replacing the presence

[fopoman]

I want to add a PPiMV and use the presence pot location. Don't want to drill another hole. Which value resistor should replace the presence control? I used the A4 schematic.

thanks,
Nathan

[HiGain]

Maybe you could make the presence control an internal trimmer so you can alter it's settings...

Jake

[fopoman]

great idea. Didn't think about that.

thanks,
Nathan

[Funkalicousgroove]

you could also do more like an AC30 with no Negative feedback, might sound cool???

[rhinson]

hello there, another idea is to use a different value cap for the presence. they say that van halen and others use a 0.68mf for a more mid type response instead of the brighter response of the standard value. something easy enough to play with. keep in mind also that the larger the neg.fb. resistor the less the presence control is noticed as an effect. you could put the presence pot on the back of the chassis by the speaker jacks. rh

[fopoman]

I've got space on the front of the chassis, so I'll just add another Pot. the only question i have using the PPIMV drawing is the ground from the pot going to the 220K's. Does that effect the bias circuit?

thanks,
Nathan

[toddyjoe]

Yes, it will alter the bias and tone. I had to rebias when I installed one in a Marshall with two 220K grid resistors, even though I used big 1M pot values. You can compensate by upping the value of the 220K grid resistors so that the effective resistance of the grid resistors in parallel with the full pot values is still "220K" ohms. For example, changing the 220K resistors to about 280K when using a 1M dual-ganged pot will give you the same effective "220K" grid resistance and will not change the loading, bias or tone as much. Figure out the proper parallel resistance for your value of pots. Good luck!