Vintage Amp Negative Feedback Help

C

Cvoxdog

Well-known member
I've got a funky 60s EKO Viscount made in Italy head that was supposedly owned by Johnny Depp that I've been tinkering with. I've successfully made some moves with the insane tonestack of this thing and made it more useable. What I'm looking to do next is to decrease the negative feedback from this circuit but I unfortunately I don't know what I'm doing. If you search on google there's detailed notes from someone that refurbished an EKO Viscount Reverb amp for his nephew (same thing just with reverb). He made up some schematics and I was wondering if someone knew by looking at this picture which resistor I could change to alter the negative feedback.
eko.jpg
 
See the 2k2 in parallel with the 2n2 capacitor? That’s your nfb resistor, however with the capacitor parallel to it, you are getting more nub of higher frequencies than low, so maybe a first step is trying to simply snip the cap, or put a resistor in series. The 2k2 value is in line with some black and silver panel fenders, however amps like tweeds and Marshall’s would use a 22k, 47k or even as much as 100k there depending on tubes.
 
Giorfida said:
See the 2k2 in parallel with the 2n2 capacitor? That’s your nfb resistor, however with the capacitor parallel to it, you are getting more nub of higher frequencies than low, so maybe a first step is trying to simply snip the cap, or put a resistor in series. The 2k2 value is in line with some black and silver panel fenders, however amps like tweeds and Marshall’s would use a 22k, 47k or even as much as 100k there depending on tubes.
Click to expand...
a low value NFB resistor means a large amount of negative feedback. this might be necessary for this circuit, i have no idea.

for flexibility, could even just add a 25 K / 50K pot in series with the 2k2 resistor (or maybe a 1K) and have variable a NFB control.

the 2n2 cap is behaving like a fixed depth mod (increasing low end response). You could take it out, or try different values to see what you like.

Or take it out and use the opportunity to add a real depth / resonance control:
1712295658895.png
(in this diagram - the depth control is 'before' the NFB resistor)
 
owlexifry said:
a low value NFB resistor means a large amount of negative feedback. this might be necessary for this circuit, i have no idea.

for flexibility, could even just add a 25 K / 50K pot in series with the 2k2 resistor (or maybe a 1K) and have variable a NFB control.

the 2n2 cap is behaving like a fixed depth mod (increasing low end response). You could take it out, or try different values to see what you like.

Or take it out and use the opportunity to add a real depth / resonance control:
View attachment 72324
(in this diagram - the depth control is 'before' the NFB resistor)
Click to expand...
I agree that the massive negative feedback was by design. Question is, is it for stability? Or simpler to keep it clean (these weren’t designed for guitar so you’d want run it cleaner for pA/hifi application).

A 50k or even 100k linear pot is a great way to tune in the value you like!
 
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