Bas Horneman said:
I thought he meant that...but still what is a presence control? Is it like a soundstage/width control?
On my guitar amp it seems to function sort of like a "loudness" control but with at least some boost across the frequency spectrum rather than concentrated at the low end. (But that is what my ears hear, maybe not what the circuit is really doing!
)
Presence control, or "variable negative feedback" (sort of, anyway) you might call it too, is not beneficial to the overall sound of an SE guitar amp. Unlike on PP-A / PP-AB, where it's almost mandatory. If you want more detailed answers go to ax 84 or Randall Aiken .
If you really need some sort of "feedback" in a SE amp, go for a pentode tube in UL mode. That'll give you local negative feedback, according to Randall Aiken. You could also wire in an NFB from the output back to the driver, but I honestly don't know if that will sound any good...
If you really need some sort of "feedback" in a SE amp, go for a pentode tube in UL mode. That'll give you local negative feedback, according to Randall Aiken. You could also wire in an NFB from the output back to the driver, but I honestly don't know if that will sound any good...
You just need to add a global feedback loop from the transformer's secondary and inject that into the cathode of your driver stage through a voltage divider and then shunt the highs to ground through a .1uF cap and a variable resistor.
Assuming that there is a common cathode driver stage, like half a 12ax7, just add a resistor to ground beneath the current cathode resistor and apply the feedback to the junction of the two through a series resistor. That will form a voltage divider to let you control the amount of feedback.
That junction is also where you put the presence control. Wire one side of a .1u cap the the junction and the other to the wiper of a pot and ground one of the pot's other terminals.
Assuming that there is a common cathode driver stage, like half a 12ax7, just add a resistor to ground beneath the current cathode resistor and apply the feedback to the junction of the two through a series resistor. That will form a voltage divider to let you control the amount of feedback.
That junction is also where you put the presence control. Wire one side of a .1u cap the the junction and the other to the wiper of a pot and ground one of the pot's other terminals.
igge said:
Feedback from the output/speaker secondary in SE guitar amps was and is very common practice. If you review Leo Fenders original Champ and Princeton schematics it was a feature I believe they switched to in the 60's.
I mention this only because yesterday I built up a cute little 1960's Blackface Clone , Heres a link that has the schematic at the bottom.
Black Face Champ
The only reason for a presence control on this type of amp would to deaden it. Its very bright with lots of punch. I can see why so many guys tore up their OEM speakers in this baby.
On My Ampeg, The presence control seems more like a tone mute.
Gene
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