Anyone ever play a 60s Sunn modded with an Orange type F.A.C. knob?

[kerlkoenig]

My old amp tech, a guy who knew his stuff and didn't do a lot of out-there mods, told me he liked to add an Orange-style F.A.C. control to the '60s Sunns, to tame some of the brightness. Has anyone here ever used an amp modded this way? Sadly, he passed away before I bought my Sentura II.

I've got to replace the Orange Drops that found there way into it at some point, and I might wire an F.A.C. up when I've got it apart, maybe in place of the reverb knob, if I can do it in a way that wouldn't be hard to switch back.

If anyone's curious, the amp tech in question was the late John Martin of Philadelphia, and aside from rebuilding Bogen Challengers into guitar amps and Hiwatt-ifying Sound City amps from the appropriate era (I know he did a lot of the former, and I think he mentioned the latter), he didn't talk about mods a lot. He was more into getting the best sound out of the standard configuration, at least as far as I could tell. Didn't know him for too long. He owned several Sunns, so maybe folks on this forum have heard of him before.

[Isaac]

I'm curious. What is an F.A.C. control?

[loudthud]

It's basically a bass rolloff at the input to the power amp implemented as a switch that selects a smallish capacitor in series with the signal.

[mckinnon audio]

   Hi there,I believe the name given to it by Orange was Frequency Amplitude Control,Mel.

[kerlkoenig]

It's a low pass filter with a dial instead of a knob. The dial adds capacitors in series which, in conjunction with a resistor, form a tone network. It's installed after the tone stack in the circuit on all the Orange schematics I've seen. The effect is much more pronounced than using a bright switch, partly because of the values used, partly because of where it is in the circuit, and partly because there are 6 positions instead of 2.

http://www.schematicheaven.com/newamps/orange_mkii1974.pdf

I would probably install it before the reverb and vibrato if I added it to the Sentura II, coming off the 220k resistor before it joins the 620k that is the last part of the effects circuit. I'd start with the Orange values and tweak from there. What I really need a suggestion on is where to run the extra ground that I'd be creating. My Sentura II seems to have original wiring in most places, with the grounds all over the place, and it's so noise-free right now that I really don't want to have to pick all the original grounding scheme apart. I was hoping someone had a working amp that had this mod already installed, so I could ask which existing ground to attach the 100k resistor (the Orange value). Failing that, does anyone have a suggestion?