Vari-mu compressor

This is like asking if anyone here has used an internal combustion engine to get to work.

The term "Variable MU®" is a registered trademark of Manley Laboratories. The process [which ironically Manley doesn't employ which is how they got the "®" trademark symbol... but I digress] refers to a "remote shutoff" of a tube [usually a triode]... this can also be called a "Delta MU" [Delta being the Greek symbol used in physics to represent "change of", a.k.a "variable"] process.

The way a remote shutoff/"Delta MU" limiter works [warning: seriously over simplified explanation to follow] is that the audio modulates the voltage to the grid of the tube. The control voltage on the grid controls the amplification factor of the tube [a.k.a "MU" of the tube], in a remote cutoff triode you have a much greater range of gain control because of the way the grid wire is wound... the spacing is non-uniform. In a sharp cutoff triode like a 12ax7 the spacing is wound uniformly and there comes a certain point where the flow of electrons is pinched off causing the tube to stop passing audio... in a good remote cutoff the tube never cuts off entirely...

So, as the quantity of audio increases [stuff gets louder, more voltage] the current to the grid of the tube decreases, thus the output level of the amplifier decreases. When you employ a "Variable Gain Amplifier" [VGA] of this nature, it creates a situation where you do indeed have less dynamic range, the point and purpose of an audio compressor/limiter. The amplification a tube provides is called the "MU" of the tube, by changing the "MU" you change the gain, by changing the gain of the tube in a manner controlled by the actual audio, you control the dynamics of the signal.

There are different tube types that can be applied for this task, some are triodes, some are pentodes. Generally, we find that triodes have a greater abundance of even order harmonics associated with their distortion characteristics so we find them "warm" and musically pleasing, however, some pentodes also fit into this category... in other words, the tube chosen for the circuit is an integral portion of the design of the circuit and will seriously affect the resultant tone of the equipment.

One of the biggest questions when choosing a tube for this application is how smoothly the tube responds to the variations in plate voltage. With all triodes the wires on the grid are wound around two metal posts. With triodes like a 12ax7 the way the grid is wound is very uniform so you can actually get the tube to "turn off". With tubes that are really suited to a remote shutoff application [like a 6386 or a 6es8] you will find a very musical, fluid response to the way the tube handles the audio. The way the grid is wound on tubes that are well suited for a "delta MU" design is that the grid windings are fatter in the middle and narrower on the edges... which means that the electric field never cuts off the flow of electrons between plate and cathode... i.e. the sound doesn't totally cut off and you have a greater control range on the attenuation. The reason I mention this is because there are some "remote shutoff" units out there that employ a 12ax7 tube because they're in plentiful supply and rather inexpensive... the problem is that they don't do the job properly... but hey, they have the right buzzwords so who cares.

So... after this long winded, sorta pedantic explanation of the process... is there like a specific question about a specific unit or are you asking if the process is valid?
 
I was wondering if there was a specific question also (hence my unusually simple reply). For the record, it was a Manley Variable-Mu®, I used just a whisker (just enough to move the needle actually) of gain reduction, and it sounded great.
 
Fletcher said:
This is like asking if anyone here has used an internal combustion engine to get to work.

The term "Variable MU®" is a registered trademark of Manley Laboratories. The process [which ironically Manley doesn't employ which is how they got the "®" trademark symbol... but I digress] refers to a "remote shutoff" of a tube [usually a triode]... this can also be called a "Delta MU" [Delta being the Greek symbol used in physics to represent "change of", a.k.a "variable"] process.

The way a remote shutoff/"Delta MU" limiter works [warning: seriously over simplified explanation to follow] is that the audio modulates the voltage to the grid of the tube. The control voltage on the grid controls the amplification factor of the tube [a.k.a "MU" of the tube], in a remote cutoff triode you have a much greater range of gain control because of the way the grid wire is wound... the spacing is non-uniform. In a sharp cutoff triode like a 12ax7 the spacing is wound uniformly and there comes a certain point where the flow of electrons is pinched off causing the tube to stop passing audio... in a good remote cutoff the tube never cuts off entirely...

So, as the quantity of audio increases [stuff gets louder, more voltage] the current to the grid of the tube decreases, thus the output level of the amplifier decreases. When you employ a "Variable Gain Amplifier" [VGA] of this nature, it creates a situation where you do indeed have less dynamic range, the point and purpose of an audio compressor/limiter. The amplification a tube provides is called the "MU" of the tube, by changing the "MU" you change the gain, by changing the gain of the tube in a manner controlled by the actual audio, you control the dynamics of the signal.

There are different tube types that can be applied for this task, some are triodes, some are pentodes. Generally, we find that triodes have a greater abundance of even order harmonics associated with their distortion characteristics so we find them "warm" and musically pleasing, however, some pentodes also fit into this category... in other words, the tube chosen for the circuit is an integral portion of the design of the circuit and will seriously affect the resultant tone of the equipment.

One of the biggest questions when choosing a tube for this application is how smoothly the tube responds to the variations in plate voltage. With all triodes the wires on the grid are wound around two metal posts. With triodes like a 12ax7 the way the grid is wound is very uniform so you can actually get the tube to "turn off". With tubes that are really suited to a remote shutoff application [like a 6386 or a 6es8] you will find a very musical, fluid response to the way the tube handles the audio. The way the grid is wound on tubes that are well suited for a "delta MU" design is that the grid windings are fatter in the middle and narrower on the edges... which means that the electric field never cuts off the flow of electrons between plate and cathode... i.e. the sound doesn't totally cut off and you have a greater control range on the attenuation. The reason I mention this is because there are some "remote shutoff" units out there that employ a 12ax7 tube because they're in plentiful supply and rather inexpensive... the problem is that they don't do the job properly... but hey, they have the right buzzwords so who cares.

So... after this long winded, sorta pedantic explanation of the process... is there like a specific question about a specific unit or are you asking if the process is valid?
Click to expand...

So basically it sounds really good and costs a whole lot. :D
 
Wow. Great explination.

Simplified worked wonders for my simple mind.

Out of curiosity, what kind of tube would the ART Pro Channel (the one that everybody got accidentally shiped instead of the VLA) have???

I see in the tech specks of various retailers it states

Tubes: 3 hand selected

???? 3 hand selected what???

I am going to assume they are 12aX7's being so cheap and (I think) a hybrid design.

What does "dual-triode" do versus "single?-triode"??


-mike
 
I was under the impression that vari-mu was a electrical term not a trademark ?...Altec also has a vari-mu (436) and there are others..thanks for the electron explanation....I was asking if anyone used a vari-mu type comp (not a specific unit) to cut vox and what folks used (specific unit wise) and their impressions of it...

thanks guys,
Ray
 
Variable MU® at one time was indeed a process, now it's a trademark... so the process is called either Delta MU or remote shutoff... both equally accurate in terms of terminology, neither is a trademark.
 
On the bright side... the Pendulum ES-8, which IMNTLBFHO sounds better and is way, way, way more flexible than the Manley, is only $3,150...
 
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