#1
Hey UG,

I have a quick question: What does a tube amp's presence control do?

Thanks in advance.
#2
It controls the upper mids.

Some people (myself included) like to raise the presence instead of the treble.
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Quote by utsapp89
^I'd let a pro look at it. Once you get into the technicalities of screws...well, it's just a place you don't want to be, friend.
#4
Isn't presence a treble control above the treble with even higher frequencies?
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#5
I think it just depends on the amp and the company and what frequency range they think is where the amp needs to be boost/cut for more/less "presence".

*shrug*
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Quote by utsapp89
^I'd let a pro look at it. Once you get into the technicalities of screws...well, it's just a place you don't want to be, friend.
#6
Yeah, I thought it controlled the frequencies higher than the treble knob controls.
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#7
Quote by Gabel
Isn't presence a treble control above the treble with even higher frequencies?


It can vary I think. On most amps it controls the upper treble frequencies, but some control the upper mids.
#8
You got me thinking. Huzzah for google:

The negative feedback loop in an amp is a sort of 'damping' (that isn't quite the right electronics term, but it will do) and makes the power section run more linearly, or 'smoothly' - by taking some of the output signal, running it 'backwards' (that's the negative bit) and re-applying it to the input, as a sort of self-regulation.

If you put what are essentially tone controls into the negative feedback loop, you can affect the degree of smoothing at those frequencies. Presence affects the top end, resonance the bottom end. Basically turning either of these 'up' actually turns those frequencies down in the loop, and allows those ranges to be less restricted. They don't sound the same as simple bass and treble controls because they affect the dynamics of those frequencies more than the 'amount' of them - which is why you tend not to hear them doing very much at low volume, but once the power section is really cranked they can become more effective than the normal tone controls.


and:

So you know that two audio waves that are "out of phase" wil cancel each other out, right?

Negative feedback exploits this. Tube amps are hardly "linear" or "flat" when it comes to frequency response. Specifically, there is usually a 'bump' in the midrange. In many hi-fi circuits (and guitar amps starting with the fender brownface amps, since Leo was always looking for a more 'hi fi' sound) there will be a small amount of the output signal "fed back" into the audio path at an earlier point with polarity inverted so that it is out of phase with the main signal.

Since we know that two signals out-of-phase will cancel, this means that a little bit of the audio gets cancelled out. This works to "flatten" the frequency response, since the frequencies that are loudest in the fed-back signal get attenuated most in the overall output. The loudest bits of the fed-back signal are doing "the most cancelling," to put it another way.

Now, if we add tone controls (like a simple capacitor treble-bleed off tone control as on your guitar) to the negative feedback circuit, we can control how much treble gets fed back and "cancelled out." By turning the presence "up" we are actually 'darkening' the signal in the negative feedback loop, making it so that the treble frequencies are "less cancelled." This gives not just a brightness, but an overall liveliness in the top end that cannot be accomplished with a simple treble boost.

You could also experiment with removing the negative feedback in your amp entirely. It only involves disconnecting one wire. For some things, the sound of the "naked" power tubes is just the thing. The speaker will be more responsive, the transition into overdrive from clean smoother, and the amp a touch louder, midrangier, rawer, and 'tweedier' with the NFB dis-engaged. On my BF deluxe, I removed the 'death cap' and routed the NFB through the now-defunct ground switch so I can switch it in or out. I think of it as a "blues/jazz" switch.
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Quote by utsapp89
^I'd let a pro look at it. Once you get into the technicalities of screws...well, it's just a place you don't want to be, friend.