RCA ba-25 6386 vari-mu compressor schematic / PRR synopsis

[rafafredd]

I don't think I would bother making an exact clone of this device. It's not exactly famous, and although original units sell for a lot of money we're not talking about holy grail dollars here.

I don´t think this is a good reason for cloning or not cloning a unit. the question is, does it sounds great?

I do like my moded STA-level A LOT, by the way.

 

[emrr]

Well, we're talking taste here. I just think if you want one enough to consider the clone proposition, you need the identical transformers for it to really sound the same, so you might as well look for a real one and refurb. it since they're not at crazy prices yet. Far easier than starting from scratch unless you have a ton of time and no $. I never have any time, personally, not that I ever have any $ either. Or clone if you really want the best possible stereo pair match, which would be difficult with original units.

I used the two I had continuously on a CD project for a paying client as a way to listen to them and burn them in, and I never found a reason NOT to use them, and I have plenty of other choices. Why did I sell them? I'm trying to consciously limit the # of limiters I own that require 6386's. Plus, I've modded every tube limiter I have to get expanded attack, release, and ratio settings that never existed on the stock units. This gives me the ability to own fewer of them since they cross over into one another's territory more than before.

 

[chrisz]

Anyone have any ideas why I would get a thump/thud like sound whenever any low end hits the compressor? I recapped the entire unit and messed with different values for c16, r39, and r40 but no luck.

 

[Rob Flinn]

Anyone have any ideas why I would get a thump/thud like sound whenever any low end hits the compressor? I recapped the entire unit and messed with different values for c16, r39, and r40 but no luck.

Most probably you have something out of balance.  The whole audio path is balanced which cancels out the control signal bleeding through on to the audio.  Vari Mu compressors rely on this to stop the thumping.  If one side of the circuit is not behaving the same as the other then you will get this sort of problem.  What you could do for a start is use a meter to see what test  voltage you are getting on the cathodes of both sides of the same valve & then the anodes.  Compare and see if they are the same.  i.e pin 2 =pin 8  & pin 4 =pin 6 on the 6386 & so on through the circuit.  This may point you to the area of the issue, but under heavy compression what looks in balance under no compression could be dramatically out.  The first valve the 12AY7 is a phase splitter which turns the unblanced input int a balanced signal to go through the rest of the balanced circuit.

 

[chrisz]

Thanks, Rob. I measured today and got 60v on p4 and 63v on p6 of the 6386. Could that little of a difference be causing it? Before I measured I tried a different 6386 but had the same sound.

 

[Rob Flinn]

probably not.  I would just check through to 2 valves after it in the same way  & see what you get.