What Exactly DOES a Presence Control Do on a Tube Amp?

That mysterious control, usually found on the far right (or sometimes left) side of the control panel; what the heck IS it, anyway?  Most of us can figure out terms like Bass, Mid, and Treble, but “presence”, well that one seems a bit ambiguous.  Fact is, it IS ambiguous!  Enough so that it can mean very different things on different amps.  For the purpose of this blog, we will only concern ourselves with the true presence control found on some tube amps.

Here is a question I received via email:

The issue is that I have my presence control cranked really high for clarity. My understanding of the "presence" control is that it is the frequencies higher than “treble."

And here was (and still is) my response:

In tube amps the "presence" control is usually a shelving type

of equalizer, and is normally implemented as a low-pass filter inside the

global negative feedback loop. By decreasing the amount of high

frequencies that are fed back, the high frequencies at the output of the

amplifier are boosted ... so not exactly "above" the treble control ...

but MUCH different, as it's after the input gain stages!

But of course the big question is: what does this actually mean in tone terms.  As is often the case, there is no universal answer.  The presence control is so tied to the amps negative feedback loop!  The amount and type of negative feedback employed in the amps design, and the frequency of the presence shelf vary greatly from one amp to another.  However, here is one universal truth concerning true presence controls in tube amps:  Since they are in the final stages of amplification, they are generally less affected by input settings, which is not the case on a typical tone stack treble control.  So, if you want to dial-in an overall amount of bite in your amp, that will remain more or less the same regardless of input gain settings, this is the place to do it.

Are presence controls “above” the frequency of the treble control?  Sometimes, sometimes not!  Oh, and then there is the issue of certain manufacturers mudding the water.  Like this solid-state Randall with a “Presence” control.  What?  My dad, the old farmer, has a saying: “like tits on a bull”. It just ain't right...

See ya all next week! 

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