Local feedback in a class AB amp?

[DuaneOh]

About 10 years ago I built a class A stereo audio amp based on a schematic by Steve Bench that sounds terrific. It is cathode biased and uses EL34’s in push pull triode mode but has a couple of interesting features. The feature I’d like to discuss is its use of local feedback on the EL34’s and no Global Negative Feedback, and how it may be used in a guitar amp. Below is its schematic:

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/PowerAmps/e34lamp.gif

My personal preferences for guitar amps are ones that don’t have GNF, or have very little. One of my favorite mods is to put in a switch which breaks the feedback loop and or halves the feedback. To me, GNF takes away the chime and touch sensitivity but it does tighten up the bass along with having a more defined line where the amp breaks into distortion.

When I built the stereo amp I asked Steve Bench why local feedback? He said “There seems to be no sonic penalty for local feedback, vs global feedback which tends to produce a more sterile but less satisfying sound quality.”

Since building this amp I have seen a few more audio amps that use local feedback, and they have adapted it in different ways:

https://www.diyaudio.com/forums/tubes-valves/246741-local-feedback-push-pull-pentode.html
http://www.tubecad.com/march2001/

I have read through these and I get the gist of what is going on but not sure about how to apply it to a guitar amp. I was thinking about attaching various size resistors between the pentodes plate and the phase inverters plate and see what happened but thought I’d throw it out here first to see if anybody had tried it or has suggestions.

Thanks for your help!

[sluckey]

The Fender Bassman AB165 did just that...

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ab165.pdf

...and that's all I know about it.

[DuaneOh]

Wow! Thanks, now I'm gong down the Bassman AB165 rabbit hole....

[sluckey]

Most people that have one convert it to something else.   

[AmberB]

I had an AB-165 that I played bass through at smaller venues for about 20 years.  I always liked the way it sounded through a 4 ohm 15 inch speaker.  It distorted way too soon for bass through an 8 ohm speaker.  Of course, they were made for a 4 ohm load. 
When I bought back in 1978 or 9, it had the Fender 2-15 cabinet with it.  I hauled that heavy bugger around for a lot of years before I got a Music Man 1-15 cabinet and put a 4 ohm speaker in it....

[AmberB]

The AB-165 has local feedback from the power tube plates to the driver plates, it also has local feedback on the triode before the phase inverter tube, and it also has global feedback...I guess they designed it to be clean sounding...

[pdf64]

Almost any cathode that’s not fully bypassed will result in some degree of local NFB.
Other than the basic paraphase, any phase splitter relies on it too.
Miller capacitance might be seen as a consequence of local NFB, ie the stage gain amplifies the grid to anode capacitance.