Sunn 200S feedback

[mresistor]

I was looking at this Sunn 200S bass amp schematic this morning, from another post..  I have never seen a feedback circuit like this. The feedback is applied to one of the Ultralinear screen grids on one of the KT88's. I searched for an answer for a while and couldn't find one.


How does this not unbalance the tubes?  How does this work? Does this get balanced out through the OT primary somehow during the PP cycle?


https://el34world.com/charts/Schematics/files/Sunn/Sunn_200s_only_schematic.pdf

[sluckey]

That feedback is not applied to the screen. It originates from the screen and is applied back to the cathode of the 6AN8 pentode, same as the main NFB loop.

The Sunn power amp is a Dynaco Mark III. Here's what the Dynaco manual says about that feedback loop. I doubt you will find any other discussion about that 390pF cap. Notice there is also a more traditional main NFB loop. See attached...

[mresistor]

Interesting - it's for high frequency compensation and imbalance of the cathodyne PI..  Thanks Sluckey..


So, does the cathode of a Cathodyne PI have less signal than the plate as the imbalance ?


OK reading about cathodyne PI's I see they are pretty balanced when set up right. So this feedback applied to the screen of the output tube must be for correction of an imbalance in frequency response or bandwidth of the cathodyne PI.

[sluckey]

Never looked to see. The Tweedle_D guys think so. They put a pot in the cathode circuit so you can perfectly balance the PI. Maybe one of them will comment.

[sluckey]

That feedback is not applied to the screen. It originates from the screen and is applied back to the cathode of the 6AN8 pentode, same as the main NFB loop.

[Willabe]

So this feedback applied to the screen of the output tube......

Reread what Sluckey wrote.

'Feedback' has to be fed backwards to earlier in the circuit. It's not fed forward in the circuit.

[mresistor]

Aha   got it..    I should know this    same for an Op amp..   feed BACK..   

[Willabe]

Yes.