"Resonance" knob, the subtlety alludes me.

What is this beast known as the Resonance knob?  Being a software and analog hardware engineer, I'm having a hard time quantifying this entity...  I twist it around and I'm not sure what is happening if anything - what is happening?  The ideal response would be a technical response, but if someone had a good musician/guitar player usage response, that might be good too.  The manual states start at 12 o'clock, CW for saturated patches, CCW for clean patches.  I sometimes think CCW for slide...  But sometimes I'm not even sure I hear/feel anything...
Thank you!

The resonance knob adds harmonics and overtones, thus adding some thickness and richness. So depending on what you want to do, adjust resonance accordingly. Too much or too little does make a difference, unless you are already quite overdriven etc.

Resonance and Presence are both EQ controls that operate on the power tubes through the negative feedback loop rather than the bass, mid treble controls which operate in the preamp stage.

A lot of power amps use a "negative feedback loop" which sends some of the output signal back into the input of the power amp (but inverted - hence negative feedback).   What this does is to slightly reduce the gain of the power amp overall but in the process stabilise the amp and make the frequency response more even across the board.

When you adjust resonance or presence what you are doing is reducing the negative feedback at low or high frequencies respectively.  This means overall there will be more bass or treble, but also as mm408 says this loss of negative feedback makes the response less flat and encourages harmonics and overtones to emerge.  Some people say the bass is less "tight" but more fat overall as you increase the resonance setting.

I've not got a resonance knob on my Switchblade worst luck, but I did have one on my last Marshall. I'm not sure if CW or CCW increases or decreases the negative feedback on the Grandmeister.

That's my understanding on the subject anyway

Good explanation ricksteruk, it helps to know more of the why, when trying to gauge the what...

ricksteruk, thank you, that was right on the money.  I was born and raised an analog h/w engineer, also with some control systems knowledge - so this is exactly the explanation I was looking for.  I am sufficiently enlightened.

Here's my bassist's explanation:
Resonance makes amp go THUD/BOOM.
Presence makes amp shiny. Ooh! Shiny!

Egads wrote:Here's my bassist's explanation:
Resonance makes amp go THUD/BOOM.
Presence makes amp shiny. Ooh! Shiny!

Cool player's analogy. Nice yet useful contrast to ricksteruk's technical comment. This thread should now answer this question for all...probably...

The basic electronics theory behind it is that a signal that is out of phase with respect to the input of the power amp (or some other point along the path) is mixed in with the original signal resulting in a reduction in signal level. This negative feedback has the same eq as the output when no presence or resonance filters are in the circuit so all frequencies are reduced equally.
The presence circuit is same as a typical guitar tone circuit. When you attach this to the negative feedback path, the highs are shunted to ground removing them from the feedback signal.The result is that the highs are not reduced by the feedback so are now higher in amplitude than the mids and lows at the amps output. The tone gets brighter.
The res circuit is similar (series cap and R instead though) that reduces the lows by blocking them. So with less lows in the feedback signal, the lows are reduced less. That is pretty much the basics of it.
A de-esser and other processors use a similar idea of eqing the feedback to make it target a particular frequency to either increase of decrease it in the final output.

And the last one,,, what you hear to be the difference... if you turn the amp up a little, strike a chord and completely turn the knob down and up,,,, you will hear the difference on both Presence and Resonance. To me Resonance is like hearing a cello (very resonant) versus a violin etc. More richness if you will. Presence is closer to a tone shaping knob in that you will hear a brightness increase.

That said - all the technical wizardry explained by the above posts, make it happen, and make it happen in a cool way, not just adding bass or treble to the tone.

I think a little suggestion from a different angle might help here.  The presence control on an amp can be implemented as just a reduction of negative feedback around the power amp at the topmost high frequencies.  H&K probably follow pretty standard lines there.

I don't think the Resonance control works in the same way but at the bottom end, purely as an additional bass boost perhaps around the feedback loop.  I think it may not be reducing negative feedback at all, that would simply increase or decrease the gain and to a lesser extent distortion which could be made frequency dependent.

I think it may be adding positive feedback, (no it's not quite the same thing as reducing negative feedback).  It may be attempting to first decrease the stability margin and second increase the output impedance reducing the damping factor to make the output ring rather than just have a higher gain in that frequency range.  It will not add enough +ve feedback to approach instability but it will add richness to the overall sound in the same way as a more ringy low damping factor valve output and speaker interaction is better for guitar sound than a perfectly controlled solid state hifi amp which is way too dead to sound "interesting".

With HK,and the amount of control and technology they put into amps,it is possible they are doing something different with the rest control. The generic res control on most tube amps is a cap with series resistor. I guess it really doesn't matter to answer the question though.
To hear the full effect of the res control, the cab has a large impact as doeshow the signal is eq'd heading to the power section. If the cab can't reproduce those frequencies, you won't hear much. I play mine through an old Marshall 4x12 and the effect is anything but subtle.

I'm using a 1x12, so maybe that's why it's subtle to me.