NFB Control Questions????

[Platefire]

Back in the Summer of 2015 I tweaked an old build, my Peavey Pacer AB763 DR conversion.
One of the tweaks is I added a 50KA NFB pot with a switch as shown in the two pictures and operation as on attached schematic.

My Question is I'm not understanding the behavior?? That being my understanding is more NFB makes the amp brighter/tighter and less NFB makes the amp darker/looser.

It seems to be working just backwards with my control. Where I got it marked "Normal Blackface"
which is suppose to be just the 820 Ohm only to the PI resistor/cap network, the sound is darker/looser at that position. Where I've got it marked "Less Clean" turning pot counterclockwise, it seems to be getting brighter/tighter?

I'm kind of Bamboozled because best I can tell, I got it hooked up right but don't seem to be responding the way I would expect in accordance with the setting I have marked.  The way I designed it to work is as follows:

1-Normal Blackface: Pot open 100% with no pot resistance/Switch on with only resistance being 820 Ohm NFB to PI resistor/cap network.

2-Turning 50K pot counterclockwise from "Normal Blacface" positions adds more resistance/less NFB more gain/harmonics/boost increasing/less bright

3-Fully counterclockwise Switch off/no NFB. Darkest/loosest position.

Please let me know if I've missed something. Why is my operations working opposite than what I expect?? Platefire   

[HotBluePlates]

You're saying the control works opposite of what you expect, but you don't say exactly what it is doing.

While I agree no NFB will be looser, I'd also call it "brighter". That's because there's more distortion, more harmonics. I personally hear that as a brighter sound.

Your wiring & control markings appear to coincide: clockwise turn should equal more NFB until you get all the way up, which shorts the 2 lugs of the pot you're using (for no added resistance, and stock NFB).

[Platefire]

Thanks HPB

Yeah at my "Normal Blackface" setting the notes are more rounder less bright to my ears. I love that setting, it sounds great---not spiky. Sounds great with my tele. I just assumed this would be the brightest setting---but not so---It's rounder tone.

As I turn it counterclockwise from that position I can hear the amp operations noise increase slightly, I suppose because the gain is increasing as less NFB is allowed by. As you say, it sounds more brighter to me reducing NFB.

So I'm not complaining at all but I guess I just thought less NFB would go darker instead of brighter. If that is normal, guess it's hooked up right! Platefire

 

[terminalgs]

One thing to note about your 50K pot:  with the 820 & 47ohm resistors making up the nfb divider, your 50k  has probably 90% of its swing where it's nearly the same (<1%):

"stock" 47/(820+47) = 5.420%

"at full swing 10 " 47/(50000+820+47) = 0.092%

at "1" : 47/(5000+820+47) = 0.801%

a smaller pot would give you more "dialability", like a 1K pot.

If you are done playing with it, and you like the stock set up, you can play around a little bit with a "Resonance" control if you  strap a .01uf or .005uf across that pot .

[Platefire]

Thanks

I have another amp with a NFB pot/switch on a SE EL84. As you turn it clockwise it gets brighter/stiffer and the gain/volume decreases. Turn it counterclockwise it get darker/looser and gain increases.

That's kind of what I was expecting with this AB763. What times I've used the Pacer/Blackface I've left it in the "Normal Blackface" Position because that sounds best to me. Recently playing over it decided to try some of the other positions on the NFB control and it just struck me that it doesn't behave like the SE EL84 NFB does.

So it will probably stay sitting in the "Normal Blackface" Position for my strat and tele guitars and for a LP or 335 maybe cut NFB to get more treble. Platefire

[HotBluePlates]

... I have another amp with a NFB pot/switch on a SE EL84. As you turn it clockwise it gets brighter/stiffer and the gain/volume decreases. ...
... it just struck me that it doesn't behave like the SE EL84 NFB does.

When you look at the back of your NFB pot, you're using the middle & right lugs. When you turn it clockwise, the resistance between them drops.

If you used the middle & left lugs, the resistance would increase as you turn clockwise, and the function of the pot would match your EL84 amp.

The problem is the switch you're using now (to disconnect the NFB altogether) would engage at the wrong end of the pot (full CCW), and go from darkest/tightest instantly to brightest/loosest.

If it would me, I'd prefer the switch action to make auditory sense with the pot's travel, which is how you have it now. Your choice.

[Platefire]

Yeah, thanks! It's good as is. My problem is my main playing with the Praise Band at Church, I have to keep my volume down and even at 22 watts I can't open it up and let it go like it needs to. With those power tubes really working a lot of that treble would go by the way side and there would be more meat but at lower volume it is ever present.

Really I could get by with my DIY Champ full time but I've got all these other amps and feel I need to put them to some use. So I'm constantly rotating. That's why I use a compressor big time now, it helps to get the feel at lower volumes. Platefire

[Platefire]

Just wanted to say thanks HotBluePlates for helping me determine I had it hooked up as I thought I had designed it. So now I don't have to rewire it or change my decals for direction of operation. Platefire