Variable NFB mod?

[john l]

Hi guys Id like to experiment with the 100k NFB resistor in my 1987. Ive heard making this variable can be very useful. Is it just a matter of unsoldering the resistor and wiring in a 100k pot or do I add a different value pot in series? Pot in parallel? What would be a good value pot to start with ? Thanks fellas.

[John_P_WI]

Throw a 50k pot in with a 50k resistor in series with the pot.

[john l]

Sorry are you saying to swap the NFB resistor to 50k and then throw a 50k pot in series with it? Would there be a way to utilize the stock 100k with a different value pot?

[Smokebreak]

It depends on what you want your sweep to be. 1-100k?
25-75? This is theoretical, but as John says, for this application your best bet is probably a 50k to 100K sweep. So replace the 100k with a 50K(or47K)Resistor . Attach one leg of a pot(or trimpot if you want to keep it inside the amp) to one end of the 50k, and then attach the wiper of the pot to the outgoing circuitry. Basically you are replacing the 100k with a 50 k and a 50k pot as a variable resistor . When pot is "off" you have 50k NFB, when it is turned all the way up you have 100k NFB. this is very useful a lot of other places. I just did it today on the Rocket CF. Works great on EXpress 150K gride leak too, to chill the gain.

In order o utilize your stock 100k, you can do the same thing with the pot, but now your sweep starts at 100k and goes to whatever value pot you put in.

You could also parallel a 100k with the existing 100k, which puts ou back at option 1

[Reeltarded]

You have the 100k on the 4R tap?

Try 27k+250k pot on 8R or 16R tap instead. 100k on 4R is very low NFB as is. 277k range on the 8 gets you more and less. On the 16 it will be similar on the low end and more on the high end of travel.

Rig up the pot and start swapping taps.

I used to think I liked it raw (100k 4R) but 56kish on 8R is very nice.

[john l]

You could also parallel a 100k with the existing 100k, which puts ou back at option 1

Thanks for the response. Ideally Id like to have a 27k-250k sweep so I can have more and less NFB than stock for versatility can I get here with with a parallel combination so I don't need to molest my PCB anymore than I have to? Any sugestions.
Also at what value does instability come into play typically with too little feedback.

[Smokebreak]

A 37K resistor paralleled to 100k would get you 27k. Your pot would need to be ~225K. Maybe Mouser has one. I would think things would get nasty above 100K but someone smarter than me would need to chime in. You can try different OT taps too.

[Reeltarded]

Instability? I use the standby switch as an NFB defeat on 4-5 stage amps.

27k and pot need to be in series. Para 27k is limiting pot range to near 27k. Eeee!

[iknowjohnny]

Ideally Id like to have a 27k-250k sweep

Thats almost exactly what i have. I have a 33k in series with a 250k pot. A really useful scenario. I usually like it toward the low NFB end and often all the way, but it is useful at times to use more NFB. I find going much lower than that limits the usefulness too much. You could even use a no load for no NFB at all on the far end of the pit. I tend not to like it any further than it goes now tho.

[Tone Lover]

Instability? I use the standby switch as an NFB defeat on 4-5 stage amps.

27k and pot need to be in series. Para 27k is limiting pot range to near 27k. Eeee!

Go with what reeltarded said here. I have found there is a sweet spot on every amp i like to start with the 27k then use the 250K pot to dial it in.
I first put this in my marshall clone thinking I would change it often , never happened once the sweet spot is found you leave it and move on to something else. It varies with the same circuit (depending for me anyway on my pickups ,speakers and which iron im using for the OT).
I just built a hot rodded 2204 and the sweetspot was 156k, odd I know but it just roared there. Now I use this as just one of my tweek points when Im dialing in that last little bit of sweetness at the end of a build.
Bill

[john l]

I don't get it. can someone baby sit me on the pot wiring please? I piggybacked a 33k on my 100k NFB resistor to get in the neighborhood of 25k then ran the NFB wire to the center lug of a 100k pot with. Looking at the back of the pot I then jumpered the right lug to the center wiper but Im still only reading 100k. I should be geting 125k "ish" right?

I was able to alligator clip the pot in parallel to go from 100k to 50k and it does sound somewhat useful to me.

[Colossal]

Try your 33k resistor in series with the 100k pot wired as a variable resistor and get that working, then go back to parallel when you figure out what you might have done wrong.

[john l]

Hi Dave! Hope you are well.

[Roe]

22k resistor in series with 100k or 220k pot

[Tone Lover]

Try this someone correct me please if im wrong . its late im tired.
Thanks Bill