VARI-MU Sidechain question (altec 436 very modified)

[Aleguitarpro]

Hi.
I have on the bench a Vari mu compressor based on Altec 436 circuit, modified in tube line-up and in other parts, like sidechain.
I don't know much of the story of this compressor: the owner bought it from a studio and the builder seems to be an amateur that worked in the studio years ago.
It's not well made (messy inside) but the most strange part for me is the sidechain, especially the part of threshold.

The compressor start compressing too early but before modifying it I'd want to understand the mods on it.
The tube line-up is:
V1 - 6BZ7
V2 - 12AT7
V3 - 6AL5

The Threshold pot is a dual 47K log pot.
The strange thing is the connection on it, so I post a scan of that part of the scheme I redraw.
Compressor works but the strange things for me is:
1. The second deck of the pot and the design that is not simmetrical, so I don't understand the need of a dual pot or this configuration
2. The absence of a point of the power supply connected to pins 1-5 (as in UA 175-176 compressors, Altec 436 and Gates Sta Level, for example)

If you have any ideas to clarify this scheme, please share!
I think to bring it back to original Altec scheme in this parts...

Thanks,
Ale

 

[emrr]

Hard to see on my phone, by think I got it. 

Connection to power supply that you do not find is threshold/ratio bias voltage.  That's the usual method for setting threshold/ratio. Without that connection both threshold and ratio are set as low as possible.  Perhaps they want that sort of very low ratio at all times and are attempting to control the signal into the sidechain rather than controlling the diode threshold itself. I've never tried this but it seems to me that it would not work very well. 

 

[Aleguitarpro]

Hard to see on my phone, by think I got it. 

Connection to power supply that you do not find is threshold/ratio bias voltage.  That's the usual method for setting threshold/ratio. Without that connection both threshold and ratio are set as low as possible.  Perhaps they want that sort of very low ratio at all times and are attempting to control the signal into the sidechain rather than controlling the diode threshold itself. I've never tried this but it seems to me that it would not work very well.

Thank you very much for your answer.
It's not bad the way in which this compressor compress (maybe due specifically to its low ratio) but if I'd want to control the diode threshold how can I calculate bias voltage?
I need the theory to understand s.c. in vari mu compressors...
I know that attack-release on a sidechain of a vari mu compressor are simple based on basic r-c theory (depends on the total resistances through the time constant cap) but I need to understand well and calculate ratio/threshold.
The amount of gain reduction depends obiouvsly on the ratio/threshold of 6AL5 but also on how the tube's remote cutoff is.
With 6BZ7 (as in this case) the tube is asleep aproximately with > -10VDC as VDC in sidechain, if I read the datasheet right, isn't it?
So, if I feed the tube with an appropriate bias voltage can I control in a better way the gain reduction (threshold of the diode change also compression' ratio)?

Any reference (papers/articles/books)?
Thanks,
A.

 

[emrr]

There are many detailed threads about this topic.  Look at the original schematic and manual, note the ratio range and the adjustable bias on the side chain rectifier.  Look at the same data for other limiters.  Easiest is to experiment and measure.  If the threshold is too high, the unit will clip during compression. If attack is very slow, the threshold has to be lower to avoid transient clipping.