If I remove presence knob from the circuit, what happens to the tone?

fatbagg

Gold Supporting Member
Messages
3,008
Does the amp get
Brighter now that I cannot control/subtract highs for audibly alter NFB?

Let’s say I have a Bogner 101b and Ihave a mod done and they remove the presence, knob has no function. Does this make the ampsound as if the knob would be maxed (essentially ‘out of the circuit’ )?
 

HotBluePlates

Member
Messages
10,654
... Let’s say I have a Bogner 101b ...
Do you have a schematic for that amp? Because it depends on how the Presence control is implemented in the circuit.

If it was a Fender amp, or a vintage Marshall amp, the result would be the same as the Presence control turned all the way down.

But a different amp with a different implementation might have a different result.
 

oneblackened

Member
Messages
1,216
Depends. In the common Fender/Marshall style NFB treble dump, disconnecting the pot would be like having the presence control all the way down.
 

Tone_Terrific

Silver Supporting Member
Messages
33,019
I have a PV Special 130 with presence control. Other PV's have them. Randall have used the term on SS amps.
The thing is, it is only a name for a frequency band, the NFB circuit in a tube amp is not required.
Even tube amps with presence controls do not necessarily configure the control that way.
 

teemuk

Member
Messages
3,178
And there are also SS amps with "power amp NFB presence", such as modern Blues Cube Artist.

And tube amps with "passive preamp presence" control, such as Dual Rectifier.

"Presence" is really just a name for upper high frequency tone control, not any sort of description about circuit architecture or topology of the said control.
 

Badside

Member
Messages
1,413
Does the amp get
Brighter now that I cannot control/subtract highs for audibly alter NFB?

Let’s say I have a Bogner 101b and Ihave a mod done and they remove the presence, knob has no function. Does this make the ampsound as if the knob would be maxed (essentially ‘out of the circuit’ )?
Depending on how the circuit is designed, and whether it's simply removed versus "bypassed", it will either be like Presence on full or Presence on zero (unless they hardwire your preferred knob position with a resistor).

If you were to just remove the knob on an old Marshall it would actually completely mute the amp since the pot acts as the resistance to ground for the phase inverter circuit.
 

pdf64

Member
Messages
7,366
If you were to just remove the knob on an old Marshall it would actually completely mute the amp since the pot acts as the resistance to ground for the phase inverter circuit.
I think that some degree of signal may still be able to be passed, as the LTP current would have a path to 0V via the series feedback resistor and OT secondary.
 




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