"Targeted" NFB on gain stage

[EFK]

I'm curious if it is possible to add NFB to a specific channel within an amp - specifically, a particular stage. I'd like to run a NFB line from the output to the second gain stage on the second channel lower channel on schematic); I assume it would be applied to the cathode (pin 8, 2.2K)? Is there a problem with the cathode being bypassed by the .1 uF cap? I've never applied NFB in this fashion before but I don;t think there would be a phase issue; I assume it would be applied on the hot side of the cathode resistor, but not sure how the bypass cap plays into this? Any information appreciated!

[EFK]

Well that didn't work. Howled and throbbed like a sick elephant.

Is there any way to apply NFB to this paraphase PI? Or how about some form of localized NFB - can I take signal from an inverted stage somewhere and apply it back to the cathode mentioned above? I really don't want it on both channels - as per the PI approach - although I could put it on a switch if need be...

[Steve Conner]

You can apply NFB to a paraphase PI just the same as any other one. Feed it into the cathode of the left-hand triode. See this schematic for an example.

http://scopeboy.com/poweramp.gif

That is a floating paraphase, but it should work the same with a non-floating one. Note the 136pF cap for compensation to stop HF oscillations.

Too many AC coupled stages inside a NFB loop will make the amp howl or motorboat for sure. The phase shifts caused by the coupling caps all add up, and when they get to 180 degrees, the negative feedback becomes positive. Three or more stages needs special care. (Remember, the classic Fender tremolo oscillator "motorboats" nicely with three RCs.)

[EFK]

Thanks for the advice Steve. Right now I'm experimenting with inserting it into the first cathode within the PI, as you note above, but I'm also playing around with sticking it to the first stage of the second channel (since I couldn't add it to the second stage of the second channel) although it's probably not nearly as effective. It does work though.