balanced tube VCA

[René Schmitz]

At 01:18 18.06.00 +1000, Paul Perry wrote:
>Looking at a tube VCA from Rene, at
>http://www.uni-bonn.de/~uzs159/vtvca.html
>
>Can anyone comment on how important it is to match the 
>tubes? It reminds me of a valve compressor from the 
>early 60s, wher it was said to be quite critical to 
>match the tubes somehow....

Hi Paul!

I didn't match the tubes at all. I used two new/unused 6J1B's that were
equally old. (These russian tubes have date stamps.) I found that the
operating points weren't equal, the negative grid biasses differed a bit.
It would be an option to work with a fixed grid bias instead of the
automatic bias generated by the cathode resistor. 

However the imbalance between the two halves is quite irrelevant. The
circuit isn't 100% symmetrical because of the simple cathodyne stage at the
input. The only thing that an imbalance causes is even order harmonics, but
this is to me one of the reasons to use tubes!

The CV-cancellation isn't perfect, but it won't be even with matched tubes,
since the rejection of the common mode signal (i.e. the CV-signal amplified
by the g2-gain of the pentodes) at the transformer isn't high enough. I
expect something like 60-70db of CMRR with such a cheap transformer like I
used. (I did this circuit to get *sound* not precision.) But there is a
trimpot to minimize this imbalance, but for the CV not audio.

Bye,
 René




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