[sdiy] Vacum tube VCA

[jhaible at debitel.net]

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.)

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. 
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.

JH.



> 
> 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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