presence/resonance circuit question

[tedmich]

I am adding an NFB presence/resonance circuit to an older PV ULTRA+, figured I'd just add the JSX circuit but wanted to use a Fender/SVT dual concentric pot to replace the odd C&K 7211 ON/ON/ON switch in the ULTRA
the resonance switch is absent from a few ULTRA schematics, so I tried to decode it:

I think my read is correct MED and LOOSE have 87k and 330k respectively across the 4.7nF cap, while the TIGHT has just the series C11 (22uF) and R84 (68k) at the PI (not shown)

The JSX circuit:

uses a 1MA and 10KL but my pot has a 1MA/50KL, while I could put 2 x 6.25K resistors from the wiper to each leg and sort of get a 10k pot, what would I expect to hear from the 50k pot vs the 10K? Since the full 10k represents the MAX presence 50K would be even more presence right? maybe a bit shrill? Could I modify the 4.7nF cap to soften the presence?

[pdf64]

Need to see a bit more of the circuit really, but my guess is that the presence control is just being used as a variable resistor, with max presence at minimum resistance. Therefore using a 50k pot will tend to push the useful range between, say, 7 to 10, with 0 - 7 not making much of a difference.
So, it will work and won't alter turnover frequencies or available boost etc, but won't have a user friendly taper.
Pete.

[tedmich]

got the prototype JSX resonance/presence control installed in my ULTRA+ 212/60


tack soldered the ground pin wire until the molex connector arrives. You gotta install the ground wire to the pre tube board connector (Grnd/NFB IN) as the two pins on the output board are common (both NFB OUT) Thats a mPP and a PS film cap along with a Nichicon EL to block pot DC noise (Gotta make sure no ground loops as pre board pin was NC!)

I think it will fit in the ULTRA+ 120 head even with less room due to 1/2 pwr switch. Haven't decided whether to go with the pretty ($!) Gotoh mini dual knob

or (more likely) cut down a PV knob

will try to record clips soon. I am thinking these controls could be real handy dialing in the right sound at LOOOW volume given the Fletcher Munson stuff...

one minor point, be careful to have amp unplugged when installing so loose ground wire doesn't contact AC line and vaporize ground trace on preamp tube board (DOH!) requiring a jumper.... ;(