Vari-mu / UA 176 questions
« on: April 13, 2010, 09:35:47 PM »
I've recently become very interested in Vari-Mu compressors, and I've been looking over the UA 176 schematic (  The dual diode tube (the 6al5) can be replaced with a pair of silicon diodes, right?  I'm assuming this will give a different response, but I don't mind as I'm not trying to build a clone.

Also, is the interstage transformer necessary? And for that matter, is the 12ax7 really necessary? I've also been looking at the am864 schematic ( which is without the interstage and extra tube, and people complain about its insane output levels all the time.

Any help is appreciated!


Re: Vari-mu / UA 176 questions
« Reply #1 on: April 13, 2010, 10:21:05 PM »
Remove 12ax7 and interstage trafo and you pretty much end up with the 436.


Re: Vari-mu / UA 176 questions
« Reply #2 on: April 13, 2010, 11:45:20 PM »
There are pages and pages to be found here about the benefits of an interstage transformer.  Dig around.   PRR, Larrchild, and NewYorkDave did most of the talking. 

The 12AX7 drives the 12BH7, which otherwise would suffer (or benefits, depending on what you want), from having a varying drive impedance as the GR tube shifts around.  And, if you lose it, you have to do away with the feedback loop in the output stage. 

I think the proper topology comparison for what you suggest by losing the 12AX7 would be the GE Unilevel/Collins 356E versus the Gates Stalevel. 

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


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