Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 06, 2019, 10:44:08 am

Title: Sunn 200S feedback
Post by: mresistor on February 06, 2019, 10:44:08 am

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

Title: Re: Sunn 200S feedback
Post by: sluckey on February 06, 2019, 11:26:40 am

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...

Title: Re: Sunn 200S feedback
Post by: mresistor on February 06, 2019, 12:01:12 pm

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.

Title: Re: Sunn 200S feedback
Post by: sluckey on February 06, 2019, 12:13:34 pm

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.

Title: Re: Sunn 200S feedback
Post by: sluckey on February 06, 2019, 12:16:59 pm

Quote from: sluckey on February 06, 2019, 11:26:40 am

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.

Title: Re: Sunn 200S feedback
Post by: Willabe on February 06, 2019, 12:44:04 pm

Quote from: mresistor on February 06, 2019, 12:01:12 pm

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.

Title: Re: Sunn 200S feedback
Post by: mresistor on February 06, 2019, 01:42:39 pm

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

Title: Re: Sunn 200S feedback
Post by: Willabe on February 06, 2019, 03:14:18 pm

Yes.  :icon_biggrin: