Removing Negative Feedback

[zozoe]

Greetings,,, on a soon to be mine Ampeg AC12 guitar amp, is removing the negative feedback as simple as just removing whichever resistor? Fear not, this will be performed by an experienced hand.....
AND, after removing the resistor, do I just leave it open, or do I need to re-complete the path, w/o the resistor?
Kindly point out on schematic...
Thnx, Kenny ॐ

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[martin manning]

I would put a pot, say 500k, in series with R38 (68k) so it could be dialed down to taste.

[Stevem]

Wouldn’t a 250K have a smother less sudden change?

[martin manning]

My thought was make it big enough to be essentially zero NFB at max resistance, actually 1/8 of the original FB voltage. Maybe 250k, or 1/5, would be enough.

[zozoe]

I think I'd favour to be able to get as close to 'zero' feedback (max gain) as possible.... So, simply take which leg of that resistor, put it to the outer leg of the pot, and the pot's middle lug continues the 'path'?? What about a ground, if necessary? Does the pot need to go 'after' the resistor, as the path goes?? Exactly where would the pot go, & could I use a 1Meg pot for even 'closer to zero' FB?
Thnx ॐ

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[martin manning]

Doesn't matter which side of the 68k. You could use a 1M, but most of the action will be in a smaller part of the rotation.

[zozoe]

very cool... I believe I get it... but let's say I just wanna clip the NFB and not have a pot or switch (which I probably will want), could I simply clip either end of the R & leave it taped off, or remove it without completing the circuit that existed previously? ॐ
thanks again~

[martin manning]

Yes, you could just lift one end of it and have no NFB at all. That might cause stability issues, though. You could lift one end of it and add a 1Meg (or whatever) in series back to the board. I think I'd temp in a pot and find out where you like it.

[syscokid]

… could I simply clip either end of the R & leave it taped off…

If you clip it, you might not be able to reconnect the resistor. I would desolder one end of the resistor and gently lift/bend the lead out of the way. Then I would test the amp with no NFB and hear how it behaves.

[zozoe]

Yèr all rockstars!! Thanks!! FWIW, this is all about my new old Ampeg AC12, which proceded their model
GU-12, (favoured by Keith Richards during Exile & other players) 'round 1969 thru 1971. From everything I've been able to dig up, the only difference between the 2, is that they yanked the NFB on the GU's & gave it separate Bass & Treble controls.... ॐ

[zozoe]

Thanks again, so just to put a lid on this baby, if I simply wanted to hear just how it sounds w/o the NFB, I could "interrupt" the wire from the spkr jack to the nfb R? And if I liked it as is, w/o any NFB, I could let it hang as is, perhaps just taping it off? thnx

[zozoe]

My on/off toggle has 2 connects,,, the end of the resistor will go to one, & a lead will go from the other 'connect' and complete the circuit at the point I clipped it? thnx

[martin manning]

Yes correct.

[zozoe]

Thanks MARTIN & everyone else!!

[zozoe]

OK,,, here's what I'm staring at, R38/68k NFB.... Being an above average solderer & welder, I'm still skittish around traces that one can easily be ruined ..... Given what I have here, depending on the clipped results, I'll probably want to insert a simple on/off switch.... If I quickly hit that lower solder point on the green side of the board & lift the resistor, I'm good, & I should make sure there's solder remaining to allow any continuity? Then from there, I could simply add leads to take me to a simple on/off switch? Thanks a million!!

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