> > > 
> > > Whats your idea designing a low voltage gain cell
> > > around tubes!
> > 
> > Well, the 6SL7 is a dual triode, but normally a pentode is required for
> VCAs,
> > 
> > so I don't see how that could work. As far as low voltage use goes, I have 
> > never tried this, but I can't see any real advantage either.
> > 
> > Take care,
> > Doug
> > ______________________
> > The Electronic Peasant
> > 
> > www.electronicpeasant.com

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