Presence control

[23]

I know this isnt the right place to put this, but I figured it would get seen more... I just bought a Jet city jca22h and really hate the presence control as it goes from subtle 0-7 and wide open 8-9, I want a more usable range, Im used to jcm 800 and the like presence controls where I dont have to go above mid way.... Any ideas on how to mod this. I emailed a guy that offers mods for this amp and offered cash for the info but he hasnt replied.

Of course I cant find a freaking schematic.......................

[Mark Hammer]

"Presence" controls can vary.  Perhaps more importantly, what gets called a presence control on amp A may produce its effect in a different manner than what was called presence on another amp.  I see amps with "Presence" controls that are essentially upper treble rolloff pots in the preamp section, rather than the classic variable negative feedback one might see on a an older Fender or Marshall.

HOWEVER, given that the Jet City is tube-based, and Soldano-designed, one might reasonably assume this presence control is of the more traditional variety, such as one finds in a....Soldano amp!  (http://www.guitarelectric.eu/sch/Firmowe/SOLDANO  ).

This type of presence control feeds back a portion of the post-output-transformer signal to the phase-splitter/driver stage just before the power tubes.  That feedback resistor, and the presence pot, act like a feedback path on an op-amp.  The more feedback signal there is, the more the output is "damped", except in the case of presence controls, that damping is relegated to the top end.  Less negative feedback = more top end and harmonic content.  More feedback = tamer top end.

Okay, so how do you adjust the amount of negative feedback?  Well, for any pot/voltage-divider, what comes off the wiper is a joint function of the resistances on each side of it.  If the resistance between the source and the wiper is much greater than than between the wiper and ground, then most of the source is conserved.  As the wiper-to-ground resistance gets lower, more and more of the source signal/voltage is dumped to ground.

So, if we look at the Soldano  Atomic 16 here - http://www.guitarelectric.eu/schematy/Firmowe/SOLDANO/Soldano_Atomic_16.gif.html#schemat - we see a 47k resistor coming back from the speaker side of the transformer,  to a 4k7 resistor to ground; in essence a 51k7 pot, turned down about 90% of the way.  In parallel with that 4k7 resistor to ground, we see a 25k pot and .1uf cap.  At full pot resistance, the feedback signal would rather travel down the 4k7 resistor to ground than via the 25k/.1uf path.  But, as we reduce the value of the pot, such that its resistance gets lower than the 4k7 pot, it provides a more efficient path to ground for the feedback signal, so more of it is dumped, providing less negative feedback (and damping) to the driver stage...but only for the frequency content above that dictated by the value of the cap.  Make the cap value smaller/lower, and the reduced negative feedback affects a smaller portion of the spectrum (higher up).

So, what about YOUR amp?  If you find the effect of the presence control is too drastic, then there are several courses of action.  One is to modify the taper of the pot, such that the control zone of most relevance to you is expanded.  You might also consider reducing the value of the pot resistance by means of a parallel resistor.  In the Atomic 16 example, you need to quickly get through the first 20k of pot resistance before that last 5k has a chance to provide any impact.  In that example, straddling the wiper and ground side of the pot with a 10k fixed resistor of suitable wattage would bring the max resistance of the pot down to just over 7k, making most of the rotation of the presence pot produce a noticeable change, rather than just a bit of it.

If you make the cap value lower/smaller, than the presence control will provide varying amounts of negative feedback only higher up in the spectrum, such that lower order harmonics are never "tamed", only higher ones.

Does all of this make sense?

[John Lyons]

One is to modify the taper of the pot, such that the control zone of most relevance to you is expanded.
If you just want a smoother transition across the pots travel then I'd do this.
Find out what the pot is and change it to a Reverse log taper. Or you could use an Audio/log pot and have it work backwards
if you can find the correct value in reverse log. Both will work the same way, just opposite. 

[Mark Hammer]

In fact, the presence control on my old tweed Bassman IS wired to work "backwards".  That is, as you turn it up, the sound gets warmer, rather than brighter.  I imagine it is a log taper pot, wired "in reverse".

Part of that, I suppose is because in the 50's distortion was a bad thing, so adding more negative feedback (hence less treble) as you went clockwise was "more" in the sense of more correction for what shouldn't be there.  Although it may have just been that reverse-log pots of that value were not easily scored. 

[23]

Actually  from 7-9 on the pot only really has any effect on the amp, so I would just drop a 10k resistor across the wiper to ground lugs to move this up on the pots rotation so that more of the pot is useful in it rotation?

and yes this amp is considered very close to the atomic 16

thanks... is this right? also what voltage would be alright?

[Mark Hammer]

1)  Be sure the pot is 25k before changing anything.

2). One watt resistors are typical for amps.

[23]

got it, thanks mark........