Hi guys,
I've got an old JCM800 with the SE Lead mod, which I totally love apart from the low end being a little loose. Not a big surprise with 4 gain stages, all 22nF couplers and 100k/4ohm NFB, though it has a 100/100uF filter cap between the B+ resistors that tighten things up a bit. I'm not sure what John (Suhr) did to the amps he modded but the Suhr SE100 amp, which is their production amp with that circuit, has a very strange NFB/resonance circuit. Not the best photo, but you can see the orange wire from the 10k/0.1uF "meeting point" to the resonance pot, and from there the NFB resistors goes to the speaker jack - which is another thing I don't get, what is the benefit of wiring it to the speaker jack? Why not wire it to the 8 ohm tap or whatever, and have consistent NFB?
Considering these amps are $4000 I would be surprised if it's a mess-up, but of course I haven't played the exact amp in the photo. You can hear in the Youtube clip that it's not super tight like a Diezel VH4 or something, not that that's the aim but mine sounds looser than that clip. I'm guessing the only way to get there is to experiment with the NFB feedback and find the best values, but I'm just wondering what's going on in that amp? I remember trying it that way back when I was a novice, but Nik from Ceriatone said it wasn't a good idea so I just ditched it and didn't think much of it.
https://youtu.be/DpEcKMmWdLI
Strange NFB/resonance circuit
Moderators: pompeiisneaks, Colossal
Strange NFB/resonance circuit
You do not have the required permissions to view the files attached to this post.
-
Xander8280
- Posts: 100
- Joined: Wed Apr 30, 2014 8:18 am
Re: Strange NFB/resonance circuit
From the photo it looks like the resonance control is tied right to the speaker jack (resistor inside heat shrink tubing). Then the orange lead goes to the PI, I see a second orange lead connecting to the same node. Looks like the presence control has an orange lead going under the board also, very likely it's that.
My guess is either it is done because it is cleaner/easier to wire or to optimize the headroom/presence/resonance for a single cab application (assuming that single cab is a 16 ohm cab).
Having the connection on the OT taps themselves makes the most sense; the feedback loop is held constant and you can get the same power amp response with whatever you plug it into.
I like when the presence/resonance on an amp has a nice range so I tend to use more feedback than most but, I always use a PPIMV so I can kill the open loop gain if I want.
My guess is either it is done because it is cleaner/easier to wire or to optimize the headroom/presence/resonance for a single cab application (assuming that single cab is a 16 ohm cab).
Having the connection on the OT taps themselves makes the most sense; the feedback loop is held constant and you can get the same power amp response with whatever you plug it into.
I like when the presence/resonance on an amp has a nice range so I tend to use more feedback than most but, I always use a PPIMV so I can kill the open loop gain if I want.