Putting a 1 Henry choke (Hammond 622 ZA) in series
with the presence control cap might be a good place to
start for this kind of tone.
 
Ray


I have done this with good results. Replace the series resistor in the NFB loop with a twin T filter. The resistor to ground leg of the HP side can be replaced with a pot to allow a change in the center freq. The notch effect gives a boost at the center.
 
jl

Ray,
Don't hold me to this but... IIRC the values were
27k for the resistors .001 on the caps and a 25k pot for
adjustment. I may have altered the values by ear but I
think it ended up around 800 to 1k Hz.
I also forgot to mention you may need to keep some
resistance in series with the filter to maintain the proper
NFB level
 
jl

Great - thanks a bunch for that info! Seems like it
would be possible to build a really killer narrow-band
active boost into almost any amp - if it didn't have NFB
to begin with, it could be added pretty easily in most
cases.
 
Ray