NFB with Ohm selector

[sjwood3]

I have this idea and a piece of paper.  I am using RobRob's NFB switch tailored to the old ground switch in my Twin build.  Link below.    I surmised the NFB should be connected to one of the three....4 Ohm terminal of the Ohm selector/4 Ohm OT tap.     

 https://robrobinette.com/AB763_Modifications.htm#3-Way_Negative_Feedback

Take a look at my (yes) pen and paper drawing and let me know what you think of the layout.   I'm not a pro, so forgive my ignorance.  IF anyone has some good theory and time, I'd love to understand it better.  I get the general purpose of the NFB, but I want both of these features to work together. 

[shooter]

I'd love to understand it better

 

[2deaf]

The stock NFB on an AB763 Twin has a Beta of 0.109 .  Let's say you put an 820 Ohm resistor for the "off" lug, a 750 Ohm resistor on another lug, and a 240 Ohm resistor on the last lug.  Now one side has a Beta of 0.107 (close to stock), the other side has a Beta about twice as big at 0.202, and the "off" position has a Beta of 0.054 which is about half of stock.

---or---

Change the tail resistor from 47 Ohms to the stock value of 100 Ohms.


   

[d95err]

The amount of NFB will vary with the position of the impedance selector switch. Is that intentional?

[pdf64]

The amount of NFB will vary with the position of the impedance selector switch. Is that intentional?

It looked to me like the NFB feed was taken from the 4 ohm tap.

[sjwood3]

The amount of NFB will vary with the position of the impedance selector switch. Is that intentional?

It looked to me like the NFB feed was taken from the 4 ohm tap.

That's the idea.  OT has the taps for 8 and 16, but the amp will be set up and used on the 4ohm tap. I just wanted the switch and options incase the cab change. Then I assumed it would need to be re configured if the lack of nfb caused issues. 

[shooter]

the amp will be set up and used on the 4ohm tap.

not a NFB guy so;
my understanding, the signal at the 4ohm tap is used to "set up" the nfb circuit, regardless of speaker impedance? 

[SnickSound]

The amount of NFB will vary with the position of the impedance selector switch. Is that intentional?

It looked to me like the NFB feed was taken from the 4 ohm tap.

That's the idea.  OT has the taps for 8 and 16, but the amp will be set up and used on the 4ohm tap. I just wanted the switch and options incase the cab change. Then I assumed it would need to be re configured if the lack of nfb caused issues.

The NFB feed is typically connected to o a specific tap, this way you can change the cabinet impedance without affecting NFB operation.

It's true that you cannot use more than one tap at the same time to feed cabinet (the math gets really complicated), but that doesn't apply to NFB since it has a high enough impedance that it makes little to no difference to the operation of the output transformer.