NFB Loop Help !

[Cejay825]

Ok guys glad I found this forum, decided to try a NFB loop on my Fender Prosonic combo, I'm not an expert on this but I'm decent. I've hooked it up a couple of different ways and it really makes the amp sound good but I'm flying by the seat of my pants and need to know a "correct" way to do it. Values of compnents and stuff, how to achieve the correct ratio with what I've got, such as do I need to change the tail resistor and I think I'm tapping 16 ohms which is causing me to use a high FB resistor value....or it will squeal. what I would like to do is put a pot at the shunt resistor to vary effect. Anyway i've hap-hazzardly thrown it together and it does work and sound good....Ive read all of the Aiken Amp articles but I just need some guidance in the right direction. I've attached a copy of the schematic.

[JazzGuitarGimp]

Have you tried reversing the BRN and BLU wires of the OT primary winding? If the transformer is phased Brown to Yellow, then you are actually applying positive feedback, with will cause the amp to squeal. One way to tell if you've got the NFB phase right, is to compare the volume level with and without NFB. You should have less volume with the NFB in the circuit. If you don't, then you've got the phase backwards, and are actually applying positive feedback.

[martin manning]

Where are you applying the feedback?

[Cejay825]

The amp is a little lower in volume with loop connected, from the factory, my speakers are 8 ohms each in series, a high FB resistor value makes the squeal stop......I have the loop connected at the tip terminal of the speaker jack

[martin manning]

I have the loop connected at the tip terminal of the speaker jack

What about the other end?

[Cejay825]

I lifted the negative side of C25 cap and the tail resistor off of the pc board and connected them along with the NFB loop on one side of a 5k pot and then, to ground through a 250 resistor.

[Cejay825]

Have you tried reversing the BRN and BLU wires of the OT primary winding? If the transformer is phased Brown to Yellow, then you are actually applying positive feedback, with will cause the amp to squeal. One way to tell if you've got the NFB phase right, is to compare the volume level with and without NFB. You should have less volume with the NFB in the circuit. If you don't, then you've got the phase backwards, and are actually applying positive feedback.

Its hard to tell the volume level because of the dynamics the loop adds, they're very close......how much of a difference in volume should it be ??? Right now I swear it sounds just a touch louder with the loop on.....

[martin manning]

I think I would reduce the size of the tail resistor by ~5k, then connect the 0.1 cap to the bottom of that new Rtail, and then put a ~5k to ground. That will keep the total resistance from the PI cathodes to ground as it was, and fixed. Then connect a lead to a pot and a series resistor (say a 250k pot and a 47k) from the top of the ~5k to the OT tap of choice. The squealing might be due to the routing of your mod.

[Cejay825]

I think I would reduce the size of the tail resistor by ~5k, then connect the 0.1 cap to the bottom of that new Rtail, and then put a ~5k to ground. That will keep the total resistance from the PI cathodes to ground as it was, and fixed. Then connect a lead to a pot and a series resistor (say a 250k pot and a 47k) from the top of the ~5k to the OT tap of choice. The squealing might be due to the routing of your mod.

I might have described it wrong.....it squeals if I try to use standard size values of components from other Fender amps like 820/100 etc. I have the squealing contained and stopped and can hear the effects of the loop but my resistors are way above the normal values.......like right now I have a 180k FB resistor, the factory 18k tail and a 250 shunt to ground. I'll see if it will work how you mentioned.....as soon as I relate what your saying to my amp

[martin manning]

Like this...

[Cejay825]

Man! That is awesome Martin !! Saved me alot of time !Thanks I'll report back

[JazzGuitarGimp]

After you follow Martin's directives, if you still have squeal, you should try swapping the Blue and Brown wires from the output tranny. I should add that in the short amount of time I've been on this forum, Martin has consistently shown he's at the top of his game. I tend to take his advice as gospel.

[Cejay825]

Well, I wired it up like the drawing....the sound effect of the NFB loop was there still and seemed the same (good). One thing I noticed was the pot wiper noise was gone, it used to have a low audible hiss in all movement .....thats gone. I wired the pot to where full counter clock wise was 250k. From 250k to 222k was okay, lots of good tones in this range, after 222k strange sounds down to 180k at which point the squeal began to come in, after this, at 180k, for a reference wiper point, the pot measured across the outer pins at 130k. At around 222k the sound of the highs began to sound like clipping ?? not distortion but a strange sound....like too much of a good thing ?? it was doing this before, if it got to a certain point, at this point the wiper reference ohms (outer pins) was 175k.

So, 250k to 222k good and useful
222k to 180k strange sound out of the highs, getting worst as goes down
less than 180k squeal

[Reeltarded]

I think it's the routing!

[martin manning]

Try reversing the OT primary leads as JazzGuitarGimp suggested.

Thanks for the vote of confidence JGG!